Question

我在整个学期都和LibGDX一起工作，并且很好地掌握了它所引发的错误，但是我无法理解这一点。

我正在使用ScrollPane类，它接受一个String []来显示，所以我正在使用一种方法从一堆已赢得的“游戏”（Not LibGDX Game）对象中提取字符串并将它们放入String []，但是当我运行这段代码时，它给了我一个包含stage.draw（）的行的NullPointer;

当我只是在数组中放置一个字符串并运行代码时，它工作正常，所以它必须是我的方法，但我不明白为什么它会这样做。我可以在分配之前按索引打印整个String []。

以下是我的代码和错误：

@Override
public void render(float delta) {
    Gdx.gl.glClearColor(0, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

    stage.act(delta);
    stage.draw(); //**this is the line that causes the error**
}

public void show() {

    stage = new Stage();

    Gdx.input.setInputProcessor(stage);

    gameNotes = new String[CampusLocation.countAllNotes(game)]; //this returns String[20]

    skin = new Skin(Gdx.files.internal("ui/menuSkin.json"),
            (new TextureAtlas("ui/atlas.pack")));

    table = new Table(skin);
    table.setFillParent(true);

    // table.debug();

    list = new List<String>(skin);

    if (!populateWonNotes()) {
        list.setItems(new String[] {"No Locations Found"});
        System.out.println("returned false");
    } else {
        list.setItems(gameNotes);
        System.out.println("returned true"); //this is what is printing, so method returns true
    }

    scrollPane = new ScrollPane(list, skin);

    table.clear();
    table.setBounds(0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
    table.add("NOTES").colspan(3).expandX().spaceBottom(50).row();
    table.add().width(table.getWidth() / 3);
    table.add().width(table.getWidth() / 3);
    table.add().width(table.getWidth() / 3).row();
    table.add(scrollPane).uniformX().left().expandY().top().left();
    table.add(games).uniformX();
    table.add(home).uniformX().bottom().right();

    stage.addActor(table);
}

private boolean populateWonNotes() {

    int i = 0;
    if (game.lstGameContainers != null) {
        for (GameContainer gc : game.lstGameContainers) {
            if (gc.isWon) { //checks if the game has been won
                Array<String> gcNotes = gc.getNotes(); //gets all notes for this game location
                for (String s : gcNotes) {
                    gameNotes[i] = gc.name + ": " + s;
                    System.out.println("gameNotes[" + i + "] is: " + gameNotes[i]);
                    i++;
                }
            } else {
                gameNotes[i] = gc.name + " Notes Locked";
                System.out.println("gameNotes[" + i + "] is: " + gameNotes[i]);
                i++;
            }
        }
        return true;
    } else
        return false;

}

在populateWonNotes（）

中输出print语句

gameNotes[0] is: Trabant Notes Locked
gameNotes[1] is: Perkins Notes Locked
gameNotes[2] is: Writing Center Notes Locked
gameNotes[3] is: UDSIS Notes Locked
gameNotes[4] is: Advising Notes Locked
gameNotes[5] is: Little Bob Notes Locked
gameNotes[6] is: Counseling Center Notes Locked
gameNotes[7] is: Dining Halls Notes Locked
gameNotes[8] is: Bus Stop Notes Locked
gameNotes[9] is: Important Dates Notes Locked

错误：

Exception in thread "LWJGL Application" java.lang.NullPointerException
    at com.badlogic.gdx.scenes.scene2d.ui.List.layout(List.java:112)
    at com.badlogic.gdx.scenes.scene2d.ui.Widget.validate(Widget.java:88)
    at com.badlogic.gdx.scenes.scene2d.ui.List.getPrefWidth(List.java:238)
    at com.badlogic.gdx.scenes.scene2d.ui.ScrollPane.getPrefWidth(ScrollPane.java:610)
    at com.badlogic.gdx.scenes.scene2d.ui.Value$3.get(Value.java:65)
    at com.badlogic.gdx.scenes.scene2d.ui.Table.computeSize(Table.java:793)
    at com.badlogic.gdx.scenes.scene2d.ui.Table.layout(Table.java:925)
    at com.badlogic.gdx.scenes.scene2d.ui.Table.layout(Table.java:719)
    at com.badlogic.gdx.scenes.scene2d.ui.WidgetGroup.validate(WidgetGroup.java:106)
    at com.badlogic.gdx.scenes.scene2d.ui.Table.draw(Table.java:100)
    at com.badlogic.gdx.scenes.scene2d.Group.drawChildren(Group.java:112)
    at com.badlogic.gdx.scenes.scene2d.Group.draw(Group.java:58)
    at com.badlogic.gdx.scenes.scene2d.Stage.draw(Stage.java:127)
    at edu.udel.patc.screen.NoteListScreen.render(NoteListScreen.java:51)
    at com.badlogic.gdx.Game.render(Game.java:46)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:206)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:114)

感谢您的帮助！

Stage.draw LibGDX上的空指针异常

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