Android基本计算器

Question

我在这里遇到问题。我是Android开发人员的初学者，所以请和我一起裸照。 我正在尝试构建一个基本的计算器。代码很好（我猜）。在运行时我没有错误，eclipse成功编译它。几乎一切都很好，但是当我在EDITTEXT视图中输入号码后，尝试通过点击“计算”做了一些计算按钮的应用程序crashes.Here是崩溃的应用程序的代码：

private OnClickListener calcListener = new OnClickListener() { 

     @Override 
     public void onClick(View v) { 

      // TODO Auto-generated method stub 
      String f = first.getText().toString(); 
      String s = second.getText().toString(); 
      float fValue = Float.valueOf(f); 
      float sValue = Float.valueOf(s); 
      if((f.length() == 0) || (s.length() == 0)) 
       Toast.makeText(MainActivity.this, "Please enter data in the first section", Toast.LENGTH_LONG).show(); 
      else{ 
       switch(sign.getCheckedRadioButtonId()) 
       { 
       case R.id.mult: 
        float res_Mult = fValue*sValue; 
        result.setText(String.valueOf(res_Mult)); 
       case R.id.plus: 
        float res_Add = fValue*sValue; 
        result.setText(String.valueOf(res_Add)); 
       case R.id.minus: 
        float res_Minus = fValue*sValue; 
        result.setText(String.valueOf(res_Minus)); 
       case R.id.div : 
        float res_Div = fValue*sValue; 
        result.setText(String.valueOf(res_Div)); 
       }// end switch 
      }// end else 
     } 
    };// end onclik 

    Listener 

The problem seems to be the calculation since I tried to output "Hello" on the textview that is supposed to display the results of the calculations once the button "calculate" is clicked and it worked. 


Button calc = null; 
    EditText first = null; 
    EditText second = null; 
    RadioGroup sign = null; 
    TextView result = null; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     //binding 
     calc = (Button) findViewById(R.id.calc);// the "calculate" button 
     first = (EditText) findViewById(R.id.first); //the first EditText 
     second = (EditText) findViewById(R.id.second);//the second EditText 
     result = (TextView) findViewById(R.id.result);// The textView that this supposed to display the result. 

     //setting listeners 
     calc.setOnClickListener(calcListener); 
     first.addTextChangedListener(watcher); 
     second.addTextChangedListener(watcher); 
    } 

    private TextWatcher watcher = new TextWatcher() { 

     @Override 
     public void onTextChanged(CharSequence s, int start, int before, int count) { 
      // TODO Auto-generated method stub 
      result.setText(" "); 
     } 

     @Override 
     public void beforeTextChanged(CharSequence s, int start, int count, 
       int after) { 
      // TODO Auto-generated method stub 

     } 

     @Override 
     public void afterTextChanged(Editable s) { 
      // TODO Auto-generated method stub 

     } 
    };// end textWatcher 


Here is the logCat: 
    08-29 19:18:29.323: D/dalvikvm(1951): Not late-enabling CheckJNI (already on) 
    08-29 19:18:31.593: D/dalvikvm(1951): GC_FOR_ALLOC freed 72K, 10% free 2892K/3192K, paused 108ms, total 111ms 
    08-29 19:18:32.183: D/libEGL(1951): loaded /system/lib/egl/libEGL_emulation.so 
    08-29 19:18:32.213: D/(1951): HostConnection::get() New Host Connection established 0xb8cfd430, tid 1951 
    08-29 19:18:32.223: D/libEGL(1951): loaded /system/lib/egl/libGLESv1_CM_emulation.so 
    08-29 19:18:32.233: D/libEGL(1951): loaded /system/lib/egl/libGLESv2_emulation.so 
    08-29 19:18:32.313: W/EGL_emulation(1951): eglSurfaceAttrib not implemented 
    08-29 19:18:32.323: D/OpenGLRenderer(1951): Enabling debug mode 0 
    08-29 19:18:50.674: D/AndroidRuntime(1951): Shutting down VM 
    08-29 19:18:50.674: W/dalvikvm(1951): threadid=1: thread exiting with uncaught exception (group=0xb0eb8648) 
    08-29 19:18:50.724: E/AndroidRuntime(1951): FATAL EXCEPTION: main 
    08-29 19:18:50.724: E/AndroidRuntime(1951): java.lang.NullPointerException 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at com.example.gets.calculator.MainActivity$2.onClick(MainActivity.java:76) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at android.view.View.performClick(View.java:4240) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at android.view.View$PerformClick.run(View.java:17721) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at android.os.Handler.handleCallback(Handler.java:730) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at android.os.Handler.dispatchMessage(Handler.java:92) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at android.os.Looper.loop(Looper.java:137) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at android.app.ActivityThread.main(ActivityThread.java:5103) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at java.lang.reflect.Method.invokeNative(Native Method) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at java.lang.reflect.Method.invoke(Method.java:525) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
    08-29 19:18:50.724: E/AndroidRuntime(1951):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

你的switch语句没有休息。这是一个错误，它会导致每个案例落到下一个案例，并在它下面的所有案例中执行每一段代码。

switch(sign.getCheckedRadioButtonId()) { 
     case R.id.mult: 
     float res_Mult = fValue*sValue; 
     result.setText(String.valueOf(res_Mult)); 
     case R.id.plus: 
     float res_Add = fValue*sValue; 
     result.setText(String.valueOf(res_Add)); 
     case R.id.minus: 
     float res_Minus = fValue*sValue; 
     result.setText(String.valueOf(res_Minus)); 
     case R.id.div : 
     float res_Div = fValue*sValue; 
     result.setText(String.valueOf(res_Div)); 
    }// end switch