java - 不幸的是，GeoQuiz 已停止在 bignerdranch 工作

Question

当我尝试在模拟器上运行我的应用程序时，我收到一条选项卡消息

"unfortunately,GeoQuiz has stopped working" .......

GeoQuiz 是我的应用程序的名称......这是 QuizActivity.java 文件中的我的 Activity 类

public class QuizActivity extends ActionBarActivity {

private Button mTrueButton;

private Button mFalseButton;


@Override
protected void onCreate(Bundle savedInstanceState) {

    mTrueButton=(Button) findViewById(R.id.true_button);
    mTrueButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(QuizActivity.this,R.string.correct_toast,Toast.LENGTH_SHORT).show();
        }
    });

    mFalseButton=(Button) findViewById(R.id.false_button);
    mFalseButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(QuizActivity.this, R.string.incorrect_toast, Toast.LENGTH_SHORT).show();
        }
    });
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_quiz);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_quiz, menu);
    return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

我的应用程序中根本没有图像这个应用程序的最小 sdk 版本是 API 16:android 4.1 (jelly bean)目标版本为API 21:android 5.0 (Lollipop)

Answer 1

您没有在正确的位置调用setContentView:您可以在 onCreate 方法的末尾调用它!您必须将这两行移至 onCreate 方法的开头:

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

因此，您会在此处收到 NullPointerException:

 mTrueButton=(Button) findViewById(R.id.true_button);
 mTrueButton.setOnClickListener(new View.OnClickListener() { // NPE