Object Class

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. 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 :

int hashCode()

When storing objects in hash tables, this method can be used to get a hash value for an object. This value is guaranteed to be consistent during the execution of the program.

boolean equals(Object obj)

Object reference and value equality are discussed together with the == and != operators (see Section 3.10, p. 68). The equals() method in the Object class returns true only if the two references compared denote the same object. The equals() method is usually overridden to provide the semantics of object value equality, as is the case for the wrapper classes and the String class.

final Class getClass()

Returns the runtime class of the object, which is represented by an object of the class java.lang.Class at runtime.

protected Object clone() throws CloneNotSupportedException

New objects that are exactly the same (i.e., have identical states) as the current object can be created by using the clone() method, that is, primitive values and reference values are copied. This is called shallow copying. A class can override this method to provide its own notion of cloning. For example, cloning a composite object by recursively cloning the constituent objects is called deep copying.

When overridden, the method in the subclass is usually declared public to allow any client to clone objects of the class.

If the overriding clone() method relies on the clone() method in the Object class, then the subclass must implement the Cloneable marker interface to indicate that its objects can be safely cloned. Otherwise, the clone() method in the Object class will throw a checked CloneNotSupportedException.

String toString()

If a subclass does not override this method, it returns a textual representation of the object, which has the following format:

"(name of the class)@(hash code value of object)"

This method is usually overridden and used for debugging purposes. The method call System.out.println(objRef) will implicitly convert its argument to a textual representation using the toString() method.

protected void finalize() throws Throwable

This method is called on an object just before it is garbage collected, so that any cleaning up can be done. However, the default finalize() method in the Object class does not do anything useful.

In addition, the Object class provides support for thread communication in synchronized code, through the following methods :

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()

A thread invokes these method on the object whose lock it holds. A thread waits for notification by another thread.