CSCE155 and CSCE155H

FAQs and Interesting Questions

Static Methods and Data Members

Q6. When is it better to use a static data member?

A6. A static data member is a class data member. It means that if you change the value of the data member in one instance, then the change is felt or reflected in all instances of the same class. It is better to use a static data member when each instance of the same class has an opportunity to handle a change to that data member that will cause all other instances of the same class to follow suit. For example, suppose you have a class called MineSweeper and you have created 4 instances/objects of this MineSweeper class. In the MineSweeper class, there is a boolean data member called mineDetected. Now, suppose you have the following:

MineSweeper robot1 = new MineSweeper(area1);

MineSweeper robot2 = new MineSweeper(area2);

MineSweeper robot3 = new MineSweeper(area3);

MineSweeper robot4 = new MineSweeper(area4);

The argument passed into the constructor means the area that the MineSweeper object will cover.

Now, suppose that as soon as one robot finds a mine, all robots terminate their sweep.

In this case, declaring the boolean data member mineDetected static is appropriate! Why? As soon as one robot detects a mine, it sets the boolean data member to true. And then all robots will notice the change in the boolean value and call off their sweeps.

Q7. When is it better to use a static method?

A7. You declare a method static only if you want it to be used by client programs without having to create the object first. Essentially, you are creating a global method when you declare a method static. For example, suppose you have the following code:

class Pen {

private String color;

private double cost;

public void computeQuality(String color) {

if (color == “red”)

System.out.println(“Quality is low.”);

else

System.out.println(“Quality is high.”);

}

}

To use the method computeQuality(), you will need to do this in your client program:

Pen myPen = new Pen();

myPen.computeQuality(“blue”);

That is, you will have to create the Pen object first before you can access its method!

However, if you declare the computeQuality() method static, then it essentially becomes a global variable. Then it is okay to do this:

Pen.computeQuality(“blue”);

Pay very close attention to the difference here. When the method is static, we do NOT have to create an object to use the method. But to use it, we do have to use the class name to designate it.

When it is better to use static methods? It is better to declare a method static if you want the method to be used freely by other client programs. When is that? It is when the method that you implement is generally useful. For example, a random generator by implemented by the Math class (i.e., Math.random).

Do not abuse the power of class methods. Most of the time, methods should be instance methods. That is, methods should be usually declared without static.