Think about mutability in Java land this way:

  // Using mutability.
  // `increment` is void, and makes 2 bigger for everyone.
  increment(2); 

  // Typical Java "safety".
  // It's still void, but now it throws a RuntimeException
  // because the developers are saving you from making everyone's 2 bigger.
  increment(2);

  // Immutable
  // Returns 3
  increment(2);