为什么哈希码对于String s1 =“cat”和String s2 = new String（“cat”）是相同的？

Question

Program :

 class JavaCode
     {
     public static void main (String[] args) throws java.lang.Exception
      {
        String s1 ="cat";
        String s2 = new String("cat");
        System.out.println(s1 == s2);
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
     }
  }

Output :

false
98262
98262

If S1 and S2 are pointing to different memory address, then Hash code should be different for them? Please Explain how they are same?

Answer 1

If S1 and S2 are pointing to different memory address, then Hash code should be different for them?

No, this is not how hash codes work. If two objects are equal , their hash code MUST be equal too. It doesn't matter "where in memory" they sit.

I recommend reading through the following article: http://www.javaworld.com/article/2074996/hashcode-and-equals-method-in-java-object---a-pragmatic-concept.html

Answer 2

A hashcode is based on the content of some object.

Whereas == compares references, or in other words: "positions" in memory.

Thus two objects can very well have the same hashcode() (because, well: same content); but belong to two different references.

And that by the way why you always always always compare Strings using the equals() method; and not ==.

Answer 3

From the documentation of hashCode() :

Returns a hash code for this string. The hash code for a String object is computed as s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] using int arithmetic, where s[i] is the ith character of the string, n is the length of the string, and ^ indicates exponentiation.

Since both string has same values in it, hashcode are same.

c has ASCII value 99, a has 97 and t has 116. So,

hashcode of s1 = 99 * 31^2 + 97 * 31^1 + 116 = 95139 + 3007 + 116 = 98262.

Also hashcode for s2 will be 98262. That's how equal values makes hashcode same.

Answer 4

hashCode() is the public instance method of String class, which return the integer value based on the content.

    class JavaCode
     {
     public static void main (String[] args) throws java.lang.Exception
      {
        String s1 ="cat";
        String s2 = new String("cat");
        System.out.println(s1 == s2);
        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
      }
     }

In the above code, the content of s1 and s2 is same ("cat"), so the integer which is being returned is same.