4.3.2 The Class Object

The standard class Object is a superclass (§8.1) of all other classes. A variable of type Object can hold a reference to any object, whether it is an instance of a class or an array (§10). All class and array types inherit the methods of class Object, which are summarized here and completely specified in §20.1:

package java.lang;

public class Object {
	public final Class getClass() { . . . }
	public String toString() { . . . }
	public boolean equals(Object obj) { . . . }
	public int hashCode() { . . . }
	protected Object clone()
		throws CloneNotSupportedException { . . . }
	public final void wait()

		throws IllegalMonitorStateException,

			InterruptedException { . . . }
	public final void wait(long millis)
		throws IllegalMonitorStateException,
			InterruptedException { . . . }
	public final void wait(long millis, int nanos) { . . . }
		throws IllegalMonitorStateException,
			InterruptedException { . . . }
	public final void notify() { . . . }
		throws IllegalMonitorStateException
	public final void notifyAll() { . . . }
		throws IllegalMonitorStateException
	protected void finalize()
		throws Throwable { . . . }
}

The members of Object are as follows:

• The method getClass returns the Class (§20.3) object that represents the class of the object. A Class object exists for each reference type. It can be used, for example, to discover the fully qualified name of a class, its members, its immediate superclass, and any interfaces that it implements. A class method that is declared synchronized (§8.4.3.5) synchronizes on the lock associated with the Class object of the class.
• The method toString returns a String representation of the object.
• The methods equals and hashCode are declared for the benefit of hashtables such as java.util.Hashtable (§21.7). The method equals defines a notion of object equality, which is based on value, not reference, comparison.
• The method clone is used to make a duplicate of an object.
• The methods wait, notify, and notifyAll are used in concurrent programming using threads, as described in §17.
• The method finalize is run just before an object is destroyed and is described in §12.6.