Why the value of hashCode is same while all are different String Object:
public class StringObj { public static void main(String[] args) { String s1="Jack"; String s2=new String("Jack"); String s3=new String("Jack"); System.out.println(s1.hashCode()); System.out.println(s2.hashCode()); System.out.println(s3.hashCode()); } } The Java documentation for Object says that if an object equals() another, it must have the same hashCode(). This makes sense, since both objects are, supposedly, representing the same thing.
From a practical standpoint, this is very important. It allows you to use a well known String to write into and read from a map instead of having to use a singleton object key.
Short answer:
The contract of hashCode and equals explicitly say that the hashcode() of the two objects should be the same if equals returns true for them.
Note that equals on these strings will return true. So it is necessary that the hashCode is the same.
Long Answer
The contract of hashCode says:
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
hashCodemethod must consistently return the same integer, provided no information used inequalscomparisons 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
hashCodemethod 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 thehashCodemethod 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.
hashCode of String is computed as:
s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]using
intarithmetic, wheres[i]is theith character of the string,nis the length of the string, and^indicates exponentiation. (The hash value of the empty string is zero.)
so, THE hashCode() of String uses the value of the String to compute the hashCode. Many objects do that. So as long as the content is the same, the hashCode will be the same. This is specifically because that is what the hashCode contract mandates.
