Java · Theory

Why String is Immutable in Java

Theory

74/270

Why String is Immutable in Java

String is immutable so the String Pool can safely reuse literals — two "Java" literals can point to the same object. If String could change, one variable would surprise the other.

Immutability also helps HashMap keys (hashCode stays stable) and security (class names, passwords not silently edited).

When you need to change text many times, use StringBuilder. Don’t fight String with + in a loop.

Let's take this on the board with one tiny Main class — no extra files. Why String is Immutable in Java — output: Hi then Hi Java. concat without assign leaves a as Hi. a = a.concat(...) creates a new String. Start from main, go line by line, and stop at each print. That output is the proof for Why String is Immutable.

Exam tip

Why? Pool + HashMap keys + security. Then say StringBuilder when you need to edit.

Example

// Why String is immutable
public class Main {
  public static void main(String[] args) {
    String a = "Hi";
    a.concat(" Java");
    System.out.println(a); // still Hi
    a = a.concat(" Java");
    System.out.println(a); // Hi Java
  }
}

Why String is Immutable in Java — output: Hi then Hi Java. concat without assign leaves a as Hi. a = a.concat(...) creates a new String.

Short notes

DefString cannot change. Methods return a new String.
RulePool can reuse literals because content never changes.
RememberSafer HashMap keys + security.
UseMany edits → StringBuilder.

Java · Theory

Why String is Immutable in Java

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Theory

74/270

Why String is Immutable in Java

String is immutable so the String Pool can safely reuse literals — two "Java" literals can point to the same object. If String could change, one variable would surprise the other.

Immutability also helps HashMap keys (hashCode stays stable) and security (class names, passwords not silently edited).

When you need to change text many times, use StringBuilder. Don’t fight String with + in a loop.

Exam tip

Why? Pool + HashMap keys + security. Then say StringBuilder when you need to edit.

Example

// Why String is immutable
public class Main {
  public static void main(String[] args) {
    String a = "Hi";
    a.concat(" Java");
    System.out.println(a); // still Hi
    a = a.concat(" Java");
    System.out.println(a); // Hi Java
  }
}

Why String is Immutable in Java — output: Hi then Hi Java. concat without assign leaves a as Hi. a = a.concat(...) creates a new String.

Short notes

DefString cannot change. Methods return a new String.
RulePool can reuse literals because content never changes.
RememberSafer HashMap keys + security.
UseMany edits → StringBuilder.

Why String is Immutable in Java

Example

Short notes

Questions

Why String is Immutable in Java

Example

Short notes

Questions