Two objects the same value (x.equals (y) == true), but it may have a different hash code, this sentence right ?

Two objects the same value (x.equals (y) == true), but it may have a different hash code, this sentence right ?
Seeking to explain
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1. the same application , the function of an object equals hashCode function does not change the situation , regardless of how many times called , it must return the same integer.
2. calls equals function if two objects are equal , then calling the hashCode function will return the same integer .
3. calls equals function if two objects are not equal , then calling the hashCode function does not require a certain return distinct integers
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this sentence is false.

two different object instances , hash code can be equal, because there is no perfect hash function .
On the contrary , it is wrong , because you can not guarantee that users of the class does not use HashSet / HashMap
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last sentence the wrong object hashCode is the only marker ; only if the two objects are equal to their equals results mark must be equal

http://blog.csdn.net/KindAzrael/article/details/4633615
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directly on the API




public boolean equals (Object obj) indicate whether some other object associated with this object " equal ."
equals method is implemented on non-null object reference equivalence relation :

reflexive : for any non-null reference values ​​x, x.equals (x) should return true.
Symmetry: For any non- null reference values ​​x and y, if and only if y.equals (x) returns true , x.equals (y) ; only should return true.
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.
consistent: for any non- null reference values ​​x and y, multiple invocations x.equals (y) consistently return true or consistently return false, provided that objects used in equals comparisons on the information has not been modified .
For any non- null reference values ​​x, x.equals (null) should return false.
Object class equals method on the object most likely equal to the difference between ; That is, for any non- null reference values ​​x and y, if and only if x and y refer to the same object, this method returns true (x == y has the value true).

Note: When this method is overridden , it is usually necessary to override the hashCode method , in order to maintain the general contract of hashCode method , which states that equal objects must have equal hash codes .


parameters:
obj - the reference object with which to compare .
Returns:
If this object is the same as the obj argument , returns true; otherwise false.


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public int hashCode () Returns the hash code value for the object . This method is supported for the hash table to provide some advantages, such as , java.util.Hashtable hash table provided .
hashCode general agreement that:

during the execution of a Java application , on the same object multiple times calling the hashCode method must consistently return the same integer , provided that equals comparisons on the object information is not used is modified. From one execution of an application to another of the same application execution , the integer need not remain consistent .
if under equals (Object) method , two objects are equal , then the two objects calling the hashCode method on each object must be produce the same integer result .
The following are not be necessary: ​​if according to equals (java.lang.Object) method , two objects are not equal, then the two objects on any object in calling the hashCode ; method must produce distinct integer results. However, the programmer should be aware that the unequal objects produce distinct integer results can improve the performance of the hash table .
In fact, the hashCode method defined by class Object does return different objects for different integer . ( This usually is done by converting the internal address of the object into an integer to be achieved, but does not require such JavaTM programming language implementation technique . )


Returns:
this object a hash code value .

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same value using the equals method to determine , equals method after rewriting is not necessarily determine whether the object is the same object
So different is normal hashcode
like I defined a class , has a name and number
I override the equals method , so that the same name is returned equal
So although I have hashcode two different objects ( their numbers may be different or the value of all the properties of two objects are the same )
but they certainly different hashcode
because there is not an object Well
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this sentence is correct

equal objects may not be the same hashcode
In turn , hashcode is not necessarily the same as equal
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right


...
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but generally override equals to override both HashCode
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this sentence is correct .
equals and hashCode are two methods that can be overridden .
So the key to how you override these two methods .
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2nd floor theory is right, but in accordance with normal business logic , unless it is neuropathy or else override equals and hashCode methods is certain as true equals , hashCode equal
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hashcode is not the only mark if the judge depend on it to carry out equal , naturally, must be equal , if not rely on it to make equal judgment, it is what is not is
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is right Mile !
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However , API stated that only one agreement , and did not force you to have to do it
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hash conflict is likely to do
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two objects are equal , hashcode certainly consistent ...
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access points friends.
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answer equals (2 F ) , but not in my reply , 2nd Floor , -----> hashCode () can of course different
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last sentence the wrong object hashCode is the only mark only if the two objects are equal to their equals results mark must be equal

http://blog.csdn.net/KindAzrael/article/details/4633615

System.out.println ("Aa". hashCode ());
System.out.println ("BB". hashCode ());
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wrong, have the same hash code.
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last sentence the wrong object hashCode is the only mark only if the two objects are equal to their equals results mark must be equal

http://blog.csdn.net/KindAzrael/article/details/4633615


Why is it wrong, you did not say that there has not been rewritten equal , rewritten hashCode is not necessarily higher than
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landlord, I tested for the Object class must have equal equals hashCode equal. Look API is easy to understand .
For the String class , the same conclusion holds.
For other custom classes , this conclusion is uncertain, equals and hashCode depends on the specific implementation.