了解 java 代码打印的内容

Question

大家好，我对此很陌生，对此代码有疑问。我不确定输出是怎样的，所以如果有人可以向初学者解释一下，我将不胜感激！这是代码：

public class NewClass{

    public static int[] first(int[] a) { // Array {1,2,3} is passed as an argument
        int[] b = new int[a.length]; 
        for (int i = 0; i < a.length; i++)
            b[i] = a[a.length - 1 - i];
        return b;
    }

    public static void second(int[] a) { // Use more descriptive names for you methods. If its aim is to reverse the array than call it reverseArray or something alike.
        for (int i = 0; i < a.length/2; i++) { // a.length = 3, a.length/2 = 1; So this loop will run only once
            int temp = a[i]; // temp = 4
            a[i] = a[a.length - 1 - i]; // a[0] = a[3 - 1 - 0] (a[2]) equals a[0] = 6
            a[a.length - 1 - i] = temp; // a[2] = temp equals a[2] = 4
        } // Array has become {6,5,4} (So it's been reversed.)
    }

    public static void main(String[] args) {
        int[][] matrix = {{1,2,3},{4,5,6}}; // Array of two elements, both elements refering to an other array with three elements
        System.out.println(matrix.length);  // This will print 2. It is a two dimensional array. 
        first(matrix[0]);   // Calling the first method and passing the {1,2,3} array as argument. It does stuff to a copy of the array (int[] b), but the returned value is never used. Array {1,2,3} is untouched.
        second(matrix[1]); // Same as with the first method
        for (int i = 0; i < 2; i++) { 
            for (int j = 0; j < 3; j++) // You should use { }. It will make the code easier to read. Only line 25 is executed inside this nested loop
                System.out.print(matrix[i][j]); // This will run 6 times (2 (outer loop) x 3 (nested loop)). It will print matrix[0][0], matrix[0][1], matrix[0][2], matrix[1][0], matrix[1][1], matrix[1][2]. Respectively 123 (next line) 456
            System.out.println();
        }
    }
}

它输出 2，然后是 1,2,3，最后是 6,5,4。

Answer 1

在您的第一个函数中，您正在创建一个新数组。返回此数组，但该值永远不会复制到原始数组中。因此，在打印矩阵时，第一个循环将保持为 1、2、3。如果你想改变这一点，请切换行：

first(matrix[0]);

收件人：

matrix[0] = first(matrix[0]);

第二个函数获取原始数组并循环一次（因为 a.length 为 3 且 3 / 2 = 1）。在这个循环中，它将 a[0] 转换为 a[a.length - 1 - i] 或 a[3 - 1 - 0]。然后它将 a[a.length - 1 - i] 或 a[2] 切换到 temp 即 a[0]。这将切换数组中的第一个和最后一个元素，即 6 和 4。

Answer 2

起初，您的代码非常混乱。尝试使用正确的缩进（每个块四个空格）。它使代码更易于阅读（您可能自己已经找到了答案）。

public class NewClass{

    public static int[] first(int[] a) { // Array {1,2,3} is passed as an argument
        int[] b = new int[a.length]; 
        for (int i = 0; i < a.length; i++)
            b[i] = a[a.length - 1 - i];
        return b;
    }

    public static void second(int[] a) { // Use more descriptive names for you methods. If its aim is to reverse the array than call it reverseArray or something alike.
        for (int i = 0; i < a.length/2; i++) { // a.length = 3, a.length/2 = 1; So this loop will run only once
            int temp = a[i]; // temp = 4
            a[i] = a[a.length - 1 - i]; // a[0] = a[3 - 1 - 0] (a[2]) equals a[0] = 6
            a[a.length - 1 - i] = temp; // a[2] = temp equals a[2] = 4
        } // Array has become {6,5,4} (So it's been reversed.)
    }

    public static void main(String[] args) {
        int[][] matrix = {{1,2,3},{4,5,6}}; // Array of two elements, both elements refering to an other array with three elements
        System.out.println(matrix.length);  // This will print 2. It is a two dimensional array. 
        first(matrix[0]);   // Calling the first method and passing the {1,2,3} array as argument. It does stuff to a copy of the array (int[] b), but the returned value is never used. Array {1,2,3} is untouched.
        second(matrix[1]); // Same as with the first method
        for (int i = 0; i < 2; i++) { 
            for (int j = 0; j < 3; j++) // You should use { }. It will make the code easier to read. Only line 25 is executed inside this nested loop
                System.out.print(matrix[i][j]); // This will run 6 times (2 (outer loop) x 3 (nested loop)). It will print matrix[0][0], matrix[0][1], matrix[0][2], matrix[1][0], matrix[1][1], matrix[1][2]. Respectively 123 (next line) 456
            System.out.println();
        }
    }
}

它输出 2，然后是 1,2,3，最后是 6,5,4。