Immutable.java

public final class Immutable {

  private final String message;

  public Mutable(String message) {
    this.message = message;
  }

  public void hello() {
    System.out.println(message);
  }
}

Mutable.java

public class Mutable {
  private String message;

  public Mutable(String message) {
    this.message = message;
  }

  public void hello() {
    System.out.println(message);
  }

  public void setMessage(String message) {
    this.message = message;
  }
}