如何在 Java 网格 GUI 中选择特定单元格？

Question

我正在开发一个 GUI 网格，我需要选择特定的单元格。我用JFrame。我的目标是用数字填充第一列和最后一行，例如 x-y 轴。

我已尝试声明一个 JButtons 数组，其中包含我需要的确切单元格位置，并设置文本。 （代码中的大写锁定注释）

import java.awt.Color;
import java.awt.GridLayout;
import javax.swing.JButton;
import javax.swing.JFrame;

public class GridLayoutTest {

    private static JButton[] arrayBtn;

    public static void main(String[] args ) {

        // the frame that contains the components
        JFrame frame = new JFrame("GridLayoutTest from JCG");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // set the size of the frame
        frame.setSize(1000, 1000);

        // set the rows and cols of the grid, as well the distances 
       between them
        GridLayout grid = new GridLayout(10,14, 0, 0);
        // what layout we want to use for our frame
        frame.getContentPane().setBackground(Color.WHITE);;
        frame.setLayout(grid);

        arrayBtn = new JButton[140];

        // add JButtons dynamically
        for(int i=0; i < arrayBtn.length; i++) {
            arrayBtn[i] = new JButton();  

            arrayBtn[i].setBackground(Color.WHITE);
            arrayBtn[i].setSize(1,1);

               //IF I RUN ONLY THIS WORKS WITH THE FIRST CELL(IF I INSERT OUT 
               // THE FOR CYCLE DOESN'T WORK)
               arrayBtn[0].setText("9");

              // IF I RUN ALSO THIS CODE ROW I HAVE THE SAME ERROR 
               arrayBtn[1].setText("9");



            frame.add(arrayBtn[i]);
        }
        frame.setVisible(true);
    }
}

例外：

Exception in thread "main" java.lang.NullPointerException
relative to the row of the specific cell selected.

Answer 1

您在每个循环迭代期间设置第二个单元格（索引 1），这意味着即使在第二个单元格未初始化的第一次迭代期间。

假设我们第一次进入循环。我们将初始化第一个按钮（索引 0）。然后将第二个按钮（索引 1）的文本设置为 "9"。这个按钮还没有被初始化，所以我们在一个仍然是null的对象上调用一个方法。这就是您收到NullPointerException 的原因。

for (int i = 0; i < arrayBtn.length; i++) {
    arrayBtn[i] = new JButton();
    arrayBtn[i].setBackground(Color.WHITE);
    arrayBtn[i].setSize(1, 1);

    // This works (set every button text to "9"):
    arrayBtn[i].setText("9");

    // This will NOT work:
    arrayBtn[1].setText("9"); 

    frame.add(arrayBtn[i]);
}

// This works (set the first button text to "9"):
arrayBtn[0].setText("9");

请记住，数组索引从索引 0 开始：

String[] array = {"first", "second", "third"};
System.out.println(array[1]); // will print "second"

因此，在您的问题的完整上下文中：也许您应该考虑为您的按钮数组使用two-dimensional array，因为这更类似于您尝试构建的网格 (10x14)。

JButton[][] buttonGrid = new JButton[10][14];
for (int y = 0; y < 10; y++) {
    for (int x = 0; x < 14; x++) {
        buttonGrid[y][x] = new JButton("Test");
        if (y == 0) {
            // code in here is only executed for the first row
            buttonGrid[y][x].setText(Integer.toString(x));
        }
        if (x == 0) {
            // code in here is only executed for the first column
            buttonGrid[y][x].setText(Integer.toString(y));
        }
    }
}

这看起来有点像这样：

[0, 1,    2,    3,    4,    5,    6,    7,    8,    9,    10,   11,   12,   13  ]
[1, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[2, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[3, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[4, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[5, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[6, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[7, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[8, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]
[9, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test, Test]