Java + Android -> 开始新活动时出现空指针异常

Question

当我从游戏活动返回到主要活动时，我得到了 NPE。在一个甚至没有被调用的函数中。 （当我试图在该行之前在控制台中写入时，它什么也不做）

Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.semanticer.unstable.presentation.GameView.showGameBoard(com.example.semanticer.unstable.domain.model.GameBoard)' on a null object reference
                  at com.example.semanticer.unstable.presentation.GamePresenter.lambda$onCreate$0(GamePresenter.java:23)

这是我想回到主要活动的地方 - 是的，我也尝试过 finish()。

@Override
public void playAgain(View view) {
    Intent intent = new Intent(this, MainActivity.class);
    startActivity(intent);
}

这是我出错的那一行

protected void onCreate(Bundle savedState) {
    super.onCreate(savedState);
    game = GameImpl.createNew(6, 4);
    **view().subscribe(view -> view.showGameBoard(game.getBoard()));**
    view().subscribe(view -> view.showCurrentPlayer(Player.FIRST_PLAYER));
    view().subscribe(view -> view.showScore(Player.FIRST_PLAYER, Player.SECOND_PLAYER, game));
    view().subscribe(view -> view.hideWinnerText());
}

整个项目在 github -> https://github.com/zdenduk/AndroidUnstableAtoms

你可以在这里找到布局 -

/zdenduk/AndroidUnstableAtoms/tree/master/app/src/main/res/layout

这里有源代码->

/zdenduk/AndroidUnstableAtoms/tree/master/app/src/main/java/com/example/semanticer/unstable

感谢您的努力:)

Answer 1

com.example.semanticer.unstable.presentation.GameView.showGameBoard(com.example.semanticer.unstable.domain.model.GameBoard)' on a null object reference

表示 showGameBoard 被抛出一个空对象引用，即名为“view”的引用似乎为空。

您可以在此处找到继承的 view 变量： https://github.com/konmik/nucleus/blob/249ab08547cb2b10ddce268dfb0b27b5013623c2/nucleus/src/main/java/nucleus/presenter/Presenter.java

所以这可能对你有帮助：

 /**
     * Returns a current view attached to the presenter or null.
     *
     * View is normally available between
     * {@link Activity#onResume()} and {@link Activity#onPause()},
     * {@link Fragment#onResume()} and {@link Fragment#onPause()},
     * {@link android.view.View#onAttachedToWindow()} and {@link android.view.View#onDetachedFromWindow()}.
     *
     * Calls outside of these ranges will return null.
     * Notice here that {@link Activity#onActivityResult(int, int, Intent)} is called *before* {@link Activity#onResume()}
     * so you can't use this method as a callback.
     *
     * @return a current attached view.
     */
    @Nullable
    public View getView() {
        return view;
}

Presenter.java 是基类。

Answer 2

像这样的 NullPointerException 通常是未定义变量的结果。

如果我不得不从你的代码中猜测（看起来我需要更多的代码来准确地提供帮助）我会说

game = GameImpl.createNew(6, 4);

需要定义或未正确定义。尝试先看那里然后从那里去。祝你好运！ :)