An immutable class must respect several requirements, such as the following:

• The class should be marked as final to suppress extensibility (other classes cannot extend this class; therefore, they cannot override methods)
• All fields should be declared private and final (they are not visible in other classes, and they are initialized only once in the constructor of this class)
• The class should contain a parameterized public constructor (or a private constructor and factory methods for creating instances) that initializes the fields
• The class should provide getters for fields
• The class should not expose setters

For example, the following Point class is immutable since it successfully passes the preceding checklist:

public final class Point {

  private final double x;
  private final double y;

  public Point(double x, double y) {
    this.x = x;
    this.y = y;
  }

  public double getX() {
    return x;
  }

  public double getY() {
    return y;
  }
}

If the immutable class should manipulate mutable objects, consider the following problems.