Learning Java, 4th Edition

The Object Class

java.lang.Object is the ancestor 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 every class, throughout all of Java. At last count, there were nine public methods and two protected 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 9. The remaining four methods are used for basic comparison, conversion, and administration.

Every object has a toString() method that can be called when it’s to be represented as a text value. PrintStream objects use toString() to print data, as discussed in Chapter 12. toString() is also used implicitly 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 can override toString() and implement its own version that provides appropriate information about itself. This is particularly helpful in debugging, where it is common to print the string value of an object to see what is going on. Two other methods, equals() and hashCode(), may also require specialization when you create a new class.

Equality and Equivalence

equals() determines whether two objects are equivalent. Precisely what that means ...