CS884 (Prasad)java11Extn1 Java 1.1 Extensions Nested classes static public/protected/private

CS884 (Prasad)java11Extn2 Nested classes StaticStatic Top-level Classes and Interfaces Like ordinary package member except for its name. May use static members of its enclosing class. Member Classes Each instance has an enclosing instance.Each instance has an enclosing instance. May use members of its enclosing classes. Local Classes Defined in a block of code. May also use final local variables and parameters. Anonymous classes One instance only.

CS884 (Prasad)java11Extn3 public class LinkedList { // static by default public interface Linkable { public Linkable getNext(); public void setNext(Linkable node); } Linkable head; public void insert(Linkable node) {...} public void remove(Linkable node) {...} }

CS884 (Prasad)java11Extn4 class LinkableInt implements LinkedList.Linkable { int i; public LinkableInt(int _i){i= _i;} LinkedList.Linkable next; public LinkedList.Linkable getNext() {return next;} public void setNext(LinkedList.Linkable node) {next = node;} }

CS884 (Prasad)java11Extn5 Member Class Every instance of a member class is internally associated with an instance of the enclosing class. The methods of a member class can implicitly refer to the fields defined within the member class, as well as those defined by any enclosing class (including private fields.) static final members permitted.

CS884 (Prasad)java11Extn6 public class BankAccount{ private long number; private int balance; private Action lastAct; public class Action { private String act; private int amount; Action(String a, int amt) { act = number + “:” + a; amount = amt; } public void deposit(int amt) { balance += amt; lastAct = new Action(“deposit”,amt); } public void withdraw(int amt) { balance -= amt; lastAct = new Action(“withdraw”,amt); }

CS884 (Prasad)java11Extn7 public class BankAccount{ private long number; private int balance; private Action lastAct; public class Action { … } public void deposit(int amt) { … } public void withdraw(int amt) { … } public void transfer(BankAccount ac, int amt) { ac.withdraw(amt); deposit(amt); lastAct = new Action(“transfer”,amt); ac.lastAct = ac.new Action(“transfer”,amt); }

CS884 (Prasad)java11Extn8 Scope of Fields public class A { public String name="a"; public class B { public String name="b"; public class C { public String name="c"; public void printNames() {…} } public static void main(String[] a){…} }

CS884 (Prasad)java11Extn9 public void printNames() { // class C System.out.println(name); // c System.out.println(this.name); // c System.out.println(B.this.name); // b System.out.println(A.this.name); // a } public static void main(String[] a) { //class A A a = new A(); B b = a.new B(); A.B.C c = b.new C(); c.printNames(); }

CS884 (Prasad)java11Extn10 javap -p A$B$C public class A.B.C extends java.lang.Object { /* ACC_SUPER bit NOT set */ private final A.B this$1; public java.lang.String name; public A.B.C(A.B); public void printNames(); }

CS884 (Prasad)java11Extn11 javap -p A.B public class A. B extends java.lang.Object { private final A this$0; public java.lang.String name; public A.B(A); static A access$0(A.B); // related to accessing private {...A.B.C...} }

CS884 (Prasad)java11Extn12 Scope vs Inheritance Top-level class can extend a member class. Class hierarchy –superclass to subclass Containment hierarchy –containing class to nested class Whenever there is a name conflict between an inherited member and a member of an enclosing class, Java requires explicit disambiguation. The name of a member class must differ from the name of the enclosing package or class.

CS884 (Prasad)java11Extn13 class A extends A.B { int i = 0; class B { int i = 1; } Pathological cases Cyclic Inheritance: illegal Static subclass of a member class: legal class A { int i = 0; class B { int i = 1; } static class C extends B { int i = 2; C(A a) { a.super(); } } public static void main(String[] args) { A a = new A(); B b = a.new B(); C c = new C(a); }

CS884 (Prasad)java11Extn14 Local Class (vs Member class) A local class is similar to a member class except that its methods can also access final local variables, final formal method parameters, and final exception parameters. An instance of a local class has private fields to hold values of final local variables, final exception parameter, and final method parameters, in addition to a private reference to the enclosing class instance.

CS884 (Prasad)java11Extn15 Enumeration Enumeration myEnumerate(final Object[] arr) { Enumeration class E implements Enumeration { int count = 0; public boolean hasMoreElements() { return count < arr.length; } public Object nextElement() { return arr[count++]; } } return new E(); } –Scope of arr is the body of myEnumerate function. –Lifetime of the array instance referenced by arr should be at least as long as the Enumeration object.

CS884 (Prasad)java11Extn16 import java.util.*; class XYZ { int i; Enumeration myEnumerate(final Object[]arr) { class E implements Enumeration { int count = i; public boolean hasMoreElements() { return count < arr.length; } public Object nextElement() { return arr[count++]; } } return new E(); }

CS884 (Prasad)java11Extn17 class XYZ$1$E extends java.lang.Object implements java.util.Enumeration { private final java.lang.Object val$arr[]; int count; XYZ$1$E(java.lang.Object[], XYZ); public boolean hasMoreElements(); public java.lang.Object nextElement(); } javap -p XYZ$1$E

CS884 (Prasad)java11Extn18 public class Weird { // cf. closure interface IntHolder { int getValue(); } public static void main(String[] args) { IntHolder[] holders = new IntHolder[10]; for(int i=0; i<10; i++) { final int fi = i; IntHolder class MyIntHolder implements IntHolder { public int getValue() { return fi; } } holders[i] = new MyIntHolder(); } // MyIntHolder invisible here for(int i=0; i<10; i++) System.out.println(holders[i].getValue()); System.out.println(holders[i].getValue()); }

CS884 (Prasad)java11Extn19 Anonymous Class Local class without a name. Combines class definition and instantiation. No constructor. Can implement an interface (by implicitly extending class Object ). new ( ) { } new ( ) { }

CS884 (Prasad)java11Extn20 import java.applet.*; import java.awt.*; import java.awt.event.*; public class Scribble extends Applet { public void init() { this.addMouseMotionListener( new MouseMotionAdapter() { public void mouseDragged(MouseEvent e) { Graphics g = getGraphics(); int x = e.getX(), y = e.getY(); g.drawLine(0, 0, x, y); } } ); }}

CS884 (Prasad)java11Extn21 class LocalVarThreads { public static void main(final String[] args) { for (int i = 0; i < args.length; i++) { final int ii = i; Runnable r = new Runnable() { public int count; public void run() { System.out.println(" > "+count++); }; } new Thread(r).start(); } }}

CS884 (Prasad)java11Extn22 Other new features Instance Initializers similar to class initializers serves as a constructor in anonymous classes “Blank” finals assign only once prior to use instead of in decl. Anonymous arrays int[] a; a = new int[] {1,2,3,4}; Class literals String.class, Integer.class, Integer.TYPE, etc In Java 1.1, class is a keyword and a field.

CS884 (Prasad)java11Extn23 Reflection Introspection Meta-programming

CS884 (Prasad)java11Extn24 What does Reflection API do? Java Runtime Environment maintains an immutable Class object that contains information about a class. Reflection API enables a program to:  Determine the class of an object.  Get information about a class's modifiers, fields, methods, constructors, and superclasses.  Find out what constants and method declarations belong to an interface.

CS884 (Prasad)java11Extn25  Create an instance of a class whose name is not known until runtime.  Get and set the value of an object's field, even if the field name is unknown to your program until runtime.  Invoke a method on an object, even if the method is not known until runtime.  Create a new array, whose size and component type are not known until runtime, and then modify the array's components.

CS884 (Prasad)java11Extn26 Applications: Construction of development tools. Tools that need to discover and use public members of a target object based on its class. –GUI builders selecting and creating a component. determining properties, methods, events, etc supported by a JavaBean. customizing a component to running a method.

CS884 (Prasad)java11Extn27 (cont’d) Tools that need to discover and use members declared in a given class. –Class browsers requires information about classes to display their fields, methods, constructors, etc. –Debuggers accessing and modifying field values. –Interpreters

CS884 (Prasad)java11Extn28 Java Core Reflection APIs package java.lang.reflect new interface: Member new classes: Field, Method, Constructor. new class: Array new utility class: Modifier package java.lang new wrapper classes: Byte, Short Classupdated class: Class Classnew objects: instances of class Class to represent primitive types.