instance_methods.md

Instance Methods

Overview

In procedural langauges like Fortran and C, the world is made of data structures and you do things to them. For example, if you have a data structure vrepresenting a two-dimensional vector, you might call functions like length(v) to determine the length of v or add(v, w) to get the sum of vectors v and w.

In object-oriented languages like C++ and Java, the world is made of objects and you ask them to do things. If v is an object (an instance of a class Vector), you would say v.length() to get the length of v. Think of this expression as saying to the object, "Tell me your length." Similarly, v.add(w) says to the object, "Tell me the result of adding you to w."

The things that an object knows how to do are called instance methods, because you call them on specific instances (objects). This is constrasted with static methods, which are called on classes, as in Math.sin(x).

Instance methods are defined inside a class, but they are not declared static. Here is the definition of Vector including a constructor and the methods described above:

public class Vector {

    double x;

    double y;

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

    double length() {
        return Math.sqrt((x * x) + (y * y));
    }

    Vector add(Vector that) {
        return new Vector(x + that.x, y + that.y);
    }
    
}

It is worth making sure you understand the definition of add. The body could have been written as

return new Vector(this.x + that.x, this.y + that.y);

to emphasize that you are adding together the value of x for this (the object on which add was called) and the value of x for that (the argument passed to the method). When there is no ambiguity, you are allowed to leave off this..

Resources

• Sedgewick and Wayne, Introduction to Programming in Java, Section 3.2
• Horstmann, Core Java, Volume I: Fundamentals, 11th Edition, Section 4.3

Questions

1. ⭐ When, if ever, is an instance method declared static?
2. ⭐⭐ Is this a reserved word? What about that?
3. ⭐⭐ When, if ever, is this == null?
4. ⭐⭐ Add a method perimeter, which takes no arguments and returns the Square's perimeter, to the class below.

   class Square {
   
       double side;
   
       Square(double s) {
           side = s;
       }
   
   }

5. ⭐⭐ Supply the missing line in the class below.

   class Snake {
   
       Snake(double length) {
           this.length = length;
       }
   
       double getLength() {
           return length;
       }
   
   }    

6. ⭐⭐ Why does the class below not compile?

   class Account {
   
       int balance;
   
       int getBalance() {
           return balance;
       }
   
       static void main(String[] args) {
           System.out.println(getBalance());
       }
   
   }

7. ⭐⭐ By adding an instance variable, modify the class below so that the instance methods don't need arguments.

   class Square {
   
       public static void main(String[] args) {
           Square s = new Square();
           double side = 3;
           System.out.println("Area: " + s.area(side));
           System.out.println("Perimeter: " + s.perimeter(side));
       }
   
       double area(double side) {
           return side * side;
       }
   
       double perimeter(double side) {
           return side * 4;
       }
   
   }

Answers

1. Never. By definition, an instance method is one that is not declared static.

2. this is a reserved word. that is not, but it is sometimes a reasonable name for a parameter of the same type as this.

3. Never. Calling an instance method on null results in a NullPointerException.

4. double perimeter() {
       return 4 * side;
   }

5. The line

   double length;

   should be added inside the class (but not inside any method).

6. The instance method getBalance cannot be called from the static method main. It would work to print new Account().getBalance().

7. class Square {
   
       double side;
   
       public static void main(String[] args) {
           Square s = new Square(3);
           System.out.println("Area: " + s.area());
           System.out.println("Perimeter: " + s.perimeter());
       }
   
       Square(double side) {
           this.side = side;
       }
   
       double area() {
           return side * side;
       }
   
       double perimeter() {
           return side * 4;
       }
   
   }