java - setBackground 不适用于第一次但第二次，为什么？

Question

我在一本书中看到过这个例子，工作正常，但出现的唯一问题是背景在第一次调用 Paint 时没有变黑，当 clearCounter 变为 == 5 时，然后屏幕被清除并再次绘画开始时，背景变为黑色。

public class apletprg extends JApplet implements ActionListener 
    {
int clearCounter;
    Timer t;
public void init(){

    setBackground(Color.black);
    clearCounter = 0;
    Timer t = new Timer(1000, this);
    t.start();

}

public void paint(Graphics g)
{
    setBackground(Color.black);
    clearCounter++;
    Graphics2D g2 = (Graphics2D) g;

    if (clearCounter == 5){
        g2.clearRect(0, 0, 500, 400);
        clearCounter=0;
    }

    for (int i = 1; i <= 40; i++) {
        Color c = chooseColor();
        g2.setColor(c);
        Font f = chooseFont();
        g2.setFont(f);
        drawJava(g2);
        }
}
public void actionPerformed(ActionEvent ae){
repaint();
}
public Color chooseColor(){
int r= (int) (Math.random() * 255);
int g= (int) (Math.random() * 255);
int b= (int) (Math.random() * 255);
Color c = new Color(r,g,b);
return c;
}
public Font chooseFont(){

    int fontChoice = (int) (Math.random() * 4) + 1;
    Font f = null;
    switch (fontChoice) {
    case 1: f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);break;
    case 2: f = new Font("SansSerif", Font.PLAIN, 17);break;
    case 3: f = new Font("Monospaced", Font.ITALIC, 23);break;
    case 4: f = new Font("Dialog", Font.ITALIC, 30);break;
    }
    return f;

}
public void drawJava(Graphics2D g2){
    int x = (int) (Math.random() * 500);
    int y = (int) (Math.random() * 400);
    g2.drawString("Adnan", x, y);   
}
}

我知道 Init () 在开始时只被调用一次，但为什么不能在开始时改变背景？

Answer 1

在您的初始化方法中，将 setBackground(Color.black) 替换为 getContentPane().setBackground(Color.black)

并添加 super.paint(g) 作为您的 paint 方法的第一行。

否则，如果您不想使用 Swing 功能，那么继续导入 java.applet.Applet 并让您类扩展 Applet 而不是 JApplet

public class NewClass extends JApplet implements ActionListener {

    int clearCounter;
    Timer t;



    public void init() {

        getContentPane().setBackground(Color.black);
        repaint();
        clearCounter = 0;
        //t = new Timer("1000", true);


    }

    public void paint(Graphics g) {
        super.paint(g);
        setBackground(Color.black);
        clearCounter++;
        Graphics2D g2 = (Graphics2D) g;

        if (clearCounter == 5) {
            g2.clearRect(0, 0, 500, 400);
            clearCounter = 0;
        }

        for (int i = 1; i <= 40; i++) {
            Color c = chooseColor();
            g2.setColor(c);
            Font f = chooseFont();
            g2.setFont(f);
            drawJava(g2);
        }
    }
    @Override
    public void actionPerformed(ActionEvent ae) {
        repaint();
    }

    public Color chooseColor() {
        int r = (int) (Math.random() * 255);
        int g = (int) (Math.random() * 255);
        int b = (int) (Math.random() * 255);
        Color c = new Color(r, g, b);
        return c;
    }

    public Font chooseFont() {

        int fontChoice = (int) (Math.random() * 4) + 1;
        Font f = null;
        switch (fontChoice) {
        case 1:
            f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);
            break;
        case 2:
            f = new Font("SansSerif", Font.PLAIN, 17);
            break;
        case 3:
            f = new Font("Monospaced", Font.ITALIC, 23);
            break;
        case 4:
            f = new Font("Dialog", Font.ITALIC, 30);
            break;
        }
        return f;

    }

    public void drawJava(Graphics2D g2) {
        int x = (int) (Math.random() * 500);
        int y = (int) (Math.random() * 400);
        g2.drawString("Adnan", x, y);
    }
}

如果你想只执行一次 super.paint() 在你的类中添加一个 boolean 变量

boolean firstTime = true;

在 paint() 中

if(firstTime) {
    super.paint(g);
    firstTime = false;
}