为什么StringBuffer / StringBuilder不覆盖equals或hashCode？

Question

Why does StringBuffer/StringBuilder does not override the equals() , hashcode() methods from object?

Please suggest me clear picture that helps the understand the problem...

Answer 1

Because StringBuffer is mutable, and its primary use is for constructing strings. If you want to compare content, call StringBuffer#toString() and compare the returned value.

It is not generally useful to override hashCode() for mutable objects, since modifying such an object that is used as a key in a HashMap could cause the stored value to be "lost."

Answer 2

Actually behind this everything depends upon the hashcode code value. To understand this concept let's take an example :

String str1 = new String("sunil");
String str2 = new String("sunil");

HashMap hm = new HashMap()
hm.put(str1,"hello");
hm.put(str2,"bye");

final hm:

hm = { sunil=bye }

In above code, str1 and str2 are two different String objects. Should they be added to the HashMap separately? The answer is NO . This is because before inserting/putting a value in HashMap, it internally checks and compares the hashCode values of str1 , str2 . Both return the same hashcode value because the String class overrides equals() and hashcode() methods. So upon executing hm.put(str2,"bye"); first key will get overriden with the new value. Now try this :

StringBuilder sb1 = new StringBuilder("sunil");
StringBuilder sb2 = new StringBuilder("sunil");

HashMap hm = new HashMap()
hm.put(sb1,"hello");//sb1 and sb2 will return different HashCode 
hm.put(sb2,"bye");// StringBuffer/StringBuilder does not override hashCode/equals methods

final hm:

{sunil=hello, sunil=bye}

Both value will be added in hashMap because sb1 and sb2 both returns different hashcode. StringBuilder/ StringBuffer does not override equals() and hashCode() method.

Sun Microsystem wanted the programmer to allow adding 2 different String kind of Values in Hashtable or any other Hash Collections likes (HashSet,HashMap…),that's the reason hashCode() and equals() were not overridden intentionally in StringBuffer,StringBuilder class.

Answer 3

Because StringBuffer is mutable . With example Try This:)

package test;

import java.util.HashMap;

public class CheckHashcodeEquals {

    public static void main(String[] args) {

        /*
         * String class override equals() and hashcode() method thats way
         * override value of HashMap
         */
        String s1 = new String("Arya");
        String s2 = new String("Arya");
        HashMap hm = new HashMap<>();
        hm.put(s1, "A1");
        hm.put(s2, "A2");
        System.out.println(hm); /* Output: {Arya=A2} */

        /*
         * String class does not override equals() and hashcode() method thats
         * way insert duplicate value
         */
        StringBuffer sb1 = new StringBuffer("Arya");
        StringBuffer sb2 = new StringBuffer("Arya");
        HashMap hm2 = new HashMap<>();
        hm2.put(sb1, "A1");
        hm2.put(sb2, "A2");
        System.out.println(hm2); /* Output: {Arya=A2, Arya=A1} */
    }

}