By default, any instance field or attribute works like a variable; therefore, we can change their values. When we create an instance of a class that defines many public instance fields, we are creating a mutable object, that is, an object that can change its state.

For example, let's think about a class named MutableVector3D that represents a mutable 3D vector with three public instance fields: X, Y, and Z. We can create a new MutableVector3D instance and initialize the X, Y, and Z attributes. Then, we can call the Sum method with their delta values for X, Y, and Z as arguments. The delta values specify the difference between the existing value and the new or desired value. So, for example, if we specify a positive value of 20 in the deltaX parameter, it means that we want to add 20 to the X value.

The following lines show pseudocode in a neutral programming language that create a new MutableVector3D instance called myMutableVector...