java displaying List element as string

Question

I am following a Stream tutorial but I do not understand what I need to do to have the List elements output the content as the numbers contained within.

Given two lists of numbers, how would you return all pairs of numbers? For example, given a list [1, 2, 3] and a list [3, 4] you should return [(1, 3), (1, 4), (2, 3), (2, 4), (3, 3), (3, 4)]. For simplicity, you can represent a pair as an array with two elements.

I have read a few posts about overriding toString() but I have no object to override. I am clearly missing something about the nature of 'pairs', is it because 'pairs' is a List of int[][]?

package testcode;
import java.util.Arrays;
import java.util.List;
import static java.util.stream.Collectors.toList;

public class TestCode {

public TestCode(){
    testStream();
}

public static void main(String[] args) {
    TestCode tC = new TestCode();
}

public void testStream(){
    List<Integer> numbers1 = Arrays.asList(1,2,3);
    List<Integer> numbers2 = Arrays.asList(3,4);
    List<int[]> pairs = numbers1.stream()
            .flatMap(i -> numbers2.stream()
                    .map(j -> new int[]{i, j})
            )
            .collect(toList());
    pairs.forEach(System.out::println);
//I have also tried 
    //String a = Arrays.toString(pairs); // but no suitable method found.
    String list = pairs.toString();
    System.out.println("list = " + list);
//tried converting pairs element to a List<String> using concatenation to force it be a string.
    List<String> stringsList  = new ArrayList<String>(pairs.size()); 
    for (int i = 0 ; i < pairs.size() ; i++) {
        stringsList.add("" + pairs.get(i));   
    }
    stringsList.forEach(System.out::println);
// or   
    System.out.println(" ** test 2");
    String[] stringsList2  = new String[pairs.size()]; 
    for (int i = 0 ; i < pairs.size() ; i++) {
        stringsList2[i] = "" + pairs.get(i);
        System.out.println(stringsList2[i]);
    }
}

Cheers for reading.

the code I used: pairs.forEach(pair -> System.out.println("(" + pair[0] + ", " + pair[1] + ")")); — Query9000
– Query9000, Commented Jan 18, 2015 at 20:45
You should consider using printf for printing you pairs: System.out.printf("(%d,%d)\n",pair[0],pair[1])). It might just be my opinion, but it looks a bit cleaner than all that string concatenation. — ankh-morpork
– ankh-morpork, Commented Jan 18, 2015 at 21:50

Malik Brahimi · Accepted Answer · 2015-01-18 18:21:36Z

1

Just use a nested for loop to cycle through the arrays and concatenate casted strings.

public static String test_method() {
    int[] array1 = {1, 2, 3};
    int[] array2 = {3, 4};

    String string = "";

    for (int i: array1) {
        for (int j: array2) {
            String sub = "(" + String.valueOf(i) + ", " 
                             + String.valueOf(j) + ") ";
            string += sub;
        }
    }

    return string;
}

answered Jan 18, 2015 at 18:21

Malik Brahimi

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1 Comment

Malik Brahimi Over a year ago

You can of course iterate through a List object too.

ankh-morpork · Accepted Answer · 2015-01-18 18:19:08Z

1

Arrays.deepToString(Object[] a) should accomplish what you need.

deepToString will recursively print the elements of a multidimensional array.

Example

Code

import java.util.Arrays;
public class Test{
    public static void main(String[] args){
        int[][] test = {{1,2,3},{4,5,6},{7,8,9}};
        System.out.println(Arrays.deepToString(test));
    }
}

Output

[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

answered Jan 18, 2015 at 18:19

ankh-morpork

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