[Chapter 5] 5.10 The Object and Class Classes
Author: Pat Niemeyer and Josh Peck
Date: Tue Jul 22 18:53:07 1997

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| Chapter 5 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.

### Equality

`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.

### Hashcodes

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.

### java.lang.Class

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();
            ...
            device.typeLine("Hello...");
        }
        catch ( Exception e ) {
    }

Inner Classes |  | Reflection