Object 的俩个方法：equals() hashcode()

hashCode

public int hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by

java.util.Hashtable

.
The general contract of

hashCode

is:

Whenever it is invoked on the same object more than once during an execution of a Java application, thehashCode method must consistently return the same integer, provided no information used inequals 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(Object) 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(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 hashtables.

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.)

Returns:a hash code value for this object.See Also:

equals(java.lang.Object)

,

Hashtable

equals

public boolean equals(Object obj)

Indicates whether some other object is "equal to" this one.
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

.

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 returns

true

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 thehashCode method, which states that equal objects must have equal hash codes.

Parameters:

obj

- the reference object with which to compare. Returns:

true

if this object is the same as the obj argument;

false

otherwise.

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String对象的equals（）代码：

public boolean equals(Object obj)

{

if(this == obj) //情况1：二个对象的引用相同，即指向的内存地址相同 。成立

return true;

if(obj instanceof String) //情况2：obj也是String的一个对象，他们的值完全一致。成立

{

String s = (String)obj;

int i = count;

if(i == s.count)

{

char ac[] = value;

char ac1[] = s.value;

int j = offset;

int k = s.offset;

while(i-- != 0)

if(ac[j++] != ac1[k++])

return false;

return true;

}

}

return false;

}

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老祖Object的equals方法：

public boolean equals(Object obj) //就是引用相同

{

return this == obj;

}

分析总结：若obj1.equals(obj2)==true ---> obj1.hascode()==obj2.hashcode(); ①
//地址是同一地址

若obj1.equals(obj2)==false -/--> obj1.hascode()!=obj2.hashcode(); ②//推不出

编程者知：对equals()不相等的俩个对象，产生不同的integer值（hashcode）是可以提升hashtables的性能的。

如果不重写equals，那么比较的将是对象的引用是否指向同一块内存地址，重写之后目的是为了比较两个对象的value值是否相等。特别指出利用equals比较八大包装对象

（如int，float等）和String类（因为该类已重写了equals和hashcode方法）对象时，默认比较的是值，在比较其它自定义对象时都是比较的引用地址

hashcode是用于散列数据的快速存取，如利用HashSet/HashMap/Hashtable类来存储数据时，都是根据存储对象的hashcode值来进行判断是否相同的。

这样如果我们对一个对象重写了euqals，意思是只要对象的成员变量值都相等那么euqals就等于true，但不重写hashcode，那么我们再new一个新的对象，

当原对象.equals（新对象）等于true时，两者的hashcode却是不一样的，由此将产生了理解的不一致，如在存储散列集合时（如Set类），将会存储了两个值一样的对象，

导致混淆，因此，就也需要重写hashcode()