Question

iv created an android app, what my problem is that when i press the + = – / * buttons before i input a number my app stops working, is there anyway i can make it that if there is no number input it doesnt stop working?

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package steven.mcilhone.calculatorcoursework;

import java.math.BigDecimal;


import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;


public class Calculator extends Activity {

EditText firstValue;
EditText secondValue;
TextView result;
Button addbtn, subtractbtn, dividebtn, multiplybtn, equalbtn, clearbtn;
BigDecimal firstNum, secondNum;



String Operator = "";

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_calculator);

    addbtn = (Button) findViewById(R.id.addbtnID);
    subtractbtn = (Button) findViewById(R.id.subtractbtnID);
    dividebtn = (Button) findViewById(R.id.dividebtnID);
    multiplybtn = (Button) findViewById(R.id.multiplybtnID);
    equalbtn = (Button) findViewById(R.id.equalbtnID);
    clearbtn = (Button) findViewById(R.id.clearbtn);
    firstValue = (EditText) findViewById(R.id.edttxt1);
    secondValue = (EditText) findViewById(R.id.edttxt1);


    clearbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
            edttxt1.setText("");
        }
    });



    addbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub

            Operator = "+";
            addandclear();
        }
    });

    subtractbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Operator = "-";             
            addandclear();
        }
    });

    dividebtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Operator = "/";
            addandclear();

        }
    });

    multiplybtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            Operator = "*";
            addandclear();
        }
    });

    equalbtn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            secondNum = new          BigDecimal(secondValue.getText().toString());
            EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
            edttxt1.setText("");
            if (Operator == "+"){
                edttxt1.setText(firstNum.add(secondNum).toString());
            }
            else if (Operator == "-"){
                edttxt1.setText(firstNum.subtract(secondNum).toString());
            }
            else if (Operator == "/"){
                edttxt1.setText(firstNum.divide(secondNum).toString());
            }   
            else if (Operator == "*"){
                edttxt1.setText(firstNum.multiply(secondNum).toString());
            }   



        }
    });

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_calculator, menu);
    return true;
}


        public void addandclear(){


            firstNum = new BigDecimal(firstValue.getText().toString());             
            EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
            edttxt1.setText("");
        }



}

---

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<EditText
    android:id="@+id/edttxt1"
    android:layout_width="250dp"
    android:layout_height="100dp"
    android:layout_alignParentRight="true"
    android:layout_alignParentTop="true"
    android:background="?android:attr/editTextBackground"
    android:ems="15"
    android:inputType="numberDecimal" 
    android:gravity="right"/>

<Button
    android:id="@+id/addbtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_below="@+id/edttxt1"
    android:layout_marginLeft="20dp"
    android:text="@string/add" />

<Button
    android:id="@+id/dividebtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_below="@+id/edttxt1"
    android:layout_toRightOf="@+id/addbtnID"
    android:text="@string/divide" />

<Button
    android:id="@+id/multiplybtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_below="@+id/edttxt1"
    android:layout_toRightOf="@+id/dividebtnID"
    android:text="@string/multiply" />

<Button
    android:id="@+id/subtractbtnID"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/multiplybtnID"
    android:layout_alignBottom="@+id/multiplybtnID"
    android:layout_toRightOf="@+id/multiplybtnID"
    android:text="@string/subtract" />

<Button
    android:id="@+id/zerobtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignLeft="@+id/addbtnID"
    android:layout_alignParentBottom="true"
    android:layout_alignRight="@+id/twobtn"
    android:text="@string/zero" />

<Button
    android:id="@+id/onebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/zerobtn"
    android:layout_alignLeft="@+id/zerobtn"
    android:text="@string/one" />

<Button
    android:id="@+id/twobtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/zerobtn"
    android:layout_toLeftOf="@+id/multiplybtnID"
    android:text="@string/two" />

<Button
    android:id="@+id/threebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/zerobtn"
    android:layout_toLeftOf="@+id/subtractbtnID"
    android:text="@string/three" />

<Button
    android:id="@+id/fourbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/onebtn"
    android:layout_toLeftOf="@+id/twobtn"
    android:text="@string/four" />

<Button
    android:id="@+id/fivebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/fourbtn"
    android:layout_alignBottom="@+id/fourbtn"
    android:layout_toLeftOf="@+id/threebtn"
    android:text="@string/five" />

<Button
    android:id="@+id/sixbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/fivebtn"
    android:layout_alignBottom="@+id/fivebtn"
    android:layout_toRightOf="@+id/fivebtn"
    android:text="@string/six" />

<Button
    android:id="@+id/sevenbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/fourbtn"
    android:layout_toLeftOf="@+id/fivebtn"
    android:text="@string/seven" />

<Button
    android:id="@+id/eightbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/fivebtn"
    android:layout_toLeftOf="@+id/sixbtn"
    android:text="@string/eight" />

<Button
    android:id="@+id/ninebtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_alignBaseline="@+id/eightbtn"
    android:layout_alignBottom="@+id/eightbtn"
    android:layout_toRightOf="@+id/eightbtn"
    android:text="@string/nine" />

<Button
    android:id="@+id/equalbtnID"
    android:layout_width="70dp"
    android:layout_height="120dp"
    android:layout_alignBottom="@+id/threebtn"
    android:layout_alignTop="@+id/sixbtn"
    android:layout_toRightOf="@+id/threebtn"
    android:text="@string/equal" />

<Button
    android:id="@+id/clearbtn"
    android:layout_width="70dp"
    android:layout_height="70dp"
    android:layout_above="@+id/equalbtnID"
    android:layout_alignLeft="@+id/equalbtnID"
    android:text="@string/clear" />

</RelativeLayout>

Answer 1

In several places, you are converting the user input to a BigDecimal without doing any error checking. In particular, when there is no input, new BigDecimal(String) will generate a NumberFormatException because “” is not a valid number. From the docs:

The string must contain at least one digit in either the integer or the fraction.

Find every place where you are using the user input and put in some error checking. For instance, in addandclear():

public void addandclear(){
    String val = firstValue.getText().toString();
    try {
        firstNum = new BigDecimal(val);
        EditText edttxt1 = (EditText) findViewById(R.id.edttxt1);
        edttxt1.setText("");
    } catch (NumberFormatException e) {
        if (val.length() == 0) {
            // blank field
        } else {
            // something wrong with the input
        }
    }
}