Android calculator previous option function

Question

I am trying to learn android development from the beginning here. So basically it will be a calculator. The way I code the calculate function is down below:

 private boolean action = false;
 private String fNum;
 private String sNum;
 private String preOp = "=";

Action as the flag to check if there is any previous option exist. fNum and sNum as the number needs to be taken care of. fNum is the one before calculate symbol button was clicked , sNum as the number after the symbol button was clicked. preOp as the storage for the last operation.

private double Calculate(String fn, String sn){
    switch (preOp) {
        case "plus":
            return Double.parseDouble(fn) + Double.parseDouble(sn);
        case "minus":
            return Double.parseDouble(fn) - Double.parseDouble(sn);
        case "multiply":
            return Double.parseDouble(fn) * Double.parseDouble(sn);
        case "device":
            return Double.parseDouble(fn) / Double.parseDouble(sn);
        case "=":
            return Double.parseDouble(sn);
        default:
            return 0;
    }

}

Function to actually run the calculate.

case R.id.buttonPlus:{
            if(action){
                fNum = text.getText().toString();
                text.setText("0");
                preOp = "plus";
                action = true;
            }
            else if (!action){
                sNum = text.getText().toString();
                text.setText(String.valueOf(Calculate(fNum, sNum)));
                preOp = "plus";
            }
        }break;
        case R.id.buttonE:{
            if(action){
                sNum = text.getText().toString();
                text.setText(String.valueOf(Calculate(fNum, sNum)));
                preOp = "=";
            }
        }break;

The button functions.

So I have got the error when I clicked the + button:

12-04 05:53:04.198 31194-31194/tian.secondproject E/AndroidRuntime: FATAL EXCEPTION: main
    Process: tian.secondproject, PID: 31194
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.trim()' on a null object reference
    at java.lang.StringToReal.parseDouble(StringToReal.java:263)
    at java.lang.Double.parseDouble(Double.java:301)
    at tian.secondproject.MainActivity.Calculate(MainActivity.java:21)
    at tian.secondproject.MainActivity.onClick(MainActivity.java:226)
    at android.view.View.performClick(View.java:5198)
    at android.view.View$PerformClick.run(View.java:21147)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

So, am I doing it with a wrong method? or there is just some simple mistake I have made?

Thank you :)

where did you call trim() in your code ? Its because string object over there is null — Nitin Mesta, Dec 04 '15 at 06:00
your are executing `.trim()` on a string object which is null. in the code you've posted no trims appear, so it is hard to say, what the problem might be. — Rich, Dec 04 '15 at 06:00
I don't think I have called trim() anywhere in my code. Should I use it somewhere(for what purpose)? — Tian, Dec 04 '15 at 06:12
MainActivity.java:226 Look at the xml associated with this line. "null object reference" means whatever 226 is referencing is null or does not exist. — SmulianJulian, Dec 04 '15 at 08:01

Android calculator previous option function

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