java - 计时码表为空，即使父类已将其初始化

我遇到了这个奇怪的问题，这困扰着我。我不明白为什么在MainActivity中初始化计时器时会收到此NullPointerException异常。如果我在SecondActivity中初始化计时器，则该程序可以正常运行。

希望有人可以解决这个问题...

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Chronometer.setOnChronometerTickListener(android.widget.Chronometer$OnChronometerTickListener)' on a null object reference

--

public class MainActivity extends AppCompatActivity {

    protected Chronometer chronometer;

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

        //chronometer gets initialised here but it's still null appearantly
        chronometer = (Chronometer) findViewById(R.id.chronometer1);

        //this works
        getSupportActionBar().setBackgroundDrawable(new ColorDrawable(Color.parseColor("#cd2626")));
    }
}

--

public class SecondActivity extends MainActivity {

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

         //if chronometer gets initialised again, program does work.
         //chronometer = (Chronometer) findViewById(R.id.chronometer1)
     }
}

最佳答案

包括setContentView（R.layout.main_layout）;在您的mainactivity中，否则findviewbyid将无法找到计时器，并且将始终显示null异常