Question

我正在学习使用java.swing库。我正在尝试创建一个非常简单的计算器布局。我添加了addNumbers方法。我试图在计算器中显示按钮，我已经使用了loops.buttons没有出现我正在获得nullpointerexception。

import java.awt.*;
import javax.swing.*;

public class Calculator extends JFrame{

    /**
     * @param args
     */
    //dEFINE WIDTH AND HEIGHT
    private static final int WIDTH = 400;
    private static final int HEIGHT = 600;

    //Values for buttons having numbers
    private JButton[] numButton;

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Calculator myCalculator = new Calculator();

    }

    public Calculator(){
        setTitle("Simple Calculator");
        setSize(WIDTH,HEIGHT);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        Container Pane = getContentPane();
        Pane.setLayout(new GridLayout(3,3));

        //Add numbers to screen now
        addNumbers(Pane);



    }

    //Function to add numbers on screen
    public void addNumbers(Container P){
        for(int i = 1; i <= 9; i++){
            numButton[i] = new JButton(String.valueOf(i));
            P.add(numButton[i]);
        }
    }

}

Answer 1

您需要初始化数组：

private JButton[] numButton = new JButton[10];

这里的10允许你的数组中有10个空格。

Answer 2

上面的解决方案可以工作并删除NullPointer，因为内存现已分配给numButton数组.... 这是一个工作代码片段，也包含（BODMAS）标志...... 唯一的区别是我将所有键标签都放入了ArrayList中并且for循环现在可以在此ArrayList上运行以获取其标签计算器... ArrayList可能不是一个好的集合。可能enum可能会更好但是这是为了展示如何使其更加动态并添加按钮标签或者新按钮会更容易...... 另请注意，您可以根据键标签的大小使GridSize动态化 array..gridSize = this.buttons.size（）/ 4 ....

Answer 3

import java.awt.Container;
import java.awt.GridLayout;
import java.util.ArrayList;
import java.util.List;

import javax.swing.JButton;
import javax.swing.JFrame;

public class Calculator extends JFrame{

    private static final int WIDTH = 400;
    private static final int HEIGHT = 600;


    List<String> buttons = new ArrayList<String>();




    JButton[] numButton = null;


    public static void main(String[] args) {
        Calculator myCalculator = new Calculator();
    }

    public Calculator(){
        setTitle("Simple Calculator");
        setSize(WIDTH,HEIGHT);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        Container Pane = getContentPane();


        this.buttons.add("1");
        this.buttons.add("2");
        this.buttons.add("3");
        this.buttons.add("4");
        this.buttons.add("5");
        this.buttons.add("6");
        this.buttons.add("7");
        this.buttons.add("8");
        this.buttons.add("9");
        this.buttons.add("0");
        this.buttons.add("+");
        this.buttons.add("=");
        this.buttons.add("%");
        this.buttons.add("#");
        this.buttons.add("*");
        this.buttons.add("/");

        int gridSize = this.buttons.size() / 4;

        Pane.setLayout(new GridLayout(gridSize,gridSize));


        this.numButton = new JButton[this.buttons.size()];

        //Add numbers to screen now
        addNumbers(Pane);
       }

    //Function to add numbers on screen
    public void addNumbers(Container P){
        for(int i = 0; i < this.buttons.size(); i++){
            numButton[i] = new JButton(this.buttons.get(i).toString());
            P.add(numButton[i]);
        }
    }

}

按钮没有出现，空指针异常

3 个答案: