Object类的
方法：
HashMap.
The general contract of hashCode is:
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.equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.equals(java.lang.Object) 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.As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
equals(java.lang.Object), System.identityHashCode(java.lang.Object)The equals method implements an equivalence relation on non-null object references:
x, x.equals(x) should return true.x and y, x.equals(y) should return true if and only if y.equals(x) returns true.x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.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.x, x.equals(null) should return false.The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returnstrue if and only if x and y refer to the same object (x == y has the value true).
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
obj - the reference object with which to compare.true if this object is the same as the obj argument; false otherwise.hashCode(), HashMap原文：http://www.cnblogs.com/hanying/p/4648095.html