在输入值之前按下操作符时，Java Android Calculator会崩溃

Question

If I press "=", "+", "-", "*", or "/" before any numbers, my app crashes! I can't find any good answers to this and I'm not sure what I need to do.

I've tried multiple different ways to make this calculator but I still get the same problem. I'm thinking that somehow an If statement that has to do with OnCalculation.

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

    public void onClick(View v) {
        Button button = (Button) v;
        String str = button.getText().toString();
        total += str;
        EditText edit = (EditText) findViewById(R.id.editText);
        edit.setText(total);
    }

    public void OnAdd(View v) {
        temp1 = Double.parseDouble(total);
        total = "";
        Button button = (Button) v;
        String str = button.getText().toString();
        sign += str;
        EditText edit = (EditText) findViewById(R.id.editText);
        edit.setText("");
    }

    public void OnCalculate(View v) {
        EditText edit = (EditText) findViewById(R.id.editText);
        String str2 = edit.getText().toString();
        temp2 = Double.parseDouble(str2);
        Double grand_total = 0.0;

        if (sign.equals("+")) {
            grand_total = temp1 + temp2;
        } else if (sign.equals("-")) {
            grand_total = temp1 - temp2;
        } else if (sign.equals("*")) {
            grand_total = temp1 * temp2;
        } else if (sign.equals("/")) {
            grand_total = temp1 / temp2;
        }
        edit.setText(grand_total + "");
        sign = "";
    }

public void OnClear(View v) {
    EditText edit = (EditText)findViewById(R.id.editText);
    edit.setText("");
    total = "";
}
}

Log:

07-13 18:45:32.831 25631-25631/com.example.shelbywallace.mycalc E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                  Process: com.example.shelbywallace.mycalc, PID: 25631
                                                                                  java.lang.IllegalStateException: Could not execute method for android:onClick
                                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                                                                                      at android.view.View.performClick(View.java:5702)
                                                                                      at android.widget.TextView.performClick(TextView.java:10888)
                                                                                      at android.view.View$PerformClick.run(View.java:22533)
                                                                                      at android.os.Handler.handleCallback(Handler.java:739)
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                      at android.os.Looper.loop(Looper.java:158)
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:7229)
                                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
                                                                                   Caused by: java.lang.reflect.InvocationTargetException
                                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                                                                                      at android.view.View.performClick(View.java:5702) 
                                                                                      at android.widget.TextView.performClick(TextView.java:10888) 
                                                                                      at android.view.View$PerformClick.run(View.java:22533) 
                                                                                      at android.os.Handler.handleCallback(Handler.java:739) 
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                                      at android.os.Looper.loop(Looper.java:158) 
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:7229) 
                                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 
                                                                                   Caused by: java.lang.NumberFormatException: Invalid double: ""
                                                                                      at java.lang.StringToReal.invalidReal(StringToReal.java:63)
                                                                                      at java.lang.StringToReal.parseDouble(StringToReal.java:267)
                                                                                      at java.lang.Double.parseDouble(Double.java:301)
                                                                                      at com.example.shelbywallace.mycalc.MainActivity.OnCalculate(MainActivity.java:54)
                                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                                                                                      at android.view.View.performClick(View.java:5702) 
                                                                                      at android.widget.TextView.performClick(TextView.java:10888) 
                                                                                      at android.view.View$PerformClick.run(View.java:22533) 
                                                                                      at android.os.Handler.handleCallback(Handler.java:739) 
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                                      at android.os.Looper.loop(Looper.java:158) 
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:7229) 
                                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

Answer 1

java.lang.NumberFormatException: Invalid double: "" at

Try initializing your total string as "0" instead of the empty String. Otherwise you get an error trying to convert nothing to a number!

---

for example Double.parseDouble(""); produces error:

Exception in thread "main" java.lang.NumberFormatException: empty String
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
    at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
    at java.lang.Double.parseDouble(Double.java:538)
    at Ideone.main(Main.java:12)

---

Alternatively you could make a method to parse the string, and return zero for empty strings:

public static int parseDouble(final String value) {
    if (value.equals("")) {
        return 0;
    }
    return Double.parseDouble(value);
}

Then you can call it like this:

EditText edit = (EditText) findViewById(R.id.editText);
String str2 = edit.getText().toString();
temp2 = parseDouble(str2);