10.2 The 'Object' Class

All classes extend the Object class, either directly or indirectly. A class declaration, without the extends clause, implicitly extends the Object class (see Section 6.1, p. 226). Thus, the Object class is always at the top of any inheritance hierarchy. The Object class defines the basic functionality that all objects exhibit and that all classes inherit. Note that this also applies for arrays, since these are genuine objects in Java.

The Object class provides the following general utility methods (see Example 10.1 for usage of some of these methods):

int hashCode()

boolean equals(Object obj)

final Class getClass()

protected Object clone() throws CloneNotSupportedException

String toString()

protected void finalize() throws Throwable

In addition, the Object class provides support for thread communication in synchronized code, through the following methods, which are discussed in Section 9.5 on page 370:

final void wait(long timeout) throws InterruptedException
final void wait(long timeout, int nanos) throws InterruptedException
final void wait() throws InterruptedException
final void notify()
final void notifyAll()

Example 10.1 Methods in the Object class

public class ObjectMethods {
    public static void main(String[] args) {
        // Two objects of MyClass.
        MyClass obj1 = new MyClass();
        MyClass obj2 = new MyClass();

        // Two strings.
        String str1 = new String("WhoAmI");
        String str2 = new String("WhoAmI");

        // Method hashCode() overridden in String class.
        // Strings with same content (i.e., are equal) have the same hash code.
        System.out.println("hash code for str1: " + str1.hashCode());
        System.out.println("hash code for str2: " + str2.hashCode() + "\n");

        // Hash codes are different for different MyClass objects.
        System.out.println("hash code for MyClass obj1: " + obj1.hashCode());
        System.out.println("hash code for MyClass obj2: " + obj2.hashCode()+"\n");

        // Method equals() overridden in the String class.
        System.out.println("str1.equals(str2): " + str1.equals(str2));
        System.out.println("str1 == str2 : " + (str1 == str2) + "\n");

        // Method equals() from the Object class called.
        System.out.println("obj1.equals(obj2): " + obj1.equals(obj2));
        System.out.println("obj1 == obj2 : " + (obj1 == obj2) + "\n");

        // The runtime object that represents the class of an object.
        Class rtStringClass  = str1.getClass();
        Class rtMyClassClass = obj1.getClass();
        // The name of the class represented by the runtime object.
        System.out.println("Class for str1: " + rtStringClass);
        System.out.println("Class for obj1: " + rtMyClassClass + "\n");
        // The toString() method is overridden in the String class.
        String textRepStr = str1.toString();
        String textRepObj = obj1.toString();
        System.out.println("Text representation of str1: " + textRepStr);
        System.out.println("Text representation of obj1: " + textRepObj + "\n");

        // Shallow copying of arrays.
        MyClass[] array1 = {new MyClass(), new MyClass(), new MyClass()};
        MyClass[] array2 = (MyClass[]) array1.clone(); // Cast required.
        // Array objects are different, but share the element objects.
        System.out.println("array1 == array2 : " + (array1 == array2));
        for(int i = 0; i < array1.length; i++) {
            System.out.println("array1[" + i + "] == array2[" + i + "] : "
                       + (array1[i] == array2[i]));
        }
        System.out.println();

        // Clone an object of MyClass.
        MyClass obj3 = (MyClass) obj1.clone();
        System.out.println("hash code for MyClass obj3: " + obj3.hashCode());
        System.out.println("obj1 == obj3 : " + (obj1 == obj3));
    }
}

class MyClass implements Cloneable {

    public Object clone() {
        Object obj = null;
        try { obj = super.clone();}        // Calls overridden method.
        catch (CloneNotSupportedException e) { System.out.println(e);}
        return obj;
    }
}

Output from the program:

hash code for str1: -1704812257
hash code for str2: -1704812257

hash code for MyClass obj1: 24216257
hash code for MyClass obj2: 20929799

str1.equals(str2): true
str1 == str2 : false

obj1.equals(obj2): false
obj1 == obj2 : false

Class for str1: class java.lang.String
Class for obj1: class MyClass

Text representation of str1: WhoAmI
Text representation of obj1: MyClass@17182c1
array1 == array2 : false
array1[0] == array2[0] : true
array1[1] == array2[1] : true
array1[2] == array2[2] : true

hash code for MyClass obj3: 16032330
obj1 == obj3 : false