String immutability and why == fails

The string pool, why concatenation in a loop is slow, and the equals rule.

12 min read · Lesson 4 of 6 · Pro

A String cannot be changed

Once a String object exists, its characters never change. Every method that looks like it edits a string actually returns a new string.

Java

public class Immutable {
    public static void main(String[] args) {
        String s = "hello";
        s.toUpperCase();                 // result thrown away
        System.out.println(s);           // hello

        s = s.toUpperCase();             // now s points at the new object
        System.out.println(s);           // HELLO

        String t = "abc";
        t.concat("def");
        System.out.println(t);           // abc, not abcdef
    }
}

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