Abstract Methods and Classes

Transcription

1 Abstract Methods and Classes abstract class vaporclass{ abstract vapormethod (String name); The keyword abstract enables you to provide a method prototype in the class body without its implementation. An abstract class can't be instantiated but it can be subclassed and its abstract methods must be overridden with methods whose implementations are provided. Interface: is a purely abstract class i.e. it contains only abstract methods (no implementations). Thus an interface defines a set of methods a class must implement. Any class that implements each of the methods listed in the interface declares that it implements the interface. A class can implement as many interfaces as it needs to. Interface types act like class types. You can declare vars to be of interface type; declare args of methods to accept interface types; return type of a method can be an interface type. E.g. interface scale { static final int BIG = 0, SMALL = 1; void setscale(int size); class box implements scale { void setscale(int size) { switch(size) { case BIG: case SMALL:

2 E.g. interface Driveable { boolean starteng(); void stopeng(); float accl(float acc); class auto implements Driveable { boolean starteng() { if (notcold) engrun = true; void stopeng() { engrun = false; float accl(float acc) { void honkhorn() { class lawnmower implements Driveable { //all three methods implemented void cutgrass() { auto toyota = new auto(); lawnmower mower = new lawnmower(); Driveable vehicle; //var of type Driveable vehicle = toyota; vehicle.starteng(); vehicle = mower; vehicle.stopeng();

3 Package: is a name for a group of related classes. A compilation unit (i.e. a file) normally contains a single class and is named for that class. E.g. myclass.java file will contain the class myclass. A class is declared to belong to a particular package with the package statement. E.g. package mytools.text; //first line in a file class Textcomp { Here, class Textcomp is placed in the package mytools.text. Class Visibility e.g. package mytools.txt; public class TextEd { Since TextEd is declared public, it can be referenced anywhere. There can only be one public class in a compilation unit (file). Classes in a package can be hidden by not using the public modifier. So packages hide classes in the way classes hide private members. Modifier public protected none private protected private Visibility All classes (outside package too) All subclasses inside or outside the package and all classes in the package. Default, i.e. all classes in the package. Subclasses only; inside or outside the packages. none By default, variables and methods of a class are accessible to members of the class itself and other classes in the same package.

4 Importing classes E.g. package somewhere.else; import mytools.text.texted; //or import mytools.text.* class myclass { texted editboy; The Object class All classes are derived from java.lang.object e.g java.net.inetaddress and java.net.url inherit from java.lang.object. The tostring() method is called to represent an object as a text value. e.g. InetAddress thiscomputer = InetAddress.getLocalHost(); System.out.println(thiscomputer.tostring()); This generates a string with the form hostname/ipaddress. The equals() method tests if two objects are equivalent. String username = "Joe"; e.g if (username.equals(suspectname)) arrest(username);

5 Array Variables int ia[] = new int[3]; or int [] ia = new int[3]; This declares that ia is an array ref that refers to an array object of 3 ints. Length of each array object is fixed. A new array of a different size can be assigned to array ref ia at any time. IndexOutofBoundsException is generated when an out of range subscript is used. e.g. Attr attr[] = new Attr[12]; //this only creates an array of 12 vars that //are initialized to null. The Attr objects are created only when: for (int i = 0; i < attr.length; i++) attr[i] = new Attr(); Array of arrays: e.g. float matrix [][] = new float[4][4]; initmatrix(); //initialize the array for (int i = 0; i <matrix.length; i++) { for(int j = 0; j <matrix[i].length; j++) System.out.println(matrix[i][j] + " "); e.g. float matrix [][] = new float[4][]; for (int i = 0; i < matrix.length; i++) matrix[i] = new float[i+1]; e.g. String animals = {"lions", "tigers"; or e.g. String animals [] = new String[3]; animals[0] = "lions"; animals[1] = "tigers";

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