Touch Calculator in Android

Question

Ich versuche, ein Taschenrechner-Design zu erstellen.Aber bekomme ich keine Kompilierzeit errors.Finally beim Ausführen des Projekts/Code eine nullpointer Ausnahme Fehler aufgetreten.

MainActivity.java:

public class MainActivity extends Activity { 
GridView mKeypadGrid; 
KeyAdapter mKeypadAdapter; 
private TextView userInputText; 
private boolean resetInput; 
private boolean hasFinalResult; 
private String mDecimalSeperator; 
private Stack<String> mInputStack; 
private Stack<String> mOperationStack; 
private double memoryValue; 
private TextView mStackText; 
private TextView memoryStatText; 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

userInputText= (TextView) findViewById(R.id.txtInput); 
mStackText=(TextView)findViewById(R.id.txtStack); 
memoryStatText=(TextView)findViewById(R.id.txtMemory); 
mKeypadGrid = (GridView)findViewById(R.id.grdButtons); 

mKeypadAdapter = new KeyAdapter(this); 


mKeypadGrid.setAdapter(mKeypadAdapter); 
mKeypadAdapter.setOnButtonClickListener(new OnClickListener() { 
    @Override 
    public void onClick(View v) { 
     Button btn = (Button) v; 

     KeypadButton keypadButton = (KeypadButton) btn.getTag(); 


     ProcessKeypadInput(keypadButton); 
     }}); 
mKeypadGrid.setOnItemClickListener(new OnItemClickListener() { 
    public void onItemClick(AdapterView<?> parent, View v,int position, long id) { 

    } 
}); 
} 

public void ProcessKeypadInput(KeypadButton keypadButton) { 

     String text = keypadButton.getText().toString(); 
     String currentInput = userInputText.getText().toString(); 

     int currentInputLen = currentInput.length(); 
     String evalResult = null; 
     double userInputValue = Double.NaN; 

     switch (keypadButton) { 
     case BACKSPACE: 
      if (resetInput) 
       return; 

      int endIndex = currentInputLen - 1; 


      if (endIndex < 1) { 
       userInputText.setText("0"); 
      } 
      else { 
       userInputText.setText(currentInput.subSequence(0, endIndex)); 
      } 
      break; 
     case SIGN: 
      if (currentInputLen > 0 && currentInput != "0") { 

       if (currentInput.charAt(0) == '-') { 
        userInputText.setText(currentInput.subSequence(1, 
          currentInputLen)); 
       } 

       else { 
        userInputText.setText("-" + currentInput.toString()); 
       } 
      } 
      break; 
     case CE: 
      userInputText.setText("0"); 
      break; 
     case C: 
      userInputText.setText("0"); 
      clearStacks(); 
      break; 
     case DECIMAL_SEP: 
      if (hasFinalResult || resetInput) { 
       userInputText.setText("0" + mDecimalSeperator); 
       hasFinalResult = false; 
       resetInput = false; 
      } else if (currentInput.contains(".")) 
       return; 
      else 
       userInputText.append(mDecimalSeperator); 
      break; 
     case DIV: 
     case PLUS: 
     case MINUS: 
      case MULTIPLY: 
      if (resetInput) { 
       mInputStack.pop(); 
       mOperationStack.pop(); 
      } else { 
       if (currentInputLen >0) { 
        if (currentInput.charAt(0) == '-') { 

       } else { 
        if(currentInput!=null) 
        mInputStack.add(currentInput); 
       } 
       mOperationStack.add(currentInput); 
      } 
      } 
      if(text!=null){ 
      mInputStack.add(text); //132nd Line 
      mOperationStack.add(text); 
      } 
      dumpInputStack(); 
      evalResult = evaluateResult(false); 
      if (evalResult != null) 
       userInputText.setText(evalResult); 

      resetInput = true; 
      break; 
     case CALCULATE: 
      if (mOperationStack.size() == 0) 
       break; 

      mOperationStack.add(currentInput); 
      evalResult = evaluateResult(true); 
      if (evalResult != null) { 
       clearStacks(); 
       userInputText.setText(evalResult); 
       resetInput = false; 
       hasFinalResult = true; 
      } 
      break; 
     case M_ADD: 
      userInputValue = tryParseUserInput(); 
      if (Double.isNaN(userInputValue)) 
       return; 
      if (Double.isNaN(memoryValue)) 
       memoryValue = 0; 
      memoryValue += userInputValue; 
      displayMemoryStat(); 

      hasFinalResult = true; 

      break; 
     case M_REMOVE: 
      userInputValue = tryParseUserInput(); 
      if (Double.isNaN(userInputValue)) 
       return; 
      if (Double.isNaN(memoryValue)) 
       memoryValue = 0; 
      memoryValue -= userInputValue; 
      displayMemoryStat(); 
      hasFinalResult = true; 
      break; 
     case MC: 
      memoryValue = Double.NaN; 
      displayMemoryStat(); 
      break; 
     case MR: 
      if (Double.isNaN(memoryValue)) 
       return; 
      userInputText.setText(doubleToString(memoryValue)); 
      displayMemoryStat(); 
      break; 
     case MS: 
      userInputValue = tryParseUserInput(); 
      if (Double.isNaN(userInputValue)) 
       return; 
      memoryValue = userInputValue; 
      displayMemoryStat(); 
      hasFinalResult = true; 
      break; 
     default: 
      if (Character.isDigit(text.charAt(0))) { 
       if (currentInput.equals("0") || resetInput || hasFinalResult) { 
        userInputText.setText(text); 
        resetInput = false; 
        hasFinalResult = false; 
       } else { 
        userInputText.append(text); 
        resetInput = false; 
       } 

      } 
      break; 

     } 

    } 

    public void clearStacks() { 
     mInputStack.clear(); 
     mOperationStack.clear(); 
     mStackText.setText(""); 
    } 

    public void dumpInputStack() { 
     Iterator<String> it = mInputStack.iterator(); 
     StringBuilder sb = new StringBuilder(); 

     while (it.hasNext()) { 
      CharSequence iValue = it.next(); 
      sb.append(iValue); 

     } 

     mStackText.setText(sb.toString()); 
    } 

    public String evaluateResult(boolean requestedByUser) { 
     if ((!requestedByUser && mOperationStack.size() != 4) 
       || (requestedByUser && mOperationStack.size() != 3)) 
      return null; 

     String left = (String) mOperationStack.get(0); 
     String operator = (String) mOperationStack.get(1); 
     String right = (String) mOperationStack.get(2); 
     String tmp = null; 
     if (!requestedByUser) 
      tmp = (String) mOperationStack.get(3); 

     double leftVal = Double.parseDouble(left.toString()); 
     double rightVal = Double.parseDouble(right.toString()); 
     double result = Double.NaN; 

     if (operator.equals(KeypadButton.DIV.getText())) { 
      result = leftVal/rightVal; 
     } else if (operator.equals(KeypadButton.MULTIPLY.getText())) { 
      result = leftVal * rightVal; 

     } else if (operator.equals(KeypadButton.PLUS.getText())) { 
      result = leftVal + rightVal; 
     } else if (operator.equals(KeypadButton.MINUS.getText())) { 
      result = leftVal - rightVal; 

     } 

     String resultStr = doubleToString(result); 
     if (resultStr == null) 
      return null; 

     mOperationStack.clear(); 
     if (!requestedByUser) { 
      mOperationStack.add(resultStr); 
      mOperationStack.add(tmp); 
     } 

     return resultStr; 
    } 

    public String doubleToString(double value) { 
     if (Double.isNaN(value)) 
      return null; 

     long longVal = (long) value; 
     if (longVal == value) 
      return Long.toString(longVal); 
     else 
      return Double.toString(value); 

    } 

    public double tryParseUserInput() { 
     String inputStr = userInputText.getText().toString(); 
     double result = Double.NaN; 
     try { 
      result = Double.parseDouble(inputStr); 

     } catch (NumberFormatException nfe) {} 
     return result; 

    } 

    public void displayMemoryStat() { 
     if (Double.isNaN(memoryValue)) { 
      memoryStatText.setText(""); 
     } else { 
      memoryStatText.setText("M = " + doubleToString(memoryValue)); 
     } 
    } 
} 

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:orientation="vertical" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"> 

    <TextView 
     android:id="@+id/txtStack" 
     android:layout_width="fill_parent" 
     android:layout_height="wrap_content" 
     android:textSize="15sp" 
     android:gravity="right" 
     android:layout_marginTop = "3sp" 
     android:layout_marginLeft = "5sp" 
     android:layout_marginRight = "5sp"/> 


    <TextView 
     android:id="@+id/txtInput" 
     android:layout_width="fill_parent" 
     android:layout_height="wrap_content" 
     android:textSize="25sp" 
     android:gravity="right" 
     android:layout_marginLeft = "5sp" 
     android:layout_marginRight = "5sp"/> 


    <TextView 
     android:id="@+id/txtMemory" 
     android:layout_width="fill_parent" 
     android:layout_height="wrap_content" 
     android:textSize="15sp" 
     android:gravity="left" 
     android:layout_marginLeft = "5sp" 
     android:layout_marginRight = "5sp"/> 

<GridView 
    android:id="@+id/grdButtons" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:columnWidth="90dp" 
    android:numColumns="5" 
    android:verticalSpacing="10dp" 
    android:horizontalSpacing="10dp" 
    android:stretchMode="columnWidth" 
    android:gravity="center"/> 
</LinearLayout> 

Ausgang:

Stack Trace:

E/AndroidRuntime(1492): FATAL EXCEPTION: main 
E/AndroidRuntime(1492): Process: com.calculator, PID: 1492 
E/AndroidRuntime(1492): java.lang.NullPointerException 
E/AndroidRuntime(1492): at com.calculator.MainActivity.ProcessKeypadInput(MainActivity.java:132) 
E/AndroidRuntime(1492): at com.calculator.MainActivity$1.onClick(MainActivity.java:50) 
E/AndroidRuntime(1492): at android.view.View.performClick(View.java:4438) 
E/AndroidRuntime(1492): at android.view.View$PerformClick.run(View.java:18422) 
E/AndroidRuntime(1492): at android.os.Handler.handleCallback(Handler.java:733) 
E/AndroidRuntime(1492): at android.os.Handler.dispatchMessage(Handler.java:95) 
E/AndroidRuntime(1492): at android.os.Looper.loop(Looper.java:136) 
E/AndroidRuntime(1492): at android.app.ActivityThread.main(ActivityThread.java:5017) 
E/AndroidRuntime(1492): at java.lang.reflect.Method.invokeNative(Native Method) 
E/AndroidRuntime(1492): at java.lang.reflect.Method.invoke(Method.java:515) 
E/AndroidRuntime(1492): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
E/AndroidRuntime(1492): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
E/AndroidRuntime(1492): at dalvik.system.NativeStart.main(Native Method) 

• Ich weiß nicht, wie die Nullpointer-Ausnahmefehler zu beheben.

• Problem ist, während die
  + Pressen (plus), - (minus), * (MUL),/(div), MC, MR, C, = (gleich) und Comma Buttons Logcat Fehler occured.Other Knöpfe wie 0,1,2,3 bis 9 sind in Ordnung.

• Ihre Antwort wird hier sehr willkommen sein.

• Danke

Answer 1

In onCreate müssen findViewById für userInputText

userInputText= (TextView) findViewById(R.id.txtInput); 

Das gleiche gilt für andere Layoutkomponenten, sonst keine Identifikation für getText()

Und hinzufügen, wenn Sie das tun, Bitte verwenden Sie nicht den gleichen TextView Code für alle Komponenten ...

userInputText= (TextView) findViewById(R.id.txtInput); 
userInputText=(TextView)findViewById(R.id.txtStack); 
userInputText=(TextView)findViewById(R.id.txtMemory); 

ist falsch.

Verwenden Sie unterschiedliche Textview für Stapel und Speicher .... wie stackInputText, memoryInputText

Answer 2

Unter Ihrer ProcessKeypadInput Methode haben Sie den folgenden Code:

case MULTIPLY: 
     if (resetInput) { 
      mInputStack.pop(); 
      mOperationStack.pop(); 
     } else { 
      if (currentInput.charAt(0) == '-') {  // <-------- Problem line 
       mInputStack.add("(" + currentInput + ")"); 
      } else { 
       mInputStack.add(currentInput); 
      } 
      mOperationStack.add(currentInput); 
     } 

I angegeben, die Zeile erscheint die verursacht werden Problem, das Sie in Logcat sehen. Sie versuchen, das erste Zeichen von currentInput zu bewerten, haben aber nichts, was sicherstellen würde, dass currentInput irgendwelche Zeichen enthält. Wenn Sie versuchen, das erste Zeichen einer Zeichenfolge auszuwerten, die überhaupt keine Zeichen enthält, würde es throw the exception you are seeing.

Eine Lösung wäre, die Linie in Frage zu ändern, und unten auf die folgenden:

... 
    if (currentInputLen > 0) { 
     if (currentInput.charAt(0) == '-') { 
      mInputStack.add("(" + currentInput + ")"); 
     } else { 
      mInputStack.add(currentInput); 
     } 
     mOperationStack.add(currentInput); 
    } 
} 

Dies, dass der Versuch verhindern würde das erste Zeichen zu bewerten, wenn die Zeichenfolge mindestens ein Zeichen enthalten, und auch null verhindern Zeigerausnahmen