Question

我正在做一些练习来理解Java和Swing API。为什么在Disegno构造函数中有nullPointerException？我想打印两个矩形的坐标，但它们似乎没有初始化。

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

public class Disegno extends JFrame{

    Disegno(){
        this.setSize(500, 500);
        this.setDefaultCloseOperation(this.EXIT_ON_CLOSE);
        MyPanel aba = new MyPanel();
        this.setContentPane(aba);
        this.setVisible(true);

        System.out.println(aba.rect.blue.x + "-" + aba.rect.blue.y);
        System.out.println(aba.rect.yellow.x + "-" + aba.rect.yellow.y);
    }

    public static void main(String[] args){
        new Disegno();
    }
}

class MyPanel extends JPanel{

    JPanel up, down;
    RectArea rect;

    MyPanel(){
        this.setLayout(new BorderLayout());

        up = new JPanel();
        this.add(up, BorderLayout.NORTH);
        up.setBackground(Color.red);
        up.setVisible(true);

        down = new JPanel();
        down.setBackground(Color.green);
        this.add(down, BorderLayout.SOUTH);
        down.setVisible(true);

        rect = new RectArea();
        this.add(rect, BorderLayout.CENTER);

        this.setVisible(true);
    }
}

class RectArea extends JPanel{

    Rectangle blue, yellow;
    boolean check = false;

    RectArea(){
        super();
        this.setVisible(true);
    }

    public void initRect(){
        blue = new Rectangle(0, 0, 100, 100);
        yellow = new Rectangle(this.getWidth(), this.getHeight(), 100, 100);
        System.out.println("ok");
    }

    @Override
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        if(check == false){
            this.initRect();
            check = true;
        }

        System.out.println(this.getWidth() + "-" + this.getHeight());
        g.setColor(Color.blue);
        g.fillRect(blue.x, blue.y, blue.width, blue.height);
        g.setColor(Color.yellow);
        g.fillRect(yellow.x - yellow.width, yellow.y - yellow.height, yellow.width, yellow.height);
    }
}

Answer 1

其他人帮助建议了检测和避免NullPointerException的方法。不幸的是，当调用paintComponent()的实现时，你不能依赖。相反，

根据当前寡妇的大小确定所需的几何图形;在下面的示例中调整窗口大小，以查看yellow似乎粘贴到右下角。

由于MyPanel不包含自己的组件，因此您应该覆盖getPreferredSize()，因为@nIcE cOw显示here。

使用pack()调整封闭式Window的大小。

构建于event dispatch thread。

附录：我无法理解为什么要覆盖方法 getPreferredSize()。

JComponent的子类覆盖getPreferredSize()，以便pack()可以调整Window的大小以适合其子组件的首选大小和布局。“这样，您就不必担心用户是否有不同的字体，例如。 MyPanel只是绘制几何形状，所以你是首选大小的老板。正如here所述，演示可能会使用setPreferredSize()以方便使用，但您应该了解这样做的局限性。

import java.awt.*; import javax.swing.JFrame; import javax.swing.JPanel; /** * @see https://stackoverflow.com/q/11376272/230513 */ public class Disegno extends JFrame { public static void main(String[] args) { EventQueue.invokeLater(new Runnable() { @Override public void run() { new Disegno(); } }); } Disegno() { this.setSize(500, 500); this.setDefaultCloseOperation(EXIT_ON_CLOSE); MyPanel aba = new MyPanel(); this.add(aba); this.pack(); this.setLocationRelativeTo(null); this.setVisible(true); } class MyPanel extends JPanel { private JPanel up, down; private RectArea rect; MyPanel() { super(new BorderLayout()); up = new JPanel(); up.setBackground(Color.red); this.add(up, BorderLayout.NORTH); rect = new RectArea(); this.add(rect, BorderLayout.CENTER); down = new JPanel(); down.setBackground(Color.green); this.add(down, BorderLayout.SOUTH); } @Override public Dimension getPreferredSize() { return new Dimension(320, 240); } } class RectArea extends JPanel { private Rectangle blue = new Rectangle(0, 0, 100, 100); private Rectangle yellow = new Rectangle(0, 0, 100, 100); @Override protected void paintComponent(Graphics g) { super.paintComponent(g); System.out.println(this.getWidth() + " x " + this.getHeight()); g.setColor(Color.blue); g.fillRect(blue.x, blue.y, blue.width, blue.height); g.setColor(Color.yellow); int dx = getWidth() - yellow.width; int dy = getHeight() - yellow.height; g.fillRect(dx, dy, yellow.width, yellow.height); } } }

Answer 2

您从未在任何地方致电rect.initRect();，这就是您在NullPointerException处收到与System.out.println() lines相关的错误的原因。为什么在每个班级中使用panelObject.setVisible(true)，首先将其添加到JPanel，然后只需拨打setVisible(...)上的JFrame即可。在这里观看修改后的代码与所述的东西，按预期工作：

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

public class Disegno extends JFrame{

    Disegno(){
        this.setSize(500, 500);
        this.setDefaultCloseOperation(this.EXIT_ON_CLOSE);
        MyPanel aba = new MyPanel();
        this.setContentPane(aba);
        this.setVisible(true);

        System.out.println(aba.rect.blue.x + "-" + aba.rect.blue.y);
        System.out.println(aba.rect.yellow.x + "-" + aba.rect.yellow.y);
    }

    public static void main(String[] args){
        new Disegno();
    }
}

class MyPanel extends JPanel{

    JPanel up, down;
    RectArea rect;

    MyPanel(){
        this.setLayout(new BorderLayout());

        up = new JPanel();
        this.add(up, BorderLayout.NORTH);
        up.setOpaque(true);
        up.setBackground(Color.red);

        down = new JPanel();
        down.setOpaque(true);
        down.setBackground(Color.green);
        this.add(down, BorderLayout.SOUTH);

        rect = new RectArea();
        rect.initRect();
        this.add(rect, BorderLayout.CENTER);
    }
}

class RectArea extends JPanel{

    Rectangle blue, yellow;
    boolean check = false;

    RectArea(){
        super();
        setOpaque(true);
    }

    public void initRect(){
        blue = new Rectangle(0, 0, 100, 100);
        yellow = new Rectangle(this.getWidth(), this.getHeight(), 100, 100);
        System.out.println("ok");
    }

    @Override
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        if(check == false){
            this.initRect();
            check = true;
        }

        System.out.println(this.getWidth() + "-" + this.getHeight());
        g.setColor(Color.blue);
        g.fillRect(blue.x, blue.y, blue.width, blue.height);
        g.setColor(Color.yellow);
        g.fillRect(yellow.x - yellow.width, yellow.y - yellow.height, yellow.width, yellow.height);
    }
}

如果您要在System.out.println()内写一个intiRect()，那么在paintComponent(...)方法本身之前会调用给出错误的行。因此，在我看来，您必须从paintComponent(...)方法中取出该逻辑并将其保留在其他位置，否则如果您不需要它们，则删除这些线。

Answer 3

您正在尝试访问aba.rect.blue构造函数中的aba.rect.yellow和Disegno，但是在调用RectArea.paintComponent之前不会对其进行初始化，因此会抛出NPE。

Answer 4

你确定你的代码给出了NullPointerException ....... ??

因为当我运行你的代码时，它工作得很好......

<强>输出：

ok
484-442

paintComponent中的初始化不起作用

4 个答案: