Var args...

Var args

When you want to define a method to take multiple values of one specific data type, then var args can be used.

Syntax:

<returnType> <methodName> (<dataType> <FixedArg>, <dataType> ...<VarArgs>)
{
    ...
}

Note 1: The following syntaxes are valid (Notice the spaces)

show(int...arr)
show(int ...arr)
show(int... arr)

Note 2: In case of overloaded methods, below is the priority

1. Exact match.

2. Higher type (i.e automatic widening)

3. Wrapper classes

4. var args.

Example 1:

Limitation by passing multiple variables of the same data type as array

class Lab4 
{
    public static void main(String[] args) 
    {
        Hello h1 = new Hello();
        h1.sum(1,2,3,4,5);
        h1.sum(1,2,3,4,5,6,7,8);
    }
}

class Hello
{
    void sum(int arr[]){
        int sum=0;

        for(int a:arr){
            sum+=a;
        }
        System.out.println(sum);
    }
}

/*

Compile Time Error:

Lab4.java:6: error: method sum in class Hello cannot be applied to given types;
                h1.sum(1,2,3,4,5);
                  ^
  required: int[]
  found: int,int,int,int,int
  reason: actual and formal argument lists differ in length
Lab4.java:7: error: method sum in class Hello cannot be applied to given types;
                h1.sum(1,2,3,4,5,6,7,8);
                  ^
  required: int[]
  found: int,int,int,int,int,int,int,int
  reason: actual and formal argument lists differ in length
2 errors

*/

Example 2

class Lab5 
{
    public static void main(String[] args) 
    {
        Hello h1 = new Hello();
        h1.sum(1,2,3);
        h1.sum(1,2,3,4,5);
        h1.sum(1,2,3,4,5,6,7,8);
    }
}

class Hello
{
    void sum(int ...arr){
        int sum=0;
        System.out.println("int... arr");
        System.out.println("Length - "+arr.length);

        for(int a:arr){
            sum+=a;
        }
        System.out.println(sum);

    }
}

/*output:
int ...arr
Length : 3
Sum :6
int ...arr
Length : 5
Sum :15
int ...arr
Length : 8
Sum :36
*/

Example 3:

There can be only one variable argument in the method. More than 1 variable argument throws C.T.E

class Lab10 
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        h.show(10);
    }
}

class Hello
{
    void show(int ...arr, int ...arr1){
        System.out.println("\nshow(int..., int...)");
    }
}

/*
Lab10.java:12: error: ')' expected
        void show(int ...arr, int ...arr1){
                            ^
Lab10.java:12: error: <identifier> expected
        void show(int ...arr, int ...arr1){
                                 ^
Lab10.java:12: error: <identifier> expected
        void show(int ...arr, int ...arr1){
                                         ^
3 errors
*/

Example 4:

Variable argument (varargs) must be the last argument.

class Lab8 
{
    public static void main(String[] args) 
    {
        Hello h1 = new Hello();
        h1.show(10);
        h1.show(10,20);
        h1.show(10,20, 30);
    }
}

class Hello
{
    void show(int a, int...ab){
        System.out.println("int, int...ab");
    }
}

/*output:
int, int...ab
int, int...ab
int, int...ab
*/

Example 5:

Another example with var args as the last argument

class Lab12 
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        h.show("Sujay", 10);
    }
}

class Hello
{
    void show(String str, int ...arr){
        System.out.println(str+"\t"+arr);
    }
}

/*output:
Sujay   [I@15db9742 //array address
*/

Example 6:

Pass 2-D array as a parameter

class Lab15 
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        int arr1[] = new int[]{1,2};
        int arr2[] = new int[]{4,5};
        h.show(arr1, arr2);
    }
}

class Hello
{
    void show(int[]... arr){
        System.out.println("show int[]...");

        for(int[] temp:arr){
            for(int a:temp){
                System.out.print(a+"\t");
            }
            System.out.println();
        }
    }
}

/*output:
show int[]...
1       2
4       5
*/

Example 7:

Note how you specifiy the 2-D array as parameters

class Lab16 
{
    public static void main(String[] args) 
    {
        new Hello().show((new int[]{1,2,3}), (new int[]{4,5,6}));
    }
}

class Hello
{
    void show(int...arr[]){
        System.out.println("int ...arr[]");

        for(int[] temp:arr){
            for(int a:temp){
                System.out.print(a+"\t");
            }
            System.out.println();
        }
    }
}

/*

Lab16.java:11: error: legacy array notation not allowed on variable-arity parameter
        void show(int...arr[]){
                           ^
1 error

*/

Example 8:

Overloaded method with exact match and var args... Exact match has the highest priority

class Lab19
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        h.show(10,20);
    }
}


class Hello
{
    void show(int...b){
        System.out.println("show(int...)");
    }

    void show(int a, int b){
        System.out.println("show(int a, int b)");
    }
}

/*output:
show(int a, int b)
*/

Example 9:

Overloaded methods... automatic widening (casting) has higher priority over vargs

class Lab20
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        h.show(10,20);
    }
}


class Hello
{
    void show(int...b){
        System.out.println("show(int...)");
    }

    void show(long a, long b){
        System.out.println("show(long a, long b)");
    }
}

/*output:
show(long a, long b)
*/

Example 10:

Ambiguity as the method call matches both the methods...C.T.E

class Lab21
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        h.show(null);
    }
}


class Hello
{
    void show(int...b){
        System.out.println("show(int...)");
    }

    void show(String...a){
        System.out.println("show(String...)");
    }
}

/*
Lab21.java:6: error: reference to show is ambiguous
                h.show(null);
                 ^
  both method show(int...) in Hello and method show(String...) in Hello match
1 error
*/

Example 11:

null, null cannot be stored in Integer array, Therefore String array method is executed.

class Lab22
{
    public static void main(String[] args) 
    {
        Hello h = new Hello();
        h.show(null, null);
    }
}


class Hello
{
    void show(int...b){
        System.out.println("show(int...)");
    }

    void show(String...a){
        System.out.println("show(String...)");
    }
}

/*
show(String...)
*/

Summary

1. To define Var-Args parameter, you need to use ellipsis [three dot(...)] after data type before the variable name.
   Ex: void sum(int... values)
2. Var-Args parameter will be converted to an array by the java compiler while generating the class file.
   Ex: void sum(int ...values) ->void sum(int[] values)
3. When you are calling a method which contains Var-Args parameter by passing zero or more values then Java compiler converts it into an array.
   Ex:
   sum(); ->sum(new int[0]{})
   sum(99, 88); ->sum(new int[99, 88]{})
   sum(99, 88, 77); ->sum(new int[99, 88, 77]{})
   
4. Var-args can be used only to define parameters of method or constructor and can't be used for instance variables, static variables or local variables.
5. Var-Args parameters is equivalent to an array. So data available in Var-Args parameter can be accessed like array.
   Ex.
   

   void sum(int ...values){
       for(int i=0;i<values.length;i++){
           System.out.println(values[i]);
       }
   }
   

6. You can pass actual values or array object to the method which is having Var-Args.
   Ex:
   

   sum(10,20);
   int arr[] = new int[]{10,20};
   sum(arr);