Why does "==" sometimes work with String.trim?

Question

I have the following code

public static void main(String... args) {

    String s = "abc";
    System.out.println(s.hashCode());
    String s1 = " abc ";
    System.out.println(s1.hashCode());
    String s2 = s.trim();
    System.out.println(s2.hashCode());
    String s3 = s1.trim();
    System.out.println(s3.hashCode());
    System.out.println();
    System.out.println(s == s1);
    System.out.println(s == s2);
    System.out.println(s == s3);
}

OP:

96354
32539678
96354
96354

false   -- Correct
true    -- This means s and s2 are references to the same String object "abc" .
false   -- s3=s1.trim()... which means s3="abc" yet s==s3 fails.. If the above conditon were to be considered (s==s2 is true..) , this should also be true.. 

Why am i getting "false" when i check s==s3 ?..

Answer 1

becuase s1.trim() will return a new instance. Strings are immutable so every time a function applied on Strings then a new instance will be returned and You are using == which compares the instance equality

Edit: As suggested By Chris Hayes and R.J.

trim is smart and returns this if there's no whitespace to trim. So in second case you have ' abc ' white spaces so in this case it is not returning this but a new instance of string.

source code of trim method

Answer 2

public String trim() {
    int len = value.length;
    int st = 0;
    char[] val = value;    /* avoid getfield opcode */

    while ((st < len) && (val[st] <= ' ')) {
        st++;
    }
    while ((st < len) && (val[len - 1] <= ' ')) {
        len--;
    }
    return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}

As you can see in your case(System.out.println(s == s2);) this is returned which is pointing to same reference which is why you get true.