The product of 4.5 by 7.2 is expressed in Java as: 4.5 * 7.2. In this expression, the symbol * represents the multiplication operation. 4.5 and 7.2 are called number literals. Using DrJava, we can type in the expression 4.5 * 7.2 directly in the interactions window and see the answer.
Now suppose we change the problem to compute the area of a rectangle of width 3.6 and height 9.3. Has the original problem really change at all? To put it in another way, has the essence of the original problem changed? After all, the formula for computing the answer is still the same. All we have to do is to enter 3.6 * 9.3. What is it that has not change (the invariant)? And what is it that has changed (the variant)?