What is the difference between String[] data and String... data in java [duplicate]

Question

I have tried with data and data1 variables. It's always calling to String ... data. So, what is the difference between String[] data and String... data in java.

public class ArrayTest {

    public static void main(String[] args) {

        ArrayTest arrayTest = new ArrayTest();
        // Option one
        String[] data = {"A", "B", "C"};
        // Option two
        String data1 = "A";
        arrayTest.test(data);


    }

    public void test(String[] ... data  ) {
        System.out.println("---From: String[] ... data---");

        for(String[] item: data) {

            for(String innerItem : item) {
                System.out.println(innerItem);
            }

        }
    }

    public void test(String ... data  ) {
        System.out.println("---From: String ... data---");
        for(String item: data) {
            System.out.println(item);
        }
    }

}

Answer 1

In test(String... data) you are passing an array of strings and in test(String[]... data) you are passing an array of arrays of strings. Check the updated code for illustration:

public class ArrayTest {

    public static void main(String[] args) {

        ArrayTest arrayTest = new ArrayTest();
        // Option one
        String[] data = { "A", "B", "C" };
        // Option two
        arrayTest.test(data);

        String[] data2 = { "D", "E" };
        arrayTest.test(data, data2);
    }

    public void test(String[]... data) {
        System.out.println("---From: String[] ... data---");

        for (String[] item : data) {

            for (String innerItem : item) {
                System.out.println(innerItem);
            }

        }
    }

    public void test(String... data) {
        System.out.println("---From: String ... data---");
        for (String item : data) {
            System.out.println(item);
        }
    }

}

Output:

---From: String ... data---
A
B
C
---From: String[] ... data---
A
B
C
D
E

In the presence of both versions of method signatures, JVM chooses the closest fit and that is why it goes to test(String... data) in case of arrayTest.test(data) while it goes to test(String[]... data) in case of arrayTest.test(data, data2).

The program will still work if you remove the following definition but then JVM will be left with only one choice, which is to go to test(String[]... data) for both the calls.

public void test(String... data) {
    System.out.println("---From: String ... data---");
    for (String item : data) {
        System.out.println(item);
    }
}

Answer 2

When using Varargs (T...) arguments are packed into an array which is passed to the method at run time. You have already answered your question with your own implementation:

For:

1) public void test(String[] ... data) -- data is packed as String[][]

2) public void test(String ... data) -- data is packed as String[]

I strongly recommend the book: Java generics and collections - By Maurice Naftalin