Careeroza

StringBuilder

A Mutable String

StringBuilder holds a resizable character buffer. append, insert, delete and reverse change the same object, so no new strings are created on every step.

Example

public class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hello");
        sb.append(" World");
        System.out.println(sb);
        sb.insert(0, ">> ");
        System.out.println(sb);
        sb.replace(3, 8, "Namaste");
        System.out.println(sb);
        sb.delete(0, 3);
        System.out.println(sb);
        sb.reverse();
        System.out.println(sb);
        System.out.println("length = " + sb.length() + ", char at 0 = " + sb.charAt(0));
        String result = sb.toString();     // back to an immutable String
        System.out.println(result.getClass().getSimpleName());
    }
}

Output

Hello World
>> Hello World
>> Namaste World
Namaste World
dlroW etsamaN
length = 13, char at 0 = d
String

Building Text in a Loop

Example

Use StringBuilder whenever you build a string inside a loop.

public class Main {
    public static void main(String[] args) {
        int[] scores = { 78, 91, 66, 85 };
        StringBuilder sb = new StringBuilder("Scores: [");
        for (int i = 0; i < scores.length; i++) {
            if (i > 0) sb.append(", ");
            sb.append(scores[i]);
        }
        sb.append("]");
        System.out.println(sb);

        StringBuilder table = new StringBuilder();
        for (int i = 1; i <= 3; i++) {
            table.append("7 x ").append(i).append(" = ").append(7 * i).append(System.lineSeparator());
        }
        System.out.print(table);
    }
}

Output

Scores: [78, 91, 66, 85]
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21