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Trail: Learning the Java Language
Lesson: Classes and Inheritance

Being a Descendent of Object

The Object (in the API reference documentation) class sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the Object class. This class defines the basic state and behavior that all objects must have, such as the ability to compare oneself to another object, to convert to a string, to wait on a condition variable, to notify other objects that a condition variable has changed, and to return the class of the object.

The following is a list of handy methods that the Object class provides:

With the exception of notify, notifyAll, and wait, these methods are covered in the sections that follow. The notify, notifyAll, and wait methods all play a part in synchronizing the activities of independently running threads in a program, which is discussed in Threads: Doing Two or More Tasks At Once (in the Learning the Java Language trail).

The clone Method

You use the clone method to create an object from an existing object. To create a clone, you write: 
aCloneableObject.clone();
Object's implementation of this method checks to see whether the object on which clone was invoked implements the Cloneable interface. If the object does not, the method throws a CloneNotSupportedException. Even though Object implements the clone method, the Object class is not declared to implement the Cloneable interface, so classes that don't explicitly implement the interface are not cloneable. If the object on which clone was invoked does implement the Cloneable interface, Object's implementation of the clone method creates an object of the same type as the original object and initializes the new object's member variables to have the same values as the original object's corresponding member variables.

The simplest way to make your class cloneable, then, is to add implements Cloneable to your class's declaration. For some classes, the default behavior of Object's clone method works just fine. Other classes need to override clone to get correct behavior.

Consider a Stack class that contains a member variable referring to an array of Objects. If Stack relies on Object's implementation of clone, the original stack and its clone refer to the same array. Changing one stack changes the other, which is undesirable behavior.

Here is an appropriate implementation of clone for our Stack class, which clones the array to ensure that the original stack and its clone do not refer to the same list: 

public class Stack implements Cloneable {
    private Object[] items;
    private int top;
    ...
    // code for Stack's methods and constructor not shown
    protected Stack clone() {
        try {
            Stack s = (Stack)super.clone(); //clone the stack
            s.items = (Object)items.clone(); //clone the array
            return s; // return the clone
        } catch (CloneNotSupportedException e) {
           //This shouldn't happen because Stack is Cloneable.
            throw new InternalError();
        }
    }
}
The implementation for Stack's clone method is relatively simple. First, it calls Object's implementation of the clone method by calling super.clone, which creates and initializes a Stack object. At this point, the original stack and its clone refer to the same list. Next, the method clones the list. Note that the method returns a Stack, which is a subclass of the type returned by the Object.clone method.
Note: The clone method should never use new to create the clone and should not call constructors. Instead, the method should call super.clone, which creates an object of the correct type and allows the hierarchy of superclasses to perform the copying necessary to get a proper clone.

The equals and hashCode Methods

The equals method compares two objects for equality and returns true if they are equal. The equals method provided in the Object class uses the identity operator (==) to determine whether two objects are equal. If the objects compared are the exact same object, the method returns true.

However, for some classes, two distinct objects of that type might be considered equal if they contain the same information. Consider this code that tests two Integers, one and anotherOne, for equality: 

Integer one = new Integer(1);
Integer anotherOne = new Integer(1);
if (one.equals(anotherOne)) {
    System.out.println("objects are equal");
}
This program displays objects are equal even though one and anotherOne reference two distinct objects. They are considered equal because the objects compared contain the same integer value.

You should override the equals method only if the identity operator is not appropriate for your class. If you override equals, override hashCode as well.

The value returned by hashCode is an int that maps an object into a bucket in a hash table. An object must always produce the same hash code. However, objects can share hash codes (they aren't necessarily unique). Writing a "correct" hashing function is easy — always return the same hash code for the same object. Writing an "efficient" hashing function — one that provides a sufficient distribution of objects over the buckets — is difficult and is outside the scope of this tutorial.

Even so, the hashing function for some classes is relatively obvious. For example, an obvious hash code for an Integer object is its integer value.

The finalize Method

The Object class provides a method, finalize, that cleans up an object before it is garbage collected. This method's role during garbage collection was discussed previously, in the section Cleaning Up Unused Objects (in the Learning the Java Language trail). The finalize method is called automatically by the system, and most classes do not need to override it. Thus, you can generally ignore this method.

The toString Method

The Object's toString method returns a String representation of the object. You can use toString along with System.out.println to display a text representation of an object, such as an instance of Double: 
System.out.println(new Double(Math.PI).toString());
The String representation for an object depends entirely on the object. The String representation of a Double object is the double value displayed as text. Thus, the previous line of code displays 3.14159.

The toString method is very useful for debugging. You should override this method in all your classes.

The getClass Method

The getClass method returns a runtime representation of the class of an object. This method returns a Class object, which you can query for information about the class, such as its name, its superclass, and the names of the interfaces it implements. You cannot override getClass. The following method gets and displays the class name of an object: 
void PrintClassName(Object obj) {
    System.out.println("The Object's class is "
                       + obj.getClass().getName());
}
One handy use of a Class object is to create a new instance of a class without knowing what the class is at compile time. The following sample method creates a new instance of the same class as obj, which can be any class that inherits from Object: 
Object createNewInstanceOf(Object obj) {
    return obj.getClass().newInstance();
}
If you already know the name of the class, you can also get a Class object from a class name. The following two lines are equivalent ways to get a Class object for the String class: 
String.class
Class.forName("String")
The first is more efficient than the second.

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