ArrayOutOfBound发生异常并导致应用程序崩溃

Question

我正在Android中制作一个简单的计算器应用程序，但当我按下应用程序崩溃时的百分比按钮并给我出现以下错误时，我遇到了一些错误。基本上我要做的是，如果任何符号出现在字符串的最后一个索引处，则EditText值应该减1，并且将分配新值。在按下百分比按钮之前按下任何其他按钮时，它正常工作。但是，如果我先按百分比按钮，然后其他按钮应用程序崩溃。这基本上是问题所在。为避免首先放置符号，我写了最后一个else块，它只是在Toast中显示一条消息，但现在它崩溃了。

下面是Java代码

public void calculate(View v){ 

    String t; 
    ma_res_txt = (TextView) findViewById(R.id.ma_res_txt); 
    String[] signs ={"%","÷","+","×","π","!","√",".","×^"}; 
    int length = ma_res_txt.getText().length(); 
    String txt = ma_res_txt.getText().toString(); 

    switch (v.getId()){ 

     case R.id.cal_btn_clear:{ 
      ma_res_txt.setText(""); 
      break; 
     } 

     case R.id.cal_btn_del:{ 
      try { 
       if(length>0){ 
        ma_res_txt.setText(txt.substring(0, length - 1)); 
       } else{ 
        Toast.makeText(MainActivity.this,"No more characters left",Toast.LENGTH_LONG).show(); 
       } 
      }catch (Exception e){ 
       Toast.makeText(MainActivity.this,"No more characters left",Toast.LENGTH_LONG).show(); 
      } 
      break; 
     } 

     case R.id.cal_btn_percent:{ 
      t = "%"; 
      String s = txt.substring(length-1,length); 

      if(length>0) { 
       if(s.equals("%") || s.equals("÷") || s.equals("+") || s.equals("×") || s.equals("^")){ 
        ma_res_txt.setText(txt.substring(0, length - 1)); 
        ma_res_txt.append(t); 
       } 
       else { 
        ma_res_txt.append(t); 
       } 
      } 
      else{ 
       Toast.makeText(MainActivity.this,"Syntax Error !",Toast.LENGTH_LONG).show(); 
      } 
      break; 
     } 
    } 
} 

这是我得到的错误。即使围绕着try catch块，应用程序仍然崩溃。

08-02 20:17:16.105 26307-26307/com.example.nadeemahmad.smartcalculator E/AndroidRuntime: FATAL EXCEPTION: main 
                         Process: com.example.nadeemahmad.smartcalculator, PID: 26307 
                         java.lang.IllegalStateException: Could not execute method for android:onClick 
                          at android.view.View$DeclaredOnClickListener.onClick(View.java:5044) 
                          at android.view.View.performClick(View.java:6205) 
                          at android.widget.TextView.performClick(TextView.java:11103) 
                          at android.view.View$PerformClick.run(View.java:23653) 
                          at android.os.Handler.handleCallback(Handler.java:751) 
                          at android.os.Handler.dispatchMessage(Handler.java:95) 
                          at android.os.Looper.loop(Looper.java:154) 
                          at android.app.ActivityThread.main(ActivityThread.java:6682) 
                          at java.lang.reflect.Method.invoke(Native Method) 
                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520) 
                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410) 
                          Caused by: java.lang.reflect.InvocationTargetException 
                          at java.lang.reflect.Method.invoke(Native Method) 
                          at android.view.View$DeclaredOnClickListener.onClick(View.java:5039) 
                          at android.view.View.performClick(View.java:6205)  
                          at android.widget.TextView.performClick(TextView.java:11103)  
                          at android.view.View$PerformClick.run(View.java:23653)  
                          at android.os.Handler.handleCallback(Handler.java:751)  
                          at android.os.Handler.dispatchMessage(Handler.java:95)  
                          at android.os.Looper.loop(Looper.java:154)  
                          at android.app.ActivityThread.main(ActivityThread.java:6682)  
                          at java.lang.reflect.Method.invoke(Native Method)  
                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)  
                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)  
                          Caused by: java.lang.StringIndexOutOfBoundsException: length=0; index=-1 
                          at java.lang.String.substring(String.java:1926) 
                          at com.example.nadeemahmad.smartcalculator.MainActivity.calculate(MainActivity.java:87) 
                          at java.lang.reflect.Method.invoke(Native Method)  
                          at android.view.View$DeclaredOnClickListener.onClick(View.java:5039)  
                          at android.view.View.performClick(View.java:6205)  
                          at android.widget.TextView.performClick(TextView.java:11103)  
                          at android.view.View$PerformClick.run(View.java:23653)  
                          at android.os.Handler.handleCallback(Handler.java:751)  
                          at android.os.Handler.dispatchMessage(Handler.java:95)  
                          at android.os.Looper.loop(Looper.java:154)  
                          at android.app.ActivityThread.main(ActivityThread.java:6682)  
                          at java.lang.reflect.Method.invoke(Native Method)  
                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)  
                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)  

XML

<EditText 
     android:layout_width="match_parent" 
     android:layout_height="211dp" 
     android:background="#fff" 
     android:id="@+id/ma_res_txt" 
     android:paddingLeft="17dp" 
     android:textSize="30dp" 
     android:inputType="none" 
     android:focusedByDefault="true" 
     android:focusableInTouchMode="true" 
     android:textIsSelectable="true" 


     /> 
    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/btn_ctrl_linear" 
     android:layout_below="@+id/ma_res_txt" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:text="Voice Control" 
      android:layout_weight="1" 
      android:id="@+id/show_voice_ctrl" 
      android:background="@drawable/custom_btn_ripple_blue_white" 
      android:paddingLeft="10dp" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:text="Camera Control" 
      android:layout_weight="1" 
      android:id="@+id/show_cam_ctrl" 
      android:background="@drawable/custom_btn_ripple_blue_white" 
      android:paddingLeft="10dp" 
      /> 
    </LinearLayout> 

    <ScrollView 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/btn_ctrl_linear" 
     > 
     <LinearLayout 
      android:layout_width="match_parent" 
      android:layout_height="wrap_content" 
      android:orientation="vertical" 
      android:weightSum="1" 
      > 
    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/cal_btn_set1" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="C" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:textColor="#027fcc" 
      android:id="@+id/cal_btn_clear" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="%" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:textColor="#027fcc" 
      android:id="@+id/cal_btn_percent" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="÷" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textColor="#027fcc" 
      android:textSize="20dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_divide" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="del" 
      android:textAllCaps="false" 
      android:textColor="#027fcc" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_del" 
      android:onClick="calculate" 
      /> 

    </LinearLayout> 
    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/cal_btn_set2" 
     android:layout_below="@+id/cal_btn_set1" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="7" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_7" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="8" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_8" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="9" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_9" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="+" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="20dp" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_plus" 
      android:onClick="calculate" 
      /> 

    </LinearLayout> 
    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/cal_btn_set3" 
     android:layout_below="@+id/cal_btn_set2" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="4" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_4" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="5" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_5" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="6" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_6" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="×" 
      android:typeface="monospace" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="24dp" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_mult" 
      android:onClick="calculate" 
      /> 

    </LinearLayout> 
    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/cal_btn_set4" 
     android:layout_below="@+id/cal_btn_set3" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="1" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_1" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="2" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_2" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="3" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_3" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="π" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="16dp" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_pi" 
      android:onClick="calculate" 
      /> 

    </LinearLayout> 
    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/cal_btn_set5" 
     android:layout_below="@+id/cal_btn_set4" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="√" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="18dp" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_root" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="0" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="18dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_0" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="." 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="24dp" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_dot" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="×^" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="18dp" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_power" 
      android:onClick="calculate" 
      /> 

    </LinearLayout> 


    <LinearLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:orientation="horizontal" 
     android:id="@+id/cal_btn_set6" 
     android:layout_below="@+id/cal_btn_set5" 
     > 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="sin" 
      android:textAllCaps="false" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="14dp" 
      android:textColor="#027fcc" 
      android:textStyle="italic" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_sin" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="cos" 
      android:textAllCaps="false" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="14dp" 
      android:textColor="#027fcc" 
      android:textStyle="italic" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_cos" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="tan" 
      android:textAllCaps="false" 
      android:layout_weight="1" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="14dp" 
      android:textStyle="italic" 
      android:textColor="#027fcc" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_tan" 
      android:onClick="calculate" 
      /> 
     <Button 
      android:layout_width="wrap_content" 
      android:layout_height="65dp" 
      android:text="!" 
      android:layout_weight="1" 
      android:textColor="#027fcc" 
      android:background="@drawable/custom_btn_ripple_blue" 
      android:textSize="20dp" 
      android:layout_margin="1dp" 
      android:id="@+id/cal_btn_ptrl" 
      android:onClick="calculate" 
      /> 

    </LinearLayout> 
      <LinearLayout 
       android:layout_width="match_parent" 
       android:layout_height="wrap_content" 
       android:orientation="horizontal" 
       android:id="@+id/cal_btn_set7" 
       android:layout_below="@+id/cal_btn_set6" 
       > 
       <Button 
        android:layout_width="wrap_content" 
        android:layout_height="65dp" 
        android:text="sin(-1)" 
        android:textAllCaps="false" 
        android:layout_weight="1" 
        android:background="@drawable/custom_btn_ripple_blue" 
        android:textSize="12dp" 
        android:textColor="#027fcc" 
        android:textStyle="italic" 
        android:layout_margin="1dp" 
        android:id="@+id/cal_btn_sinIn" 
        android:onClick="calculate" 
        /> 
       <Button 
        android:layout_width="wrap_content" 
        android:layout_height="65dp" 
        android:text="cos(-1)" 
        android:textAllCaps="false" 
        android:layout_weight="1" 
        android:background="@drawable/custom_btn_ripple_blue" 
        android:textSize="12dp" 
        android:textColor="#027fcc" 
        android:textStyle="italic" 
        android:layout_margin="1dp" 
        android:id="@+id/cal_btn_cosIn" 
        android:onClick="calculate" 
        /> 
       <Button 
        android:layout_width="wrap_content" 
        android:layout_height="65dp" 
        android:text="tan(-1)" 
        android:textAllCaps="false" 
        android:layout_weight="1" 
        android:background="@drawable/custom_btn_ripple_blue" 
        android:textSize="12dp" 
        android:textStyle="italic" 
        android:textColor="#027fcc" 
        android:layout_margin="1dp" 
        android:id="@+id/cal_btn_tanIn" 
        android:onClick="calculate" 
        /> 
       <Button 
        android:layout_width="wrap_content" 
        android:layout_height="65dp" 
        android:text="=" 
        android:layout_weight="1" 
        android:background="@drawable/custome_btn_ripple_grey" 
        android:textSize="30dp" 
        android:textColor="#fff" 
        android:layout_margin="1dp" 
        android:id="@+id/cal_btn_equal" 
        android:onClick="calculate" 
        /> 

      </LinearLayout> 




     </LinearLayout> 
    </ScrollView> 
</RelativeLayout> 

Answer 1

当你做String s = txt.substring(length-1,length);你没有检查，如果如果你第一次，因此你的指数是按％按钮，您的长度> 0，你TXT是空的out of bound