java - Chronometer 为空，即使父类初始化了它

Question

我遇到了这个奇怪的问题，它困扰着我。我不明白为什么我在 MainActivity 中初始化计时器时会得到这个 NullPointerException。如果我在 SecondActivity 中初始化 chronometer，程序运行良好。

希望有人能解决这个问题......

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)
     }
}

Answer 1

包括setContentView(R.layout.main_layout);在您的主要 Activity 中，否则 findviewbyid 将无法找到计时表，并且它将始终显示空异常