Methods in Java

Explains how to write and call methods in Java program.

Prerequisites

• Understand Java primitive types
• Understand execution flow and the following Java control structures:

  • Sequence
  • Selection (if and switch statements)
  • Repetition (while, for, and do while statements)

Objectives

• Write and call (use) methods
• Pass arguments to a method
• Return a value from a method
• Understand how calling a method alters execution flow

Writing a Method

• The main reason for writing code inside methods (and functions), is to make the code resuable.
• Writing code inside methods also helps to organize a program.
• A method consists of a header and a body.
• The header is the first line of the method and includes modifiers, a return type, a name, and a parameter list.
• The body begins and ends with curly braces and usually includes a return statement immediately before the closing curly brace.
• Syntax:

  modifiers returnType methodName(param1, param2, …, paramN)
  {
      method body;
      return value;
  }

• Example:

  public static double average(double a, double b, double c)
  {
      double sum;
      sum = a + b + c;
      return sum / 3.0;
  }

Calling a Method

• The computer will not execute the code in a method until the method is called.
• To call a method, simply type the method name followed by an argument list to match the method’s parameter list.
• Syntax:

      varName = methodName(arg1, arg2, …, argN);

• Examples:

      double c = 5.4;
      double d = 2.3;
      double avg = average(3.1, c, d);
      double avg2 = average(6, 2, 3);

Complete Example

public class MethodTest
{
    public static void main(String args[])
    {
        double num1 = 4.1;
        double num2 = 4.2;
        double num3 = 5.7;
        double avg = average(num1, num2, num3);
        System.out.println("average is " + avg);
    }

    public static double average(double a, double b, double c)
    {
        double sum;
        sum = a + b + c;
        return sum / 3.0;
    }
}

Method Overloading

• When two or more methods in the same class have the same name but different parameter lists, we say the methods are overloaded.
• Method overloading is a convenience that means a programmer isn’t required to remember different function names.
• For example in the C programming language there are six functions to compute the absolute value of a number. Because C doesn’t have method overloading each function has a slightly different name.

  int abs(int x);
  long labs(long x);
  long long llabs(long long x);
  float fabsf(float x);
  double fabs(double x);
  long double fabsl(long double x);

  The C programmer must remember which function name to use based on the type of variable for which he needs the absolute value.
• In the Java Math class, there are four functions to compute the absolute value of a number, but all four of the functions have the same name.

  static int Math.abs(int x)
  static long Math.abs(long x)
  static float Math.abs(float x)
  static double Math.abs(double x)

  The Java programmer has to remember only one function name to compute the absolute value: Math.abs