为什么paint（Graphics g）没有绘制图像？ [英] Why is paint(Graphics g) not drawing image?

问题描述

以下是我将图像绘制到屏幕的代码：

Here is my code to draw image to screen:

public class Main extends Applet implements Runnable, KeyListener{


private URL base;
private Image image, image1,image2, image3, background;
private Graphics second;
String imageString;

@Override
public void init() {

    // Setting Size of screen
    setSize(800, 600);

    setBackground(Color.BLACK);
    // Set game focus
    setFocusable(true);

    addKeyListener(this);

    // Import frames
    Frame frame = (Frame) this.getParent().getParent();
    frame.setTitle("show image");

    try {
        base = getDocumentBase();
    } catch (Exception e) {
        // TODO: handle exception
    }

    // Image Setups

    // Background
    background = getImage(base, "data/background.png");
    //Images
    image1 = getImage(base, "data/image1.png");
    image2 = getImage(base, "data/image2.png");
    image3 = getImage(base, "data/image3.png");


}

@Override
public void start() {


    Thread thread = new Thread(this);
    thread.start();
}

@Override
public void run() {
    // TODO Auto-generated method stub
    imageString = randomString();
    repaint();

    try {
        Thread.sleep(17);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

@Override
public void update(Graphics g) {
    if (image == null) {
        image = createImage(this.getWidth(), this.getHeight());
        second = image.getGraphics();
    }

    second.setColor(getBackground());
    second.fillRect(0, 0, getWidth(), getHeight());
    second.setColor(getForeground());
    paint(second);
    paint(g);

    g.drawImage(image, 0, 0, this);


}

public void paint(Graphics g) {

        g.drawImage(background, 0, 0, this);
        if (imageString == "color of green"){
            g.drawImage(image1, 0, 0, this);
        } else if (imageString == "red of color"){
            g.drawImage(image2, 0, 0, this);
        } else if (imageString == null){
            g.drawImage(image3, 0, 0, this);
        }
}



@Override
public void keyReleased(KeyEvent arg0) {
    // TODO Auto-generated method stub

}

@Override
public void keyTyped(KeyEvent arg0) {
    // TODO Auto-generated method stub

}

@Override
public void keyPressed(KeyEvent e) {

    }

public String randomString(){
    int randNum;
    String randString = null;
    String randString1 = null;

    randNum = (int)(Math.random()*2 + 1);
    if (randNum == 1){
        randString = "color";
        randString1 = "green";
    } else if (randNum == 2){
        randString = "red";
        randString1 = "color";
    }

    return randString + " of " + randString1;


}

}

没有任何内容屏幕，为什么？我相信它不会识别randString +的randString +因为如果我让它只返回randString，并在我的paint（图形g）中设置我的if语句，它可以工作，但是这段代码不...为什么？



谢谢！

Nothing is drawn on the screen, why? I believe its not recognizing "randString + " of " randString1" because if I made it return randString only, and set my "if statements" in my paint (Graphics g), it works, but this code does not... why?

Thanks!

推荐答案

首先这是错误的方法：它使'红色'

First this is around the wrong way: it makes 'red of color'

randString = "red";
        randString1 = "color";

其次，使用==比较字符串是不正确的。 ==仅当对象相同时才为真。要测试字符串的'值'是否相等，你需要.Equals（）。



我认为更好的方法是改变布尔颜色，整数或枚举颜色字符串。同样在randomString中，不需要有2个字符串变量，只需要一个。

Secondly, using == to compare strings is incorrect. == will be true only if the objects are identical. To test if the 'value' of Strings are equal you need .Equals().

I think a better approach is to make the colors Booleans, ints or enums instead of Strings. Also in randomString there's no need to have 2 string variables, you just need one.

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