1	CS202 Introduction to Java Introduction to Java Philosophy and approach Similarities between C++ and Java Differences between C++ and Java Examine classes, data types, operations, functions and their arguments, arrays, inheritance, and dynamic binding
2	CS202 Introduction to Java Like C++, Java is a hybrid language Which means the syntax is not strictly limited to OOP constructs, although it is assumed that you want to do OOP using Java (e.g., exception handling is not an OOP feature) The benefit is that the initial programming effort should be simpler to learn and use than many other OOP languages One of Java’s primary goals is to make programming less error prone; for example, Java meets this goal by performing bounds checking by not having explicit pointers by providing automatic garbage collection Much of the foundation of C and C++ has been taken as a foundation in Java, with modifications. This is good news for us! On the other hand, unlike C++, Java does not maintain compatibility with the other languages, so you will find larger variations when moving from C or C++ to Java.
3	CS202 Introduction to Java In Java, we treat everything as an object We can have objects of primitive types (like int, float, char) or objects of class types. Objects of primitive types can be created in the same way that we do in C++ (e.g., int object;) Objects of class types cannot be created this way. First, we must create identifiers for objects that we desire – these are actually references to objects Then, we must allocate memory on the heap for instances So, when we say: List obj; we have created only a reference not an object. If you want to send a message to obj (i.e., call a member function), you will get an error because obj isn’t actually pointing to anything: List obj = new List(); So, for a string object we could say: String s = “CS202!”; or String s = new String(“CS202!”);
4	CS202 Introduction to Java When we create a reference, we want to connect it to a new object String s = new String (“CS202!”); //or List obj = new List(); //default constructor… We do this with the new keyword This allocates memory for a new string and provides an initial value for the character string And, like in C++ this causes the constructor for the class type to be implicitly invoked New places the objects on the heap (which is not as flexible as allowing objects to be allocoated on the stack) It is not possible to request local objects of a class or to pass objects of a class by value to a function (or as the return value). This is because we are always working with a reference to the object – which is what gets passed (by value)
5	Classes in Java Everything we do in Java is part of a class This means that none of our functions can be “global” like they can be in C or C++ Classes in Java specify types, as they do in C++ and allow us to create abstractions Classes must be specified as public, or not. Only public classes are available for the outside world to create objects of If the keyword public doesn’t precede a class, then it is “friendly” and only classes from within this file or package (a group of files) can create objects Every package (or file to begin with) has a public class Inside of a class, Java supports public, protected, and private access (or nothing – which means “friendly” access) But unlike C++, it requires that they be specified in front of each member rather than specifying categories! This means that everything has some kind of access specified for it.
6	Class Access Visibility Unlike C++, a class can be specified as public, or not. A public class within a library specifies which class(es) are available to a client programmer The public keyword just has to be placed somewhere before the opening brace ({) of the class body There can be only one public class per compilation unit They must be the same name as the file Without the public qualifier, a class becomes “friendly”, available to the other classes in the library to which it belongs Classes cannot be private or protected. If you don’t want anyone access to a class, then make the constructors private!
7	Members of a Class Like C++, classes in Java can have member functions (called methods) and data members These look the same as in C++ except for the access control specifier and The implementation of the member functions is provided (in most cases) in the class itself – not elsewhere Each member can be specified as public, private, or protected. If a member has no access control (public, private or protected), they are treated as “friendly” the default means “friendly”: all other classes in the current package have access but all classes outside of this package are private This allows us to make data members and member functions semi “global” – the scope is somewhat broader than C++ static global but restricted from actually be global in nature. It allows us to create a library (a package) and allow related classes to access members directly This helps us to organize classes together in a meaningful way
8	Class Access Specifiers What is the meaning in Java of public, private, and protected? As is expected, public means that the member following is public to everyone using this library Private is not available – only other methods within the class can access that member Helper methods should be private Protected members are available within this class and a derived class (same as C++) Recommendation: limit your use of the default friendly access (Please note, a derived class may not be able to access its parent’s “friendly” members if they are not in the same package! This is because it is possible to inherit from a class that is not in the same package.)
9	Member Accessibility External access public protected (default) private package Same package yes yes yes no Derived class in yes yes no no another package (inheritance only) User code yes no no no
10	Similarities to C++ You will find much of the syntax similar to C++ (primitive types, compound blocks, loops (all), built-in operators (most), switch, if-else), static data members, casting for primitive types, scope of your loop control variables in a for, allowing definitions of a variable to occur where you need them, use of the ++ and – operators, Break and continue
11	Similarities to C++ Other similarities between Java and C++: Two types of comments (// and /* */) Support for function overloading (unlike C) Static member functions (equivalent to C’s global functions) Global functions are not permitted in Java They don’t have a this pointer They can’t call a non-static member function without an object of the class Don’t overuse them if you are doing OOP!
12	CS202 Introduction to Java Minor Differences You cannot define the same named variable in different – inner vs outer blocks (unlike C++ allows identifiers in an inner block to hide those in an outer scope Primitive types in Java are guaranteed to have an initial value (i.e., not garbage!) Java determines the size of each primitive type (they don’t change from one machine architecture to another – unlike C and C++) All numeric types are signed – they do not support the unsigned type. No semicolon is required at the end of a class definition
13	CS202 Introduction to Java Minor Differences No const – instead we use “final” to represent memory that cannot be changed final int I = 10; //a constant value final list object = new list(); //reference is constant – so it can’t reference another object! However, the object itself can be modified This means that class objects in fact can’t be constant! Final doesn’t require that the value of the variable/object be known at compile time final list obj; //says it is a “blank” final reference Blank finals must be initialized in the constructor where the blank final is a member Arguments can also be “final” by placing the keyword in the argument list – which means the method cannot change the argument reference
14	CS202 Introduction to Java Minor Differences Although data members are initialized automatically (and so are arrays, variables of primitive types used in a function (i.e., local variables) are not automatically initialized (e.g., int var;) You are responsible for assigning an appropriate value to your local variables If you forget, you will get an error message indicating that the variable may not be initialized. Also…ints are not bools in Java, you can’t use an int as part of a conditional expression like we are used to. So saying (while (x=y)) can’t happen! Because the result of the expression is not a boolean and the compiler expects a boolean and won’t convert from an int So, unlike C++ you will get an error if you make this mistake!
15	CS202 Introduction to Java Minor Differences Remember the comma operator in C++? In Java it can only be used in for loops to allow for multiple increment steps There is no operator overloading Which means you cannot compare strings with > >= etc. You cannot assign objects to do a complete copy (=) You cannot read and write using >> or << You cannot cast class types! To convert you must use special methods (i.e., function calls) But, you can assign data members values – directly: class list { int I = 100; video v = new video();
16	Java Identifiers A Java identifier must start with a letter or underscore or dollar sign, and be followed by zero or more letters (A-Z, a-z), digits (0-9), underscores, or dollar signs. VALID age_of_dog taxRateY2K HourlyEmployee ageOfDog NOT VALID (Why?) age# 2000TaxRate Age-Of-Dog
17	What is an Identifier? An identifier names a class, a method (subprogram), a field (a variable or a named constant), or a package in a Java application Java is a case-sensitive language; uppercase and lowercase letters are different Using meaningful identifiers is a good programming practice
18	51 Java Reserved Words abstract boolean break byte case catch char class const continue default do double else extends false final finally float for goto if implements import instanceof int interface long native new null package private protected public return short static strictfp super switch synchronized this throw throws transient true try void volatile while
19	Simplest Java class
20
21	What’s in a class heading?
22	Syntax for Declarations Variable Declaration Modifiers TypeName Identifier , Identifier . . . ; Constant Declaration Modifiers final TypeName Identifier = LiteralValue;
23	Operators? Almost all operators work only with primitives (not class types) And the operators are those that you know (except there is no sizeof operator) =, == and != work on all objects of any type (even class types!!!!) But, if you use them with a reference to an object – you are just manipulating the references. = causes two object references to point to the same object (feels like shallow copy!) == and != compares two references to see if they are pointing to the same object (or not)! And, since there is no operator overloading – we can’t change this to do a deep copy! This is because Java allows us to use references truly as aliases. You can cause a deep copy to happen simply (?) by coping each of the members directly that are part of a class or calling a member function to do this The String class also supports + and +=
24	Operators? Equals() method If you want to do a deep comparison you “must” (can?) call a method (equals()) that exists for all objects of class type. Of course, the default behavior of equals() is to just compare the references So you “must” (should) override the equals() so that it actually compares the memory contents of the object I recommend you always override this!
25	What about Arrays? Arrays are available in Java, But unlike C and C++, one of Java’s primary goals is safety. So, a Java array is guaranteed to be initialized and it cannot be accessed outside of its range Range checking requires a small amount of memory overhead on each array as well as index verification at run time. And, as shown on the previous slide, argument passing with arrays are considerably different (look where the [] go!)
26	Arrays of Objects When you create an array of objects You are really creating an array of references Which are automatically initialized to null Java interprets a null as being a reference which isn’t pointing to an object You must assign an object to each reference before you use it! And, if you try to use a reference while it is still null, you will get a run-time error reported Plus, Java provides for range checking – so that arrays cannot be accessed outside of range
27	Arrays are defined… int [] array_name; int array_name []; You don’t specify the size of an array because no space is allocated for the elements at this point All we have is a reference to an array (like in C++ where the name of the array is the starting address of the first element, now in Java the name of the array is a reference) To allocate memory we must specify an initialization expression (which unlike C++ can happen anywhere in your code int [] array_name = {1,2,3,4,5}; //starting with element zero A reference can then be used to also access this array: int [] reference; reference = array_name; We can also allocate arrays on the heap reference = new int [size]; All arrays have an implicit member that specifies how many elements there are (its length)
28	Arrays of class type… All arrays of class types must be defined using new (with an exception of the String class….) list [] array = new list[size]; But, such arrays are actually arrays of references to our objects – not instances! (like an array of pointers in C++) If you forget to allocate objects for the elements, you will get an exception List [] array = new list[] {new list(1), new list (2), new list(3)}; Or, do this explicitly with a loop Unlike C and C++, Java allows the return type of functions to be an array
29	Arrays Arrays are data structures consisting of related data items all of the same type An array type is a reference type; contiguous memory locations are allocated for an array, beginning at the base address The base address is stored in the array variable A particular element in the array is accessed by using the array name together with the position of the desired element in square brackets; the position is called the index or subscript
30	double[] salesAmt; salesAmt = new double[6];
31	Array Definitions Array A collection of homogenous elements, given a single name Length A variable associated with the array that contains the number of locations allocated to the array Subscript (or index) A variable or constant used to access a position in the array: The first array element always has subscript 0, the second has subscript 1, and the last has subscript length-1 When allocated, the elements are automatically initialized to the default value of the data type: 0 for primitive numeric types, false for boolean types, or null for references types.
32	Another Example Declare and instantiate an array called temps to hold 5 individual double values. double[ ] temps = new double[ 5 ]; // declares and allocates memory
33	Declaring and Allocating an Array Operator new is used to allocate the specified number of memory locations needed for array DataType SYNTAX FORMS DataType[ ] ArrayName = new DataType [ IntExpression ];
34	Assigning values to array elements double[] temps = new double[5]; // Creates array int m = 4; temps[2] = 98.6; temps[3] = 101.2; temps[0] = 99.4; temps[m] = temps[3] / 2.0; temps[1] = temps[3] - 1.2; // What value is assigned?
35	What values are assigned? double[] temps = new double[5]; // Allocates array int m; for (m = 0; m < temps.length; m++) temps[m] = 100.0 + m * 0.2; What is length?
36	Now what values are printed? final int ARRAY_SIZE = 5; // Named constant double[] temps; temps = new double[ARRAY_SIZE]; int m; . . . . . for (m = temps.length-1; m >= 0; m--) System.out.println(“temps[“ + m + “] = ” + temps[m]);
37	Initializer List int[] ages = {40, 13, 20, 19, 36}; for (int i = 0; i < ages.length; i++) System.out.println(“ages[“ + i + “] = ” + ages[i]);
38	Passing Arrays as Arguments In Java an array is a reference type. The address of the first item in the array (the base address) is passed to a method with an array parameter The name of the array is a reference variable that contains the base address of the array elements The array name dot length returns the number of locations allocated
39	Passing an Array as Arguments public static double average(int[] grades) // Calculates and returns the average grade in an // array of grades. // Assumption: All array slots have valid data. { int total = 0; for (int i = 0; i < grades.length; i++) total = total + grades[i]; return (double) total / (double) grades.length; }
40	Memory allocated for array
41	Parallel arrays Parallel arrays Two or more arrays that have the same index range, and whose elements contain related information, possibly of different data types final int SIZE = 50; int[] idNumber = new int[SIZE]; float[] hourlyWage = new float[SIZE];
42	final int SIZE = 50 ; int [ ] idNumber = new int [ SIZE ] ; // parallel arrays hold float [ ] hourlyWage = new float [ SIZE ] ; // related information
43	Partial Array Processing length is the number of slots assigned to the array What if the array doesn’t have valid data in each of these slots? Keep a counter of how many slots have valid data and use this counter when processing the array
44	More about Array Indexes Array indexes can be any integral expression of type char, short, byte, or int It is the programmer’s responsibility to make sure that an array index does not go out of bounds. The index must be within the range 0 through the array’s length minus 1 Using an index value outside this range throws an ArrayIndexOutOfBoundsException; prevent this error by using public instance variable length
45	String[] groceryItems = new String[10];
46	String[] groceryItems = new String[10];
47	Date[] bigEvents = new Date[10];
48	Garbage Collection and Objects Another difference with Java is that You never need to destroy an object!!!!!!! Java simplifies the need to manage the lifetime of our objects and manages the cleanup work implicitly! When you create an object using new, it actually exists past the end of the block in which it was defined (although the reference to it ends) This is because Java has a garbage collector
49	Garbage Collection and Objects Remember the problems of returning local objects in C++ where the lifetime has ended? We don’t have this type of problem in Java because objects created with new exist for as long as we need them and we don’t have to worry about destroying them Java has a garbage collector, which looks at all of the objects created with new and determines which ones are not being referenced anymore – then it can release the memory for those objects at that point so the memory can be used for new objects. Please keep in mind that although the garbage collector can release the memory when no more references point to the memory, it may not if the memory is not needed elsewhere On the other hand, this means that you never need to worry about reclaiming memory yourself Simply create objects, and when you no longer need them they will go away by themselves whenever necessary There are no memory leaks!
50	Garbage Collectors: Efficiency? Why doesn’t C++ have garbage collection? There is price to it: run time overhead C++ allows objects of a class to be created on the stack, not available in Java for class objects These are automatically cleaned up Providing the most efficient way of allocating storage Allocating memory on the heap using new is more expensive We have done it in 163/202 to get experience But, in fact it shouldn’t be exclusively used! And, it requires that we allocate and deallocate our memory in C++ But, in Java, this memory need not be deallocated The main issue with garbage collection is that you never really known when it is going to start up or how long it will take This means there is an inconsistency in the rate of execution Which can be important for some real-time software problems
51	Clarifying References Let’s clarify our creation of objects in Java Instances of a primitive type (int, float, etc.) are not references and don’t need to be created using new In fact, we can’t create them using new (except for the case of an array of primitive types) When we create an object of a user defined type (i.e., a class type) we are in fact creating references which means new must be used to actually allocate memory for the instance of the type expected We can then use references to an object using the (.) between the object reference and the member name: Objectreference.member
52	Types in Java Just like C++, we use the keyword class to mean that we are creating a new type class Mytype {…} creates a new data type And, creating objects of this type is done using new: Mytype object = new Mytype(); Like C++, our classes have data members (fields) and member functions (methods) Just like objects outside of a class, data members can be of a primitive type or can be references to another user defined class type (requiring the use of new to actually create an instance of them) Unlike C++, primitive types can be initialized directly at the point of definition in the class and references can be initialized to connect to objects in the class as well
53	Primitive Wrapper Classes To get a primitive type on the heap, you have to use a wrapper class: (Boolean, Character, Byte, Short, Integer, Long, Float, Double, Void) Character Reference = new Character(‘z’); But, since there is no operator overloading We must use methods instead of operators when working with them
54	Three Categories of Data Instance data is the internal representation of a specific object. It records the object’s state. Class data is accessible to all objects of a class. Local data is specific to a given call of a method.
55	Categories of Responsibilities Constructor An operation that creates a new instance of a class Copy constructor An operation that creates a new instance by copying an existing instance, possibly altering its state in the process Transformer An operation that changes the state of an object Observer An operation that allows us to observe the state of an object without changing it Iterator An operation that allows us to process all the components of an object one at a time
56	Instance Data Instance data is the internal representation of a specific object. public class Name { // Instance variables String first; String middle; String last; . . . }
57	Class Data Class data is accessible to all objects of a class. Fields declared as static belong to the class rather than to a specific instance. public class Name { // Class constant static final String PUNCT = “, ”; . . . }
58	Local Data Local data is specific to a given call of a method. Memory for this data is allocated when the method is called and deallocated when the method returns. public int compareTo(Name otherName) { int result; // Local variable . . . return result; }
59	Functions (ahhh Methods!) Functions in Java are called methods (OOP terminology) and can only be defined as part of a class Luckily, they have the same format we are used to – with return types, argument lists, bodies and return abilities Formal arguments have a data type followed by the argument’s identifier Unlike C++, you do not get to select whether they are passed by value or by reference. Technically, you could argue that everything is passed by value. Primitive types are passed by value on the stack (you have no choice) and Object references are also passed by value on the stack (keep in mind this is the reference not the object), which “feels” like pass by reference Again, for user defined types, they are actually references automatically (no – you don’t put the & or the * in Java in your argument lists!)
60	Functions (ahhh Methods!) For example: int my_func(String s) { return s.length(); } The length method returns the number of characters in the string s is actually a reference to the calling routine’s string object void is available in Java as it is in C++ to return nothing from the function While object references are placed on the stack when a function is called – the objects to which they refer are not (never). There is no support of a “pass by value” concept with objects of a class. Therefore, we will never perform a deep copy as part of a function call
61	Method Declaration Syntax Method Declaration Modifiers void Identifier (ParameterList) { Statement . . . }
62	Methods Method heading and block void setName(String arg1, String arg2) { first = arg1; second = arg2; } Method call (invocation) Name myName; myName.setName(“Nell”, “Dale”);
63	Some Definitions Instance field A field that exists in ever instance of a class String first; String second; Instances method A method that exists in every instance of a class void setName(String arg1, String arg2); myName.setName(“Chip”, “Weems”); String yourName; yourName.setName(“Mark”, “Headington”);
64	More Definitions Class method A method that belongs to a class rather than it object instances; has modifier static Date.setDefaultFormat(Date.MONTH_DAY_YEAR); Class field A field that belongs to a class rather than its object instances; has modifier static Will cover class fields in later chapters
65	More Definitions Constructor method Special method with the same name as the class that is used with new when a class is instantiated public Name(String frst, String lst) { first = frst; last = lst; } Name name; name = new Name(“John”, “Dewey”); Note: argument cannot be the same as field
66	Void Methods Void method Does not return a value System.out.print(“Hello”); System.out.println(“Good bye”); name.setName(“Porky”, “Pig”); object method arguments
67	Value-Returning Methods Value-returning method Returns a value to the calling program String first; String last; Name name; System.out.print(“Enter first name: “); first = inData.readLine(); System.out.print(“Enter last name: “); last = inData.readLine(); name.setName(first, last);
68	Value-returning example public String firstLastFormat() { return first + “ “ + last; } System.out.print(name.firstLastFormat()); object method object method Argument to print method is string returned from firstLastFormat method
69	The This “Reference” When memory for an object is allocated, a reference to that object is created and called the “this” reference Like C++, it is the first implicit argument to each method Unlike C++, it is not a pointer but rather a reference! list func() { return this; } Which means we do not need to dereference it It allows member concatenation: Obj.func().func().func(); //etc.
70	Constructors Like C++, constructors are implicitly invoked They allow us to initialize data members to other values than their zero equivalent Note, unlike C++ data members are automatically initialized prior to a constructor invokation (to their zero equivalent) --- even if you provide a constructor The default constructor has no arguments If you write a constructor with arguments, then the default constructor is not provided automatically and you cannot create objects without arguments specified class list { list () { ///blablabla} list (int arg) { //blablabla } //we create objects via; list l = new list(10); //uses the int arg version Yes, you can overload multiple constructors just so the argument lists are unique
71	Differences with Constructors When you write multiple constructors, sometimes we like to have the contructors call another function to actually get the work done (to minimize duplication of code) In C++ we do this by writing named member functions In Java we do this by having one constructor call another constructor with a special usage of the this pointer! This can only happen once within a constructor It must be the first thing a constructor does list (int i) { //first constructor which does the real work} list (int i, int j) {this(i); //calls the constructor with an int }
72	No Destructors!? Since Java provides garbage collection There are no destructors But…have you ever had a destructor do something other than memory deallocation? If you need this – you must write a named function and call it explicitly!! (maybe called : void cleanup()?) If for some reason you do need some kind of garbage collection done that the garbage collector doesn’t know about (like C or C++ memory allocation is being used-not recommended!) You can write a method called “finalize()” which the garbage collector will implicitly call if it is provided prior to releasing memory – and then on the next garbage collection pass it will reclaim the object’s memory
73	finalize() is not a destructor! But! This is not a destructor. Java objects do not always get garbage collected – The garbage collector is only run after all references to an object have been released and memory is insufficient (or running low). It may just automatically return the memory to the operator system after execution! So, use finalize() for releasing memory that the garbage collector cannot predict, but you may need to explicitly cause the garbage collector to be exeucted: System.gc() Bottom line, finalize() cannot be relied upon. Even functions that look like they should cause finalize to be used are problematic and at times buggy. Its invokation is not guaranteed!
74	Inheritance Since one of our primary goals with Java is to perform OOP We always create inheritance hierarchies! In fact, every class, unless otherwise requested, is derived from Java’s standard root class Object To derive a class from a base class in Java means that you are “extending” it class list { //members} public class ordered_list extends list { //more members – replacing old, adding new }
75	Accessing Base Class Members If a derived class has the same named member as the base class It can be accessed by using the super keyword. If we have a “cleanup” type function to be executed at the end of an object’s lifetime, it would need to use the super keyword to invoke it’s base class’s (I recommend that you first cleanup your derived class prior to invoking the base class’ cleanup public class ordered_list extends list { public void member() { super.member(); //calls base class member } }
76	Is there Hiding? Yes and No Hiding exists like it does for C++ for data members (fields) But, a derived class member function with the same name as a base class member function will not hide the base class’ member! This means that funciton overloading in Java works between classes in a hierarchy Which is what we “wished” happened in C++!
77	Constructors in Hierarchies Default constructors for base classes are implicitly invoked from the derived class’ constructor As with C++, from the base class “outward” However, when we have constructors with arguments, this gets more complex (but of course is handled differently than C++!) In Java, we must explicitly write the calls to the base class constructor using the super keyword, followed by the appropriate arguments: This must be the first thing that is done in your derived class constructor Luckily, Java will complain if you don’t do this! Unlike C++. public class ordered_list extends list { ordered_list(int i) { super(i); //causes base class constructor with an int to be called
78	If you have a finalize()… Within a hierarchy, if you need finalize() in a derived class and base class It is important to remember to call the base class’ version of finalize() Otherwise, the base class finalization will not happen! //In the derived class protected void finalize() { super.finalize(); }
79	A Java Application Must contain a method called main() Execution always begins with the first statement in method main() Any other methods in your program are subprograms and are not executed until they are sent a message
80	Where do we get started? main Unlike C++, stand alone programs must have at least one class it must have the same name as the file and within that class must be a method called main! public static void main(String[] args) The public keyword means that the member function (method) is available to the outside world The static keyword means that this is a static member function which does not need a object of its class inorder to be invoked The argument is required (whether or not it is used) which holds the command line arguments. In C++ the command line arguments are optional as part of main int main (int argc, char * argv[]); Where argv is a “ragged array” in C and C++ (an array of arrays of characters) In Java args is an array of string object references
81	Main in which class? Now that we have a hierarchy, where does main go? Well, you can put it in each class so that you can independently test The appropriate main is invoked based on which class name is used on the command line I
82	Final Methods – a special case Final Methods? Means that any inheriting class cannot change its meaning It means that the method cannot be overridden Allows for any calls to this method to be inline for better efficiency Turns off dynamic binding All private members are implicitly “final” Because if you can’t access a private method so you couldn’t override it! public final void func() { //body of the function}
83	CS202 Introduction to Java Final Classes? Means that no classes can be derived from this class (or inherit from this class) For security reasons you do not want any subclassing… Or, you want to make it as efficient as possible Therefore, all methods are implicitly final
84	More Definitions Override When an instance method in a derived class has the same form of heading as an instance method in its superclass, the method in the derived class overrides (redefines) the method in the superclass Hide When a field in a derived class has the same name as one in its superclass or a class method has the same form of heading as a class method in its superclass, the field or class hide the corresponding component in the superclass Say again?
85	An example public class Example { char letter; public static String lineIs(); … } public class ExtExample extends Example { char letter; public static String lineIs(); … } Hiding or overriding?
86	Another Example public class Example { char letter; public String lineIs(); … } public class ExtExample extends Example { String letter; public String lineIs(); … } Hiding or overriding?
87	Class Syntax Derived Class Syntax ClassModifier class Identifier extends ClassName { ClassDeclaration . . . }
88	Overriding vs. Hiding We override an instance method of a superclass by providing an instance method in a derived class with the same form of heading We hide a data field of a superclass by providing a field in a derived class with the same name
89	Polymorphism Polymorphism is the ability of a language to have duplicate method names in an inheritance hierarchy and to decide which method is appropriate to call depending on the class of the object to which the method is applied.
90	Dynamic Binding All methods are bound in Java using run-time dynamic binding Unless the method (or class) is “final” So, we can use upcasting as we did in C++ to produce desired dynamic binding effects: list obj = new ordered_list(); Here, an ordered list object is created and the reference is assigned to a list reference obj.display(); //won’t call List’s display but rather ordered_lists!
91	Dynamic Binding Java, like C++, has some rules to get dynamic binding to work for us The methods must be defined in the base class (to which we use a reference to) and they must be anything BUT private (public, protected, or “friendly” are all ok) We must invoke the function thru a reference to the base class, but have it refer to an object of the proper class to which we are interested The argument lists, function names, and return types must be identical The only difference is we don’t need the “virtual” keyword (that was C++)
92	Overriding or Overloading? When you derive from a base class and implement a method that is in the base class If the arguments and return type are the same you are overriding it If the arguments are different, you are overloading! This is very hard to debug since no other mechanism establishes dynamic binding
93	Abstract Base Classes Abstract base classes can help with this issue Because if the methods from them are ever directly called you will find out immediately that something is wrong The intent, as with C++, is to create a common interface So that the derived classes can express their uniqueness! All derived class methods that match the signature of the base class will be called using dynamic binding This is created by making one or more abstract methods in the base class: abstract void func(); //with no body
94	Abstract Base Classes If a class has just one of these abstract methods, the class must be qualified as “abstract”, otherwise you get an error: abstract class list{ public abstract void display(); } To inherit from an abstract class (and you want objects to exist of your class), you must implement all of the functions that are abstract in the base class An abstract class without any abstract methods means that you just can’t create any objects of that class!
95	Interfaces in Java The interface keyword creates a completely abstract class One that provides for no implementation Makes it “pure” It allows us to specify the method names, argumetn lists, and return types – but no bodies It can include data members, but they are always implicitly static and final It provides a “form” rather than an implemented class Use the “interface” keyword instead of the class keyword All of the members are automatically “public” even if you don’t use the keyword. They are never “friendly” and cannot be protected or private!
96	“Implements” in Java The implements keyword allows classes to “derive” from a completely abstract class or to “implement” the code for a pure abstract class The implementation becomes an ordinary class which can be extended in the regular way Except that members must all be defined as public interface list { void display(); } class ordered_list implements list { Public void display();}
97	Multiple Inheritance! Since an interface has no memory and has not implementation, There is nothing that prevents us from having classes implement more than one interface! If you inherit from a non-interface, you can inherit from only one The “extended” class comes first and the implementation of interfaces must come second, in a class doing both: class ordered_list extends list implements one, two, three { ….}
98	What to use? So many choices! Even if you are not using multiple inheritance Interfaces are preferable to abstract classes which in turn are preferable to concrete classes when thinking about a common base class As we discussed in C++, if you are doing dynamic binding, it is best if all methods are dynamically bound – otherwise you will get stuck with having to know the data type you are dealing with at run time (RTTI) An interface ensures that this is the case
99	Shadowing Shadowing A scope rule specifying that a local identifier declaration blocks access to an identifier declared with the same name outside the block containing the local declaration A shadowed class member can be accessed by using keyword this together with the class member
100	Input and Output To perform I/O in Java requires invoking a method as part of the System class out is a static PrintStream object Because it is static, you do not need to reference it through an object of class System (but can reference it via the class name instead) The println method displays the information followed by a newline System.out.println(“stuff”);
101	Reference Types: A Review
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104	Shallow Copy vs. Deep Copy Shallow copy All class data fields, including references are copied; any objects referred to by data fields are not copied Deep copy All class data fields are copied, and all objects referred to are copied
105	What’s the difference? A shallow copy shares nested objects with the original class object A deep copy makes its own copy of nested objects at different locations than in the original class object
106	Separate deep copy
107	Copy Constructor: Different… A copy constructor is a constructor that creates a deep copy of an object that can be used for other purposes, such as creating a new instance of an immutable object from an old one
108	Java String Class A string is a sequence of characters enclosed in double quotes. string sample values “Today and tomorrow” “His age is 23.” “A” (a one character string) The empty string contains no characters and is written as “”
109	Actions of Java’s String class String operations include joining one string to another (concatenation) converting number values to strings converting strings to number values comparing 2 strings
110	"Why is String uppercase and..." Why is String uppercase and char lower case? char is a built in type String is a class that is provided Class names begin with uppercase by convention
111	Assignment Statement Syntax Variable = Expression; First, Expression on right is evaluated. Then the resulting value is stored in the memory location of Variable on left. NOTE: The value assigned to Variable must be of the same type as Variable.
112	String concatenation (+) Concatenation uses the + operator. A built-in type value can be concatenated with a string because Java automatically converts the built-in type value for you to a string first.
113	Concatenation Example final int DATE = 2003; final String phrase1 = “Programming and Problem “; final String phrase2 = “Solving in Java “; String bookTitle; bookTitle = phrase1 + phrase2; System.out.println(bookTitle + “ has copyright “ + DATE);
114	Using Java output device METHOD CALL SYNTAX These examples yield the same output. System.out.print(“The answer is, ”); System.out.println(“Yes and No.”); System.out.println(“The answer is, Yes and No.”);
115	Java Input Devices More complex than Output Devices Must set one up from a more primitive device InputStreamReader inStream; inStream = new InputStreamReader(System.in); // declare device inData BufferedReader inData; inData = new BufferedReader(inStream)
116	Using a Java Input Device // Get device in one statement inData = new BuffredReader(new InputStreamReader(System.in)); String oneLine; // Store one line of text into oneLine oneLine = inData.readLine(); Where does the text come from?
117	Interactive Input readLine is a value-returning method in class BufferedReader readLine goes to the System.in window and inputs what the user types How does the user know what to type? The program (you) tell the user using System.out
118	Interactive Output continued BufferedReader inData; inData = new BufferedReader(new InputStreamReader(System.in)); String name; System.out.print(“Enter name: ”); name = inData.readLine(); Name contains what the user typed in response to the prompt
119	Inputting Numeric Values If readLine inputs strings, how can we input numbers? We convert the strings to the numbers they represent. “69.34” becomes 69.34 “12354” becomes 12354 Yes, but how?
120	Predefined Numeric Classes Built-in Type Class int Integer long Long float Float double Double parseInt, parseLong, parseFloat, parseDouble are class methods for translating strings to numeric values
121	Converting Strings to Numbers int intNumber; System.out.println(“Enter an integer: “); intNumber = Integer.parseInt(inData.readLine()); class method Buffered- method Reader object
122	Converting a String to a Double Value double price ; price = Double.parseDouble(inData.readLine());
123	Java Program
124	Java program continued
125	Additional String Methods Method length returns an int value that is the number of characters in the string String name = “Donald Duck”; numChars; numChars = name.length(); instance method length is an instance method
126	String Methods Continued Method indexOf searches a string to find a particular substring, and returns an int value that is the beginning position for the first occurrence of that substring within the string Character positions begin at 0 (not 1) The substring argument can be a literal String, a String expression, or a char value If the substring could not be not found, method indexOf returns value -1
127	String Methods Continued Method substring returns a substring of a string, but does not change the string itsel The first parameter is an int that specifies a starting position within the string The second parameter is an int that is 1 more than the ending position of the substring Remember: positions of characters within a string are numbered starting from 0, not from 1.
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129	Relational operators w/Strings? Remember that strings are reference types myString = “Today”; yourString = “Today”; myString == yourSring returns what?
130	String methods Method Parameter Returns Operation Performed Name Type equals String boolean compareTo String int
131	"String" String myState; String yourState; myState = “Texas”; yourState = “Maryland”; EXPRESSION VALUE myState.equals(yourState) false 0<myState.compareTo(yourState) true myState.equals(“Texas”) true 0>myState.compareTo(“texas”) true
132	More String Methods Method Parameter Returns Operation Performed Name Type toLowerCase none String toUpperCase none String
133	String Method compareTo When comparing objects with ==, the result is true only if both references refer to the same object in memory String method compareTo uses a dictionary type comparison of strings and returns 0 if they have the same letters in the same order a negative number if the instance string is less than the string passed as a parameter a positive number if the instance string is greater than the string that is passed
134	Values of each expression
135	Example of If statements (same use of {}, if and else) if (creditsEarned >= 90) System.out.println(“Senior Status”); else if (creditsEarned >= 60) System.out.println(“Junior Status”); else if (creditsEarned >= 30) System.out.println(“Sophomore Status”); else System.out.println(“Freshman Status”);
136	A sentinel-controlled loop Requires a “priming read” “Priming read” means you read one data value (or set of data values) before entering the while loop Process data value(s) and then read next value(s) at end of loop
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138	An end-of-file controlled loop depends on fact that readLine returns null if there is no more data
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140	Flag-controlled loops Use meaningful name for the flag Initialize flag (to true or false) Test the flag in the loop test expression Change the value of the flag in loop body when the appropriate condition occurs
141	A flag-controlled loop Count and sum the first 10 odd numbers in a data file Initialize flag notDone to true Use while(notDone) for loop test Change flag to false when 10 odd numbers have been read or if EOF is reached first
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143	Exception Handling Handling errors is always a difficult problem Many times we ignore error handling Think back to your 162 project where this was important? What did it do to your design? A major problem with most error handling schemes is that they rely on the programmer’s vigilance and an agreed upon convention ahead of time – which may not be enforced by the language Exception handling in Java is handled directly as part of the language And, you are forced to use it to get anywhere If you don’t write your code to properly handle exceptions, you will get error messages! This consistency makes error handling easier Side note: Error handling is not an object oreiented feature!
144	Exceptions An exception is an unusual situation that occurs when the program is running. Exception Management Define the error condition Enclose code containing possible error (try). Alert the system if error occurs (throw). Handle error if it is thrown (catch).
145	Three Part Exception Handling Defining the exception Extend type Exception and supply a pair of constructors that call super Raising(generating) the exception Use of the throw statement Handling the exception Forward the exception or use try-catch-finally statement to catch and handle the exception.
146	try-catch with Built-In Exception
147	try-catch with Exception Class
148	Class DataException public class DataException extends Exception { public DataException() { super(); } public DataException(String message) { super(message); } }
149	Execution of try-catch
150	Precedence Operator Associativity () Left to right unary: ++ -- ! + - (cast) Right to left * / % Left to right + - Left to right < <= > >= Left to right == != & Left to right ^ Left to right \| Left to right && Left to right \|\| Left to right ? : Right to left = += -= *= /= Right to left
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152	"// class List" // class List public class List { // Data fields protected String[] listItems; // Array to hold list items protected int numItems; // Number of items currently in list protected int currentPos; // State variable for iteration . . . }
153	Unsorted and Sorted Lists
154	Methods for Class List public List() // Default Constructor // Result: List instantiated for 100 items { numItems = 0; listItems = new String[100]; currentPos = 0; } public List(int maxItems) // Constructor // Result: List instantiated for maxItems items { numItems = 0; listItems = new String[maxItems]; currentPos = 0; }
155	Observer Methods public boolean isEmpty() // Returns true if no components; false otherwise { return (numItems == 0) } public int length() // Returns the number of components in the list { return numItems; }
156	Observer Methods Contd. public boolean isFull() // Returns true if no more room; false otherwise { return (numItems == listItems.length); }
157	Transformer Method Insert public void insert(String item) // Result: If the list is not full, puts item in // the last position in the list; otherwise list // is unchanged. { if (!isFull()) { listItems[numItems] = item; numItems++; } }
158	Before Inserting 64 into an Unsorted List
159	After Inserting 64 into an Unsorted List
160	Observer Method isThere public boolean isThere(String item) // Returns true if item is in the list; false otherwise { int index = 0; while (index < numItems && listItems[index].compareTo(item) != 0) index++; return (index < numItems); }
161	Transformer Method Delete Find the position of the element to be deleted from the list Eliminate the item being deleted by shifting up all the following list elements Decrement numItems
162	"public void delete(String item" public void delete(String item) // Result: Removes item from the list if it is // there; otherwise list is unchanged. { int index = 0; boolean found = false; while (index < numItems && !found) { if (listItems[index].compareTo(item) == 0) found = true; else index++; }
163	"// If item found" // If item found, shift remainder of list up if (found) { for (int count = index; count < numItems - 1; count++) listItems[count] = listItems[count + 1]; numItems--; } }
164	Iterator Methods public void resetList() // Initialize iterator by setting currentPos to 0 { currentPos = 0; } public String getNextItem() // Returns current item; increments currentPos circularly // Assumption: No transformers invoked since last call { String next = listItems[currentPos]; if (currentPos == numItems - 1) currentPos = 0; else currentPos++; return next; }
165	Straight Selection Sort Examines the entire list to select the smallest element; places that element where it belongs (with array index 0) Examines the remaining list to select the smallest element from it; places that element where it belongs (array index 1) Continues process until only 2 items remain in unsorted portion Examines the last 2 remaining list elements to select the smallest one; place that element where it belongs in the array (index numItems-2, leaving the last item in its proper place as well.
166	Selection Sort Algorithm FOR passCount going from 0 through numItems - 2 Find minimum value in listItems [ passCount] . . listItems[numItems-1 ] Swap minimum value with listItems [ passCount ]
167	Selection Sort Code public void selectSort() // Sorts array into ascending { String temp; int passCount; int sIndex; int minIndex; // index of minimum so far for(passCount = 0; passCount < numItems-1; passCount++) { minIndex = passCount; // find index of smallest remaining for(sIndex = passCount + 1; sIndex < numItems; sIndex++) if(listItems[sIndex].compareTo(listItems[minIndex])<0) minIndex = sIndex; temp = listItems[minIndex]; // swap listItems[minIndex] = listItems[passCount]; listItems[passCount] = temp; } }
168	"public void insert(String item" public void insert(String item) // If the list is not full, puts item in its proper // place; otherwise list is unchanged. // Assumption: item is not already in the list. { if (! isFull()) {// find proper location for new element int index = numItems - 1; while (index >= 0 && item.compareTo(listItems[index]) < 0) { listItems[index + 1] = listItems[index]; index--; } listItems[index +1] = item; // insert item numItems++; } }
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171	"public boolean isThere(String item" public boolean isThere(String item) // Assumption: List items are in ascending order // Returns true if item is in the list; false otherwise { int first = 0; int last = numItems - 1; int middle; boolean found = false; while (last >= first && !found) { middle = (first + last) / 2; if (item.compareTo(listItems[middle]) == 0) found = true; // Item has been found else if (item.compareTo(listItems[middle] < 0) last = middle - 1; // Look in first half else first = middle + 1; // Look in second half } return found; }
172	Comparable Interface Is part of the standard Java class library Any class that implements the Comparable interface must implement method compareTo String implements the Comparable interface
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176	Initializing Static Members This is very different from C++ Remember, there is only one instance of a static data member no matter how many objects there are of the class We initialize them in a “static block” class list { static int max_lists; static { max_lists = 100; } A static block is executed only once: the first time you make an object of the class or access a static member.
177	Libraries To use a library we must import it: import java.util.*; This brings in the entire utility library And, it manages the namespace since it only brings in those classes that are requested When we have multiple source files, each file should have a public class that is the same name as the file (including capitalization but excluding the file extension) Multiple files can create a “package” If you place package list_library; As the first non-commented line in your file – you are saying that this compilation unit is part of a larger library that you are building SO, there need not be a public class in each file since we are part of a larger unit. And, then such packages can be imported! If there are collisions in the libraries and packages that you import, then Java will require you to explicitly specify the “classpath” from which the class exists