Rules For Defining Immutable Objects

Below are some rule for creating Immutable Objects in Java, Immutable Objects are those objects: we can't change the state of the object after construction. Example of Immutable object is String Class.

1. Don't provide "setter" methods — methods that modify fields or objects referred to by fields.
2. Make all fields final and private.
3. Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
5. If the instance fields include references to mutable objects, don't allow those objects to be changed:
Don't provide methods that modify the mutable objects.
Don't share references to the mutable objects. Never store references to external,
mutable objects passed to the constructor; if necessary, create copies, and store
references to the copies. Similarly, create copies of your internal mutable objects
when necessary to avoid returning the originals in your methods.

final public class ImmutableRGB {

//Values must be between 0 and 255.
final private int red;
final private int green;
final private int blue;
final private String name;

private void check(int red, int green, int blue) {
if (red < 0 || red > 255
|| green < 0 || green > 255
|| blue < 0 || blue > 255) {
throw new IllegalArgumentException();
}
}

public ImmutableRGB(int red, int green, int blue, String name) {
check(red, green, blue);
this.red = red;
this.green = green;
this.blue = blue;
this.name = name;
}


public int getRGB() {
return ((red << 16) | (green << 8) | blue);
}

public String getName() {
return name;
}

public ImmutableRGB invert() {
return new ImmutableRGB(255 - red, 255 - green, 255 - blue,
"Inverse of " + name);
}
}