在Java中開發游戲時線程“ main”中的異常java.lang.NullPointerException

Question

我正在嘗試為游戲制作字體，但是每當我加載它時，都會出現此錯誤：

Exception in thread "main" java.lang.NullPointerException
at image.ImageFont.load(ImageFont.java:78)
at image.ImageFont.<init>(ImageFont.java:47)
at image.ImageFontTest.init(ImageFontTest.java:26)
at image.GameCore.run(GameCore.java:65)
at image.ImageFontTest.main(ImageFontTest.java:11)

我也有文件：

ImageFontTest

public class ImageFontTest extends GameCore {

public static void main(String[] args) {
    new ImageFontTest().run();
}

private static final long TOTAL_TIME = 6500;

private ImageFont bigFont;
private ImageFont medFont;
private long remainingTime;
private CharMovement[] charMovement;

public void init() {
    super.init();

    remainingTime = TOTAL_TIME;
    // load image fonts
    bigFont = new ImageFont("fonts/big");
    medFont = new ImageFont("fonts/medium");

    String message = "Good Times!";
    int stringWidth = medFont.stringWidth(message);
    charMovement = new CharMovement[message.length()];
    for (int i=0; i<message.length(); i++) {
        charMovement[i] = new CharMovement(message, i,
            (screen.getWidth() - stringWidth) / 2,
            screen.getHeight() / 2);
    }
}

public void update(long elapsedTime) {
    remainingTime -= elapsedTime;
    if (remainingTime <= 0) {
        stop();
    }
}

public void draw(Graphics2D g) {
    // erase background
    g.setColor(Color.BLACK);
    g.fillRect(0,0,screen.getWidth(), screen.getHeight());

    // draw some aligned text
    medFont.drawString(g, "Left", 0, 0,
        ImageFont.LEFT | ImageFont.TOP);
    medFont.drawString(g, "Center", screen.getWidth()/2, 0,
        ImageFont.HCENTER | ImageFont.TOP);
    medFont.drawString(g, "Right", screen.getWidth(), 0,
        ImageFont.RIGHT | ImageFont.TOP);

    // draw seconds remaining
    String timeLeft = "" + (remainingTime / 1000);
    bigFont.drawString(g, timeLeft, 0, screen.getHeight());

    // draw moving characters
    double p = (double)(TOTAL_TIME - remainingTime) / TOTAL_TIME;
    for (int i=0; i<charMovement.length; i++) {
        charMovement[i].draw(g, p);
    }
}

/**
    Simple animation class to animate a character along a
    path.
*/
public class CharMovement {
    char ch;
    Point[] path;

    public CharMovement(String s, int charIndex, int x, int y) {
        int stringWidth = medFont.stringWidth(s);
        for (int i=0; i<charIndex; i++) {
            x+=medFont.charWidth(s.charAt(i));
        }
        ch = s.charAt(charIndex);

        path = new Point[4];

        // start offscreen
        path[0] = new Point(x-2000, y);

        // move to the center of the screen and pause there
        path[1] = new Point(x, y);
        path[2] = path[1];

        // "explode" at a random angle far away
        double angle = Math.random() * 2 * Math.PI;
        double distance = 1000 + 1000*Math.random();
        path[3] = new Point(
            (int)Math.round(x + Math.cos(angle) * distance),
            (int)Math.round(y + Math.sin(angle) * distance));
    }

    /**
        Draws this character in the path, where p is the
        location in the path from 0 to 1.
    */
    public void draw(Graphics g, double p) {
        int points = path.length - 1;
        int index = (int)(p*points);
        p = p * points - index;
        Point start = path[index % path.length];
        Point goal = path[(index + 1) % path.length];
        int x = (int)Math.round(goal.x * p + start.x * (1-p));
        int y = (int)Math.round(goal.y * p + start.y * (1-p));
        medFont.drawChar(g, ch, x, y);
    }
}
}

圖像字體

/**
The ImageFont class allows loading and drawing of text
using images for the characters.

Reads all the png images in a directory in the form
"charXX.png" where "XX" is a decimal unicode value.

Characters can have different widths and heights.
*/
 public class ImageFont {

public static final int HCENTER = 1;
public static final int VCENTER = 2;
public static final int LEFT = 4;
public static final int RIGHT = 8;
public static final int TOP = 16;
public static final int BOTTOM = 32;

private char firstChar;
private Image[] characters;
private Image invalidCharacter;


/**
    Creates a new ImageFont with no characters.
*/
public ImageFont() {
    this(null);
    firstChar = 0;
    characters = new Image[0];
}


/**
    Creates a new ImageFont and loads character images from
    the specified path.
*/
public ImageFont(String path) {
    if (path != null) {
        load(path);
    }

    // make the character used for invalid characters
    invalidCharacter =
        new BufferedImage(10, 10, BufferedImage.TYPE_INT_RGB);
    Graphics g = invalidCharacter.getGraphics();
    g.setColor(Color.RED);
    g.fillRect(0,0,10,10);
    g.dispose();
}


/**
    Loads the image files for each character from the
    specified path. For example, if "../fonts/large"
    is the path, this method searches for all the images
    names "charXX.png" in that path, where "XX" is a
    decimal unicode value. Not every character image needs
    to exist; you can only do numbers or uppercase letters,
    for example.
*/
public void load(String path) throws NumberFormatException {
    // in this directory:
    // load every png file that starts with 'char'
    File dir = new File(path);
    File[] files = dir.listFiles();

    // find min and max chars
    char minChar = Character.MAX_VALUE;
    char maxChar = Character.MIN_VALUE;
    for (int i=0; i<files.length; i++) {
        int unicodeValue = getUnicodeValue(files[i]);
        if (unicodeValue != -1) {
            minChar = (char)Math.min(minChar, unicodeValue);
            maxChar = (char)Math.max(maxChar, unicodeValue);
        }
    }

    // load the images
    if (minChar < maxChar) {
        firstChar = minChar;
        characters = new Image[maxChar - minChar + 1];
        for (int i=0; i<files.length; i++) {
            int unicodeValue = getUnicodeValue(files[i]);
            if (unicodeValue != -1) {
                int index = unicodeValue - firstChar;
                characters[index] = new ImageIcon(
                    files[i].getAbsolutePath()).getImage();
            }
        }

    }

}


private int getUnicodeValue(File file)
    throws NumberFormatException
{
    String name = file.getName().toLowerCase();
    if (name.startsWith("char") && name.endsWith(".png")) {
        String unicodeString =
            name.substring(4, name.length() - 4);
        return Integer.parseInt(unicodeString);
    }
    return -1;
}


/**
    Gets the image for a specific character. If no image for
    the character exists, a special "invalid" character image
    is returned.
*/
public Image getImage(char ch) {
    int index = ch - firstChar;
    if (index < 0 || index >= characters.length ||
        characters[index] == null)
    {
        return invalidCharacter;
    }
    else {
        return characters[index];
    }
}


/**
    Gets the string width, in pixels, for the specified string.
*/
public int stringWidth(String s) {
    int width = 0;
    for (int i=0; i<s.length(); i++) {
        width += charWidth(s.charAt(i));
    }
    return width;
}


/**
    Gets the char width, in pixels, for the specified char.
*/
public int charWidth(char ch) {
    return getImage(ch).getWidth(null);
}


/**
    Gets the char height, in pixels, for the specified char.
*/
public int charHeight(char ch) {
    return getImage(ch).getHeight(null);
}


/**
    Draws the specified string at the (x, y) location.
*/
public void drawString(Graphics g, String s, int x, int y) {
    drawString(g, s, x, y, LEFT | BOTTOM);
}


/**
    Draws the specified string at the (x, y) location.
*/
public void drawString(Graphics g, String s, int x, int y,
    int anchor)
{
    if ((anchor & HCENTER) != 0) {
        x-=stringWidth(s) / 2;
    }
    else if ((anchor & RIGHT) != 0) {
        x-=stringWidth(s);
    }
    // clear horizontal flags for char drawing
    anchor &= ~HCENTER;
    anchor &= ~RIGHT;

    // draw the characters
    for (int i=0; i<s.length(); i++) {
        drawChar(g, s.charAt(i), x, y, anchor);
        x+=charWidth(s.charAt(i));
    }
}


/**
    Draws the specified character at the (x, y) location.
*/
public void drawChar(Graphics g, char ch, int x, int y) {
    drawChar(g, ch, x, y, LEFT | BOTTOM);
}


/**
    Draws the specified character at the (x, y) location.
*/
public void drawChar(Graphics g, char ch, int x, int y,
    int anchor)
{
    if ((anchor & HCENTER) != 0) {
        x-=charWidth(ch) / 2;
    }
    else if ((anchor & RIGHT) != 0) {
        x-=charWidth(ch);
    }

    if ((anchor & VCENTER) != 0) {
        y-=charHeight(ch) / 2;
    }
    else if ((anchor & BOTTOM) != 0) {
        y-=charHeight(ch);
    }
    g.drawImage(getImage(ch), x, y, null);
}
}

我看到它說問題是在ImageFont中加載的，但我不知道如何解決它。

Answer 1

Exception in thread "main" java.lang.NullPointerException
at image.ImageFont.load(ImageFont.java:78)

您有一個NPE（空指針異常）：這意味着您使用了尚未初始化的變量或對象。

例：

Button b;
b.setText("Text in my button"); // NULL POINTER EXCEPTION: b is a button but which button?!?

解

Button b = new Button();
b.setText("Text in my button"); // b now is a brand new button and it can be used.

您先生有這種問題:)