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  1. 1. CompSci 230 Software Construction Java Implementation: Part 3 Version 1.1 of 20 March 2013: corrected contract for hashCode() Version 1.2 of 20 March 2013: added slide 6
  2. 2. Agenda  Topics:    Enum Types Memory allocation: another view of Java’s type system Object Identity, Assignment, Equality, and Copying     Nested Classes     Enum Types and Nested Classes pages, in the Classes and Objects Lesson. Object as a Superclass page, in the Interface and Inheritance Lesson. Equality, Relational, and Conditional Operators page, in the Language Basics Lesson. For reference:  2 What and Why Reading, in The Java Tutorials:   The Object class Overriding equals() and toString() Cloning The 3 things you should know about hashCode(), Eclipse Source Developer, available 20 March 2013. COMPSCI 230: S7
  3. 3. Enum Types  ―An enum type is a special data type that enables for a variable to be a set of predefined constants.   The variable must be equal to one of the values that have been predefined for it. Common examples include     ―Because they are constants, the names of an enum type's fields are in uppercase letters. ―… define an enum type by using the enum keyword.   For example, you would specify a days-of-the-week enum type as: public enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY } ―You should use enum types any time you need to represent a fixed set of constants.   3 compass directions (values of NORTH, SOUTH, EAST, and WEST) and the days of the week. That includes natural enum types such as the planets in our solar system and data sets where you know all possible values at compile time—for example,   the choices on a menu, command line flags, and so on.‖ COMPSCI 230: S7
  4. 4. int i = 20; Ball b1 = new Ball( 10, i, Color.RED ); Memory Allocation  We use a reference variable to refer to instantiated objects.  The value in a reference variable is, essentially, a pointer to an object.    A special value (null) indicates that there is no object. The runtime system (the JVM) interprets reference values as an index into a heap – an area of memory that is set aside, by the JVM, for storing instantiated objects. Formally: the range of allowable values for a reference variable is defined by its reference type. This is a static property. Object o1 = b1; The reference type of o1 is Object. This means it can point to any instance of Object, or to any instance of any subclass of Object. static  The new operator allocates sufficient memory on the heap to store dynamic type all the fields of the object 0xfe100140 :Ball type it is instantiating. o1 :Object = i :int = 20 -class :Class = Ball 0xfe100140 b1 :Ball = 0xfe100140 xPos :int =10 pointsTo pointsTo yPos :int = 20 4 COMPSCI 230: S7 color :Java.awt.Color = 
  5. 5. A model of Java’s type system (for reference) 5 Source: Kollman, R. and Gogolla, M., ―Capturing Dynamic Program Behaviour with UML Collaboration Diagrams‖, Proc. CSMR, 2001. COMPSCI 230: S7
  6. 6. Variables, revisited  ―The Java programming language defines the following kinds of variables: … ‖ [Variables page of the Language Basics Lesson] Lifetime Class Variables Loading: Created when a class is loaded (usually when the app or applet is loaded); destroyed when a class is reloaded (rare), or when the app/applet terminates. By default. (An explicit initialisation is generally preferred. ) Instance Variables Instantiation: Created when an object By default. (An explicit is instantiated; destroyed when an initialisation is generally preferred. object is garbage-collected. ) Local Variables 6 Initialisation Invocation: Created when a method (or a brace-delimited block of code, such as a loop body) is entered; destroyed when a method is exited. Must be initialised explicitly! Parameter s Invocation: Created when a method is entered; destroyed when a method is exited. The implicit parameter (this) is the target of the invoking message. The values of explicit
  7. 7. Object Identity  If two reference variables have the same value, they are pointing to the same object.   This relationship is called ―object identity‖. You can test it with the == operator. 0xfe100140 :Ball -class :Class = Ball xPos :int =10 pointsTo yPos :int = 20 color :Java.awt.Color = b1 :Ball = 0xfe100140 RED pointsTo Ball b1 = new Ball( 10, 20, Color.RED ); Object o1 = b1; System.out.println( o1 == b1 ); System.out.println( (String) o1==b1 ); o1 :Object = 0xfe100140 true false pointsTo 7 :String = 0xba301030 0xba301030 :String -class :Class = String value :char[] = null COMPSCI 230: S7
  8. 8. Equality test: object identity System.out.println( (3+4) == 7 ); System.out.println( new Integer(3+4) == new Integer(7) );  A box that contains 7 items is not identical to any other box that contains 7 items.   true false But… we would say ―3 + 4 equals 7‖. If we want to know whether two boxes are equivalent (= have the same value), we might have to open up the boxes and look inside.   The equals() method is implemented as == in Object. You should override equals(), if you define a subclass in which the ―natural definition‖ for equality differs from the equals() it inherits. System.out.println( (new Integer(3+4)).equals(new Integer(7)) ); System.out.println( (new Integer(7)).equals(3+4) ); System.out.println( (3+4).equals(new Integer(7)) ); true true System.out.println( ((Integer)(3+4)).equals(new Integer(7)) ); true 8 COMPSCI 230: S7
  9. 9. Object The hashCode() Method  ―The Object class, in the java.lang package, sits at the top of the class hierarchy tree.      Every class is a descendant, direct or indirect, of the Object class. Every class you use or write inherits the instance methods of Object. You need not use any of these methods, but, if you choose to do so, you may need to override them with code that is specific to your class. ―The value returned by hashCode() is the object's hash code, which is the object's memory address in hexadecimal. ―By definition, if two objects are equal, their hash code must also be equal.    … If you override the equals() method, you change the way two objects are equated and Object's implementation of hashCode() is no longer valid. Therefore, if you override the equals()method, you must also override the hashCode() method as well.‖ The hashCode() method returns an int.  Hashcodes are used in HashSet, HashMap, and some other Collection classes which use a hashing algorithm.  9  These classes will give incorrect results, if equal instances in a Collection have different hashcodes. They will have poor performance, if many unequal instances share the same
  10. 10. String Equality – be careful…  Strings are immutable.   None of the String methods will modify the value of an existing instance; instead, a new String instance is created, and returned. Some strings are ―interned‖ (= accessible by a hash lookup, at runtime).  You may get a reference to an existing String instance when you ask for a new String. Then again, you might not… String s1 = "Apple"; String s2 = "Apple"; System.out.println("s1==s2:" + (s1==s2)); System.out.println("s1.equals(s2):" + s1.equals(s2)); String s3 = new String("Apple"); String s4 = new String("Apple"); System.out.println("s3==s4:" + (s3==s4)); System.out.println("s3.equals(s4):" + s3.equals(s4)); 10  True True False True Moral: you should use equals(), and not ==, to test Strings for equality. COMPSCI 230: S7
  11. 11. Other Overridable Object Methods  Object has two other methods you might want to override   toString(): returns a String representation of the object clone(): create a copy of an existing object public class Object { ... public boolean equals(Object obj) { return (this == obj); } public String toString() { return getClass().getName() ... } protected Object clone() throws CloneNotSupportedException { ... } } 11 COMPSCI 230: S7
  12. 12. The getClass() method  You cannot override getClass().  Can you see why this isn’t allowed? public class Object { ... // Returns the runtime class of an object public final Class getClass() { ... } ... } Point p1 = new Point(10, 20); Class c = p1.getClass (); System.out.println(c.getName()); System.out.println(c.getSuperclass().getName()); 12 Point java.lang.Object COMPSCI 230: S7
  13. 13. Cloning  The clone() method in the Object class    Throws an exception, if the class of this object does not implement the interface Cloneable Creates an object of the same type as the original object Initialises the clone’s instance variables to the same values as the original object's instance variables   If an object references another object, then you might want to override clone() so that   13 This is a shallow copy: any objects that are referenced by instance variables will not be cloned. It always throws an exception (i.e. is uncloneable), or It clones the other object, and references it from the clone of the original -- so that the clone of the original can be modified or destroyed without affecting the original. COMPSCI 230: S7
  14. 14. Nested Classes   Definition: A class defined inside another class. Motivation: Some classes only make sense in the context of another enclosing class. Examples:   An Enumeration or Iterator object cannot exist by itself. It makes sense only in association with a collection being enumerated/iterated. A GUI event handler cannot exist by itself. It makes sense only in association with the GUI component for which it handles events.   Reference: the Writing an Event Listener Lesson of the Java Tutorials. Nested classes define, and enforce, a composition relationship between the outer class and its inner classes: Outer class public class MyRegularClass { ... class MyInnerClass { ... } } 14 Inner class COMPSCI 230: S7
  15. 15. Nested Classes: Some Details  ―A nested class is a member of its enclosing class.    ―As a member of [its outer class], a nested class can be declared private, public, protected, or package private.   Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. (Recall that outer classes can only be declared public or package private.)‖ ―There are two additional types of inner classes.  You can declare an inner class within the body of a method.   15 Such a class is known as a local inner class. You can also declare an inner class within the body of a method without naming it.  These classes are known as anonymous inner classes. COMPSCI 230: S7
  16. 16. Review  Topics:    Enum Types Memory allocation: another view of Java’s type system Object Identity, Assignment, Equality, and Copying  The Object class Overriding equals() and toString()  Cloning   Nested Classes   End of Theme A: The OO Programming Paradigm  16 What and Why We took a top-down approach: use-case analysis  class design  implementation. COMPSCI 230: S7