Why is the array showing null? What is wrong with my code?

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import java.util.*;
class Main {
    public static Scanner scanner=new Scanner(System.in);
    public static int[] arr;
    public static void main(String[] args) {
        InsertElements(arr);
        PrintUnsortedArray(arr);
    }
    public static void InsertElements(int[] barr){
        System.out.println("Enter the size of array:");
        int size=scanner.nextInt();
        barr=new int[size];
        System.out.println("Enter the array elements:");
        for(int i=0;i<barr.length;i++){
            barr[i]=scanner.nextInt();
        }
    }
    public static void PrintUnsortedArray(int[] arr){
        System.out.println("The unsorted array is:"+Arrays.toString(arr));
    }
}

/*My output is:

Enter the size of array:
5
Enter the array elements:
5 4 3 2 1
The unsorted array is:null*/

What I have tried:

//I know this is right

import java.util.*;
class Main {
    public static Scanner scanner=new Scanner(System.in);
    public static int[] arr;

    public static void main(String[] args) {
        System.out.println("Enter the size of array:");
        int size=scanner.nextInt();
        arr=new int[size];
        InsertElements(arr);
        PrintUnsortedArray(arr);
    }

    public static void InsertElements(int[] barr){
        System.out.println("Enter the array elements:");
        for(int i=0;i<barr.length;i++){
            barr[i]=scanner.nextInt();
        }
    }

    public static void PrintUnsortedArray(int[] arr){
        System.out.println("The unsorted array is:"+Arrays.toString(arr));
    }
}

Posted 26-Dec-21 8:13am

Sourabh Chavan

Updated 26-Dec-21 8:48am

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OriginalGriff · Accepted Answer · 2021-12-26T08:48:00

Java uses pass by value for parameters - which means when you call InsertElements with arr as the parameter:

Java

public static void main(String[] args) {
    InsertElements(arr);
    PrintUnsortedArray(arr);
}

the value in the variable is copied - null - and passed to the function.
The function then overwrites that value with the new array, fills it and returns - which doesn't affect the external variable at all.

If you think about it, that's the only sensible way to do it. What would happen if it didn't work like that, but the variable was passed instead?

Java

void Foo(int x) {
   x = x + 10;
}
...
int val = 56;
Foo(val);
System.out.println(val);

Would work fine - 66 would be printed.
But ... if we do this:

Java

void Foo(int x) {
   x = x + 10;
}
...
Foo(56);
System.out.println(56);

What happens? Does the value of a constant get changed? Logically, yes, it would - and that would make coding very, very difficult!

So just remember: Java always copies the content of a variable and passes that to a function, not the variable itself.