String immutability

Definition

A Java String can never be changed after it is created. Methods like toUpperCase() or replace() return a new String and leave the original alone. Use StringBuilder when you build text in a loop.

Example code

String methods return new strings; StringBuilder changes in place

class Demo {
    public static void main(String[] args) {
        String name = "asha";
        name.toUpperCase();                         // result thrown away: name is unchanged
        System.out.println(name);

        String upper = name.toUpperCase();          // keep the new String
        System.out.println(upper);

        StringBuilder builder = new StringBuilder();   // mutable, ideal for loops
        for (int i = 1; i <= 3; i++) builder.append(i).append(' ');
        System.out.println(builder.toString().trim());
    }
}

// Output:
// asha
// ASHA
// 1 2 3