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Exploring Java

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Objects in Java

5.10 The Object and Class Classes

java.lang.Object is the mother of all objects; it's the primordial class from which all other classes are ultimately derived. Methods defined in Object are therefore very important because they appear in every instance of any class, throughout all of Java. At last count, there were nine public methods in Object. Five of these are versions of wait() and notify() that are used to synchronize threads on object instances, as we'll discuss in Chapter 6, Threads. The remaining four methods are used for basic comparison, conversion, and administration.

Every object has a toString() method that is called when it's to be represented as a text value. PrintStream objects use toString() to print data, as discussed in Chapter 8, Input/Output Facilities. toString() is also used when an object is referenced in a string concatenation. Here are some examples:

MyObj myObject = new MyObj(); 
Answer theAnswer = new Answer(); 
System.out.println( myObject ); 
String s = "The answer is: " + theAnswer ; 

To be friendly, a new kind of object should override toString() and implement its own version that provides appropriate printing functionality. Two other methods, equals() and hashCode(), may also require specialization when you create a new class.


equals() compares whether two objects are equivalent. Precisely what that means for a particular class is something that you'll have to decide for yourself. Two String objects, for example, are considered equivalent if they hold precisely the same characters in the same sequence:

String userName = "Joe"; 
if ( userName.equals( suspectName ) )
    arrest( userName );

Note that using equals() is *not* the same as:

// if ( userName == suspectName )      // Wrong! 

The above code tests to see if the two String objects are the same object, which is sufficient but not necessary for them to be equivalent objects.

A class should override the equals() method if it needs to implement its own notion of equality. If you have no need to compare objects of a particular class, you don't need to override equals().

Watch out for accidentally overloading equals() when you mean to override it. equals() takes an Object as an argument and returns a boolean value. While you'll probably want to check only if an object is equivalent to an object of its own type, in order to properly override equals(), the method should accept a generic Object as its argument. Here's an example of implementing equals():

class Sneakers extends Shoes { 
    public boolean equals( Object arg ) { 
        if ( (arg != null) && (arg instanceof Sneakers) ) { 
            // compare arg with this object to check equivalence 
            // If comparison is okay... 
            return true; 
        return false; 

A Sneakers object can now be properly compared by any current or future Java classes. If we had instead used a Sneakers type object as the argument to equals(), all would be well for classes that reference our objects as Sneakers, but methods that simply use Shoes would not see the overloaded method and would compare Sneakers against other Sneakers improperly.


The hashCode() method returns an integer that is a hashcode for a class instance. A hashcode is like a signature for an object; it's an arbitrary-looking identifying number that is (with important exceptions) generally different for different instances of the class. Hashcodes are used in the process of storing objects in a Hashtable, or a similar kind of collection. The hashcode is essentially an index into the collection. See Chapter 7, Basic Utility Classes for a complete discussion of Hashtable objects and hashcodes.

The default implementation of hashCode() in Object assigns each object instance a unique number to be used as a hashcode. If you don't override this method when you create a new class, each instance of the class will have a unique hashcode. This is sufficient for most objects. However, if the class has a notion of equivalent objects, then you should probably override hashCode() so that equivalent objects are given the same hashcode.


The last method of Object we need to discuss is getClass(). This method returns a reference to the Class object that produced the object instance.

A good measure of the complexity of an object-oriented language is the degree of abstraction of its class structures. We know that every object in Java is an instance of a class, but what exactly is a class? In C++, objects are formulated by and instantiated from classes, but classes are really just artifacts of the compiler. Thus, you see only classes mentioned in C++ source code, not at run-time. By comparison, classes in Smalltalk are real, run-time entities in the language that are themselves described by "meta-classes" and "meta-class classes." Java strikes a happy medium between these two languages with what is, effectively, a two-tiered system that uses Class objects.

Classes in Java source code are represented at run-time by instances of the java.lang.Class class. There's a Class object for every class you use; this Class object is responsible for producing instances for its class. This may sound overwhelming, but you don't have to worry about any of it unless you are interested in loading new kinds of classes dynamically at run-time.

We can get the Class associated with a particular object with the getClass() method:

String myString = "Foo!"
Class c = myString.getClass();

We can also get the Class reference for a particular class statically, using the special .class notation:

Class c = String.class;

The .class reference looks like a static field that exists in every class. However, it is really resolved by the compiler.

One thing we can do with the Class object is to ask for the name of the object's class:

String s = "Boofa!"; 
Class strClass = s.getClass(); 
System.out.println( strClass.getName() ); // prints "java.lang.String" 

Another thing that we can do with a Class is to ask it to produce a new instance of its type of object. Continuing with the above example:

try { 
    String s2 = (String)strClass.newInstance(); 
catch ( InstantiationException e ) { ... } 
catch ( IllegalAccessException e ) { ... } 

newInstance() has a return type of Object, so we have to cast it to a reference of the appropriate type. A couple of problems can occur here. An InstantiationException indicates we're trying to instantiate an abstract class or an interface. IllegalAccessException is a more general exception that indicates we can't access a constructor for the object. Note that newInstance() can create only an instance of a class that has an accessible default constructor. There's no way for us to pass any arguments to a constructor.

All this becomes more meaningful when we add the capability to look up a Class by name. forName() is a static method of Class that returns a Class object given its name as a String:

try { 
    Class sneakersClass = Class.forName("Sneakers"); 
catch ( ClassNotFoundException e ) { ... } 

A ClassNotFoundException is thrown if the class can't be located.

Combining the above tools, we have the power to load new kinds of classes dynamically. When combined with the power of interfaces, we can use new data types by name in our applications:

interface Typewriter { 
    void typeLine( String s ); 
class Printer implements Typewriter { 
class MyApplication { 
    String outputDeviceName = "Printer"; 
    try { 
        Class newClass = Class.forName( outputDeviceName ); 
        Typewriter device = (Typewriter)newClass.newInstance(); 
    catch ( Exception e ) { 

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