Intro to methods:    re-using computer statements

  • Consider the program statements that compute the sum of integers from 1 to 10: 

     int i, sum = 0;

 for ( i = 1; i <= 10; i++ )
    sum += i;

  • Consider the program statements that compute the sum of integers from 23 to 36: 

     int i, sum = 0;

 for ( i = 23; i <= 36; i++ )
    sum += i;

  • Observation:

    • The program statements to do the different tasks are similar

Intro to methods:    re-using computer statements

Java program that compute the sums of the numbers 1 to 10 and the numbers 23 to 36 

public class Sum
{
    public static void main(String[] args) 
    {
        int i, sum;
        
        sum = 0;
        for ( i = 1; i <= 10; i++ )    // Sum numbers from 1 - 10
            sum += i;
        System.out.println(sum);
            
        sum = 0;
        for ( i = 23; i <= 36; i++ )   // Sum numbers from 23 - 36
            sum += i;  
        System.out.println(sum);
    }
}

DEMO: demo/06-methods/01-intro/Sum.java

Better solution:   define a method once and invoke (= run) the method multiple times !

 Method  

 

  • A method is used to group program statements together that perform an useful task

  • A method has parameter variables that can be used to change its behavior

Defining and invoking (= running) methods

  • How to define a method called sum that compute the sum of integers from a to b: 

    public static int sum(int a, int b) 
{
   int i, result = 0;

   for (int i = a; i <= b; i++)
      result += i;

   return result;
}

  • How to invoke (= run) the sum( ) method: 

     int s1, s2;

 s1 = ClassName.sum(1, 10);  // Compute the sum from 1 to 10
 s2 = ClassName.sum(23, 36); // Compute the sum from 23 to 36

DEMO: demo/06-methods/01-intro/SumMethod.java     

Intro to methods:    re-using computer statements

Example:   a Java program with a method definition and two method invocations: 

public class SumMethod 
{
    // Method definition<
    public static int sum(int a, int b) 
    {
        int i, result = 0;

        for (i = a; i <= b; i++)
            result += i;

        return result;
    }

    public static void main(String[] args) 
    {
        int s1, s2;
        
        s1 = SumMethod.sum(1,10);   // Method invocation
        s2 = SumMethod.sum(23,36);  // Method invocation
    }   
}

Notice: You can define a method (= code) just once and invoke (= run) the method (code) multiple times

The default classname in a method call

When you invoke a method from inside the same class, we can omit the class name: 

public class SumMethod 
{
    // Method definition<
    public static int sum(int a, int b) 
    {
        int i, result = 0;

        for (i = a; i <= b; i++)
            result += i;

        return result;
    }

    public static void main(String[] args) 
    {
        int s1, s2;
        
        s1 = SumMethid.sum(1,10);   // This is the official syntax
        s2 =           sum(23,36);  // Classname optional if invoked in same class
    }   
}

(Because Java will add the classname (= " SumMethod) to the method call by default).

Make your own library of useful methods

You can also save your methods in a dedicated class (with a meaningful class name):

The Tools class containing useful methods Java program that uses the method(s) 
public class Tools 
{
    public static int sum(int a, int b) 
    {
        int i, result = 0;

        for (i = a; i <= b; i++)
            result += i;

        return result;
    }
} 
public class UseTools 
{
    public static void main(String[] args) 
    {
        int s1, s2;
        
        s1 = Tools.sum(1,10);
        s2 = Tools.sum(23,36);
    }
}

You must use the classname Tools. before a method to invoke methods that are defined in a different class

Note: this is how Java provides the Mathematical functions to you, such as Math.pow( )

 

DEMO: demo/06-methods/01-intro/Tools.java   and   UseTools.java