数组索引处的打印值返回哈希码

Question

Just playing around with displaying values in a two dimensional array and noticed my code prints some hashcodes and then the values. But I am not printing the array object itself (as done in the post here ) or at least explicitly calling the toString or hashcode method of the array object. Instead, I'm directly accessing and printing the values in the array using arrayObj[i] and arrayObj[i][j] .

Here's my code:

class PrintTwoDimArray {
    public int [][] createArray () {
        int counter = 0;
        int[][] intArray = new int [2][4];
        for (int i = 0; i < intArray.length; i++) {
            for (int j = 0; j < intArray[i].length; j++) {
                intArray[i][j] = ++counter;
            }
        }
        return intArray;
    }
    public void printArray ( int [][] arrayObj ) {
        for (int i = 0; i < arrayObj.length; i++) {
            System.out.print(arrayObj[i] + " ");
            for (int j = 0; j < arrayObj[i].length; j++) {
                System.out.print(arrayObj[i][j] + " ");
        }
        System.out.println();
        }
    }
}

class TestPrintTwoDimArray {
    public static void main (String[] args) {

    PrintTwoDimArray twoDim = new PrintTwoDimArray();
    int [][] multiArray = new int [2][4];
    multiArray = twoDim.createArray();
    twoDim.printArray(multiArray);
    }
}

My output is as follows:

It seems that my code is somehow calling the toString or hashcode method of the array. Thoughts? How can I modify to print just the values?

javac and java version 1.8.0

Answer 1

A two dimensional array is an array of arrays. The array you create ( int[2][4] ) looks like this

[ 0 ] -> [ 1, 2, 3, 4 ]
[ 1 ] -> [ 5, 6, 7, 8 ]

So when you only access the first dimension you will get the array that holds the second dimension.

int[][] arr = createArray();
System.out.println(arr[0]);

will output something like

[I@1db9742

To print an array's values you can use Arrays.toString(arr) . In this case you can omit the inner loop, because Arrays.toString() will do it for you.

 public void printArray ( int [][] arrayObj ) {
    for (int i = 0; i < arrayObj.length; i++) {
        System.out.println(Arrays.toString(arrayObj[i]));
    }
}

Answer 2

The hash values come from your first System.print sentence.

System.out.print(arrayObj[i] + " ");

With this sentence you are printing an Object (an array) and therefore Java is invoking the default toString method.

Answer 3

When you print

arrayObj[i]

you get the default Object toString() from an array. You could use Arrays.toString(int[]) to make that a String like

System.out.println(Arrays.toString(arrayObj[i]));

Alternatively, you can use Arrays.deepToString(Object[]) to print your multi-dimensional array without a loop

System.out.println(Arrays.deepToString(arrayObj));

Edit

You could use formatted output to build your table.

public static void printArray(int[][] arrayObj) {
    for (int i = 0; i < arrayObj.length; i++) {
        System.out.printf("%03d: ", i + 1);
        for (int val : arrayObj[i]) {
            System.out.printf("% 4d ", val);
        }
        System.out.println();
    }
}

public static void main(String[] args) {
    int[][] table = { { 3, 1, 2 }, { 5, 1, 4 }, { 100, 200, 300 } };
    printArray(table);
}

Output is

001:    3    1    2 
002:    5    1    4 
003:  100  200  300