The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment:

Number n = 0;
Class<? extends Number> c = n.getClass();

The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

An equivalence relation partitions the elements it operates on into equivalence classes; all the members of an equivalence class are equal to each other. Members of an equivalence class are substitutable for each other, at least for some purposes.

 x.clone() != x

 x.clone().getClass() == x.getClass()

 x.clone().equals(x)

By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == x.getClass().

By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.

The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.

This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:

• By executing a synchronized instance method of that object.
• By executing the body of a synchronized statement that synchronizes on the object.
• For objects of type Class, by executing a synchronized static method of that class.

Only one thread at a time can own an object's monitor.

The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

In all respects, this method behaves as if wait(0L, 0) had been called. See the specification of the wait(long, int) method for details.

In all respects, this method behaves as if wait(timeoutMillis, 0) had been called. See the specification of the wait(long, int) method for details.

The current thread must own this object's monitor lock. See the notify method for a description of the ways in which a thread can become the owner of a monitor lock.

This method causes the current thread (referred to here as T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Note that only the locks on this object are relinquished; any other objects on which the current thread may be synchronized remain locked while the thread waits.

Thread T then becomes disabled for thread scheduling purposes and lies dormant until one of the following occurs:

• Some other thread invokes the notify method for this object and thread T happens to be arbitrarily chosen as the thread to be awakened.
• Some other thread invokes the notifyAll method for this object.
• Some other thread interrupts thread T.
• The specified amount of real time has elapsed, more or less. The amount of real time, in nanoseconds, is given by the expression 1000000 * timeoutMillis + nanos. If timeoutMillis and nanos are both zero, then real time is not taken into consideration and the thread waits until awakened by one of the other causes.
• Thread T is awakened spuriously. (See below.)

The thread T is then removed from the wait set for this object and re-enabled for thread scheduling. It competes in the usual manner with other threads for the right to synchronize on the object; once it has regained control of the object, all its synchronization claims on the object are restored to the status quo ante - that is, to the situation as of the time that the wait method was invoked. Thread T then returns from the invocation of the wait method. Thus, on return from the wait method, the synchronization state of the object and of thread T is exactly as it was when the wait method was invoked.

A thread can wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. See the example below.

For more information on this topic, see section 14.2, "Condition Queues," in Brian Goetz and others' Java Concurrency in Practice (Addison-Wesley, 2006) or Item 69 in Joshua Bloch's Effective Java, Second Edition (Addison-Wesley, 2008).

If the current thread is interrupted by any thread before or while it is waiting, then an InterruptedException is thrown. The interrupted status of the current thread is cleared when this exception is thrown. This exception is not thrown until the lock status of this object has been restored as described above.