Question

当一个对象在创建之前使用类静态实例时，我遇到了这个问题。例如：

class Chair {
    public static Chair mychair = new Chair();
    public void foo() {}
}

class Table {
    public Table() {
        Chair.mychair.foo();
    }
}

所以如果我调用mychair.foo（），我会得到一个NullPointerException。我知道，在真正需要课程之前，甚至不会执行静态操作。但是我如何强制Java实际加载类，因此它将被创建？

原始程序的Stacktrace：

Exception in thread "main" java.lang.ExceptionInInitializerError
    at hu.intelligames.game.level.Level.<init>(Level.java:34)
    at hu.intelligames.game.Game.initialize(Game.java:64)
    at hu.intelligames.game.Game.<init>(Game.java:43)
    at hu.intelligames.game.Game.main(Game.java:80)
Caused by: java.lang.NullPointerException
    at hu.intelligames.game.level.tiles.Tile.<clinit>(Tile.java:25)
    ... 4 more

Level类的构造函数（来自第25行）：

public Level(int width, int height) {
        this.width = width;
        this.height = height;

        tiles = new Tile[width * height];

        for (int i = 0; i < tiles.length; i++) {
            if (i % 3 == 0)
                tiles[i] = Tile.GRASS;
            else if (i % 3 == 1)
                tiles[i] = Tile.GRASS;
            else
                tiles[i] = Tile.STONE;
        }

        init();
    }

Tile类的整个代码：

public class Tile {

    public static Spritesheet tileSheet = new Spritesheet("/tiles.png");

    public static final Tile VOID = new Tile(0, 0, tileSheet, 0x00000000);
    public static final Tile GRASS = new Tile(1, 0, tileSheet, 0xff00ff00);
    public static final Tile STONE = new Tile(2, 0, tileSheet, 0xffaaaaaa);
    public static final Tile ROAD = new Tile(3, 0, tileSheet, 0xffabcdef);

    private Spritesheet sheet;
    private int x;
    private int y;
    private int colorCode;

    private static ArrayList<Tile> tileList;

    static {
        tileList.add(VOID);
        tileList.add(GRASS);
        tileList.add(STONE);
        tileList.add(ROAD);
    }

    public Tile(int x, int y, Spritesheet sheet, int colorCode) {
        this.sheet = sheet;
        this.x = x;
        this.y = y;
        this.colorCode = colorCode;
    }

    public void render(int xPos, int yPos, int xOff, int yOff, Screen screen) {
        screen.render(xPos, yPos, xOff, yOff, x, y, sheet);
    }

    public int getColorCode() {
        return colorCode;
    }

    public static Tile getTileByColorCode(int code) {
        for (Tile t : tileList) {
            if (t.getColorCode() == code) return t;
            break;
        }

        return VOID;
    }

}

Answer 1

private static ArrayList<Tile> tileList;

保持无效

static {
    tileList = new ArrayList<>();
    tileList.add(VOID);
    tileList.add(GRASS);
    tileList.add(STONE);
    tileList.add(ROAD);
}

Answer 2

您的字段tileList未初始化，因此静态初始化代码会抛出NPE。

这应该可以解决问题：

private static ArrayList<Tile> tileList = new ArrayList<>();

Java在静态之前执行实例命令

2 个答案: