简单的井字游戏

Question

我创建了一个简单的井字游戏。 gradle 构建没有错误，并且生成了 apk。

但是在我的 phone 中安装后它显示如下...

java.lang.RuntimeException:无法启动activity

ComponentInfo{com.codec.dnv.tictactoe/com.codec.dnv.tictactoe.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2814) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1613) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:173) at android.app.ActivityThread.main(ActivityThread.java:6634) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:822) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at com.codec.dnv.tictactoe.MainActivity.onCreate(MainActivity.java:42) at android.app.Activity.performCreate(Activity.java:7074) at android.app.Activity.performCreate(Activity.java:7065) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1215) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2767)>

如何克服这个问题？

Answer 1

您在此处为您的 OnClickListener 设置了空值

android.widget.Button.setOnClickListener(android.view.View$OnClickListener)

我会尝试确定它为什么为空，也许它从未被初始化。我们需要查看完整的代码块才能进一步帮助您。

Answer 2

public class MainActivity 扩展 AppCompatActivity 实现 View.OnClickListener {

private Button[][] buttons = new Button[3][3];

private boolean player1Turn = true;

private int roundCount;

private int player1Points;
private int player2Points;

private TextView player1;
private TextView player2;



@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    player1 = findViewById(R.id.player1);
    player2 = findViewById(R.id.player2);


    for (int x = 0; x < 3; x++) {
        for (int y = 0; y < 3; y++) {
            String buttonID = "button_" + x + y;
            int resID = getResources().getIdentifier(buttonID, "id", getPackageName());
            buttons[x][y] = findViewById(resID);
            buttons[x][y].setOnClickListener(this);
        }
    }

    Button buttonReset = findViewById(R.id.button_reset);
    buttonReset.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            resetGame();
        }
    });
}

@Override
public void onClick(View v) {
    if (!((Button) v).getText().toString().equals("")) {
        return;
    }

    if (player1Turn) {
        ((Button) v).setText("X");
    } else {
        ((Button) v).setText("Y");
    }

    roundCount++;

    if (checkWin()) {
        if (player1Turn) {
            player1Wins();
        } else {
            player2Wins();
        }
    } else if (roundCount == 9) {
        draw();
    } else {
        player1Turn = !player1Turn;
    }

}

private boolean checkWin() {
    String[][] field = new String[3][3];

    for (int x = 0; x < 3; x++) {
        for (int y = 0; y < 3; y++) {
            field[x][y] = buttons[x][y].getText().toString();
        }
    }

    for (int x = 0; x < 3; x++) {
        if (field[x][0].equals(field[x][1])
                && field[x][0].equals(field[x][2])
                && !field[x][0].equals("")) {
            return true;
        }
    }

    for (int x = 0; x < 3; x++) {

        if (field[0][x].equals(field[1][x])
                && field[0][x].equals(field[2][x])
                && !field[0][x].equals("")) {
            return true;
        }
    }


    if (field[0][0].equals(field[1][1])
            && field[0][0].equals(field[2][2])
            && !field[0][0].equals("")) {
        return true;
    }

    return field[0][2].equals(field[1][1])
            && field[0][2].equals(field[2][0])
            && !field[0][2].equals("");

}


private void player1Wins() {
    player1Points++;
    Toast.makeText(this, "Player 1 wins!", Toast.LENGTH_SHORT).show();
    updatePointsText();
    resetBoard();
}

private void player2Wins() {
    player2Points++;
    Toast.makeText(this, "Player 2 wins!", Toast.LENGTH_SHORT).show();
    updatePointsText();
    resetBoard();
}

private void draw() {
    Toast.makeText(this, "Draw!", Toast.LENGTH_SHORT).show();
    resetBoard();
}


private void updatePointsText() {

    player1.setText(getString(R.string.player_1_0, player1Points));
    player2.setText(getString(R.string.player_2_0, player2Points));
}

private void resetBoard() {
    for (int x = 0; x < 3; x++) {
        for (int y = 0; y < 3; y++) {
            buttons[x][y].setText("");
        }
    }

    roundCount = 0;
    player1Turn = true;
}

private void resetGame() {
    player1Points = 0;
    player2Points = 0;
    updatePointsText();
    resetBoard();
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);

    outState.putInt("roundCount", roundCount);
    outState.putInt("player1Points", player1Points);
    outState.putInt("player2Points", player2Points);
    outState.putBoolean("player1Turn", player1Turn);
}

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);

    roundCount = savedInstanceState.getInt("roundCount");
    player1Points = savedInstanceState.getInt("player1Points");
    player2Points = savedInstanceState.getInt("player2Points");
    player1Turn = savedInstanceState.getBoolean("player1Turn");
}

}

代码在这里...

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