java将列表元素显示为字符串

Question

我正在学习 Stream 教程，但我不明白需要做什么才能让 List 元素将内容输出为其中包含的数字。

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.

我读过一些关于重写 toString() 的文章，但我没有要重写的对象。我显然遗漏了一些关于“pairs”本质的东西，是因为“pairs”是一个 int[][] 列表吗？

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]);
    }
}

为阅读干杯。

Answer 1

只需使用嵌套的 for 循环即可循环遍历数组并连接强制转换的字符串。

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;
}