Android - 天文钟启动和停止

Question

我想创建一个天文台，我可以启动和停止一个按钮。用我的代码下面我只是得到一个空指针异常。我怎样才能避免它，或无论如何设置两个功能的按钮？

public class MainActivity extends ActionBarActivity { 
    Chronometer focus; 
    Button start, stop, reset; 
    long timeElapsed = SystemClock.elapsedRealtime() - focus.getBase(); 
    int x; 


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

    timeElapsed = x; 
    start = (Button) findViewById(R.id.button1);  
    focus = (Chronometer) findViewById(R.id.chronometer1); 

    start.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      // TODO Auto-generated method stub 
      if(x == 0){ 
      focus.start(); 
      } 

      if(x != 0){ 
      focus.stop();  
      } 
     } 
    }); 

} 

} 

Answer 1

您在初始化之前使用变量。 'focus'和'x'在它们被初始化之前被使用。请使用此代码来避免空指针异常：

 public class MainActivity extends ActionBarActivity { 
    Chronometer focus; 
    Button start, stop, reset; 
    long timeElapsed = 0; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    start = (Button) findViewById(R.id.button1);  
    focus = (Chronometer) findViewById(R.id.chronometer1); 
    // to get the elapsed time use setonChronometerTickListener 
    focus.setOnChronometerTickListener(new OnChronometerTickListener() { 
     @Override 
     public void onChronometerTick(Chronometer chronometer) { 
      timeElapsed = SystemClock.elapsedRealtime() - focus.getBase(); 
     } 
    }); 
    start.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      // TODO Auto-generated method stub 
      if(timeElapsed == 0){ 
      focus.setBase(SystemClock.elapsedRealtime()); 
      focus.start(); 
      } 

      if(timeElapsed != 0){ 
     timeElapsed = 0; 
      focus.stop();  
      } 
     } 
    }); 

} 

Answer 2

与您的代码的问题是这一行：

long timeElapsed = SystemClock.elapsedRealtime() - focus.getBase(); 

由于焦点变量尚未初始化你空指针。 你应该在初始化之后调用getBase（）。

此外，您应该使用ToggleButton而不是按钮为这个应用程序。