在Android中触摸计算器

我正在尝试创建一个计算器设计.但我没有得到任何编译时错误.最后在运行Project /代码时出现nullpointer异常发生错误.

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中：

<?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>

输出：

堆栈跟踪：

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)

>我不知道如何修复NullPointer异常错误.
>问题在于按下
(加号),– (减号),*(mul),/(div),MC,MR,C,=(等于)和逗号按钮Logcat错误发生.其他按钮如0,1,2,3到9其罚款.
>欢迎您的回答.
>谢谢

解决方法

在onCreate中,需要为userInputText添加findViewById

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

对于其他布局组件也是如此,否则getText()没有标识

当你这样做时,请不要在代码中为所有组件使用相同的TextView ……

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

是不正确的.

对堆栈和内存使用不同的TextView ….就像stackInputText,memoryInputText

原文链接：https://www.f2er.com/android/309640.html

在Android中触摸计算器

解决方法

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