• To understand the var args concept , let us solve the following problem

• Write a class that has a method called sum( ) which can add and return the sum of all the integers passed to it as argument .

• The method should be able to accept 2 3 or 4 arguments

• One way to solve this problem is Using method overloading

class MyMath {
    public static int sum(int a, int b) {
        return a + b;
    }

    public static int sum(int a, int b, int c) {
        return a + b + c;
    }

    public static int sum(int a, int b, int c, int d) {
        return a + b + c + d;
    }
}

class UseMyMath {
    public static void main(String args[]) {
        System.out.println(MyMath.sum(10, 20));
        System.out.println(MyMath.sum(10, 20, 30));
        System.out.println(MyMath.sum(10, 20, 30, 40));
    }
}
Output : 30 60 100

Problems With The Code

• Programmer has to design multiple methods even though they are performing the same task

• Code is not flexible , i.e. in future if we want to add 5 integers it will not work

• We cannot pass array as argument to any of these methods

Solution ?

• To solve this problem , from JDK 5 Java has included a feature called var args which stands for variable length arguments

What Is Var Args

• Var args is a way of creating methods

• It allows a method to accept zero or more arguments of a specified type .

• This feature is very useful when the number of arguments passed to a method can vary

Syntax Of Var Args

• A variable length argument is specified by three dots (…) called as ellipses , in the method’s formal argument list .

• For Example,

  public static void fun(int arr ) { // method body }

  This syntax tells the compiler that fun( ) can be called with zero or more arguments .

• We can even pass an array as argument to this method while calling it.

Example

class Sample {
    static void fun(int... a) {
        System.out.println("Number of arguments: " + a.length);

        for (int x : a)
            System.out.print(x + " ");

        System.out.println();
    }
}

class UseSample {
    public static void main(String args[]) {
        Sample.fun(100);                   // one parameter
        Sample.fun(1, 2, 3, 4);            // four parameters
        Sample.fun();                      // no parameter
        Sample.fun(new int[]{10, 20, 30, 40, 50}); // array as parameter
    }
}

Output

Number of arguments: 1
100

Number of arguments: 4
1 2 3 4

Number of arguments: 0

Number of arguments: 5
10 20 30 40 50

Internal Working OF Var Args

• Consider the following method public int sumNumber (int x, int nums ) { // method body }

• The syntax tells the Java compiler that the method can be called with varying number of arguments

• Now when we invoke a method with variable arguments , java compiler matches the arguments from left to right

• Once it reaches to the last varargs parameter, it creates an array of the remaining arguments and pass it to the method

Rules About Var Args

• The ellipses used to denote var args always have to be before the argument name and together Predict which one is correct ?

  public static int sum(int … arr)

  // correct!

  public static int sum(int … arr)

  // correct!

  public static int sum(int… arr)

  // correct!

  public static int sum(int arr)

  // incorrect!

  public static int sum(int .arr..)

  // incorrect!

  public static int sum(int . .. arr )

  // incorrect!

  public static int sum(int .. . arr)

  // incorrect!

• We can combine any number type of arguments with var args , but var args should be the last argument

  Predict which one is correct ?

  public static void display(int n,int … arr)

  // correct!

  public static void display(String str,int … arr )

  // correct!

  public static void display(double z, int… arr,String str)

  // incorrect!

• There can be only one var args in a method

  Predict which one is correct ?

  public static void display(int … arr)

  // correct!

  public static void display(char ch,int … arr )

  // correct!

  public static void display(double … arr , int …)

  // incorrect!

• From Java 5 onwards , we can have main method with var args as shown below public

  public static void main(String … args)

  Now , when we will run the program , java will internally call the main(String … args ) method in the same way as it called main(String[ ] args)