Why String class is defined as immutable?

Question

Why String class is defined as immutable? what can be the reason behind it?

Answer 1

Do try this at home. :)
public class Main {
    public static void main(String... args) {
        System.out.println("Greetings.");
        System.out.println("Goodbye.");
    static {
        // translates into Japanese. NOT a good idea!
        try {
            final Field field = String.class.getDeclaredField("value");
            field.setAccessible(true);
            char[] chars = new char[8];
            "Sayonara".getChars(0,8,chars,0);
            field.set("Goodbye.", chars);
            chars = new char[10];
            "Konichi-wa".getChars(0,10,chars,0);
            field.set("Greetings.", chars);
        } catch (Exception e) {
            throw new AssertionError(e);
}