Calculator with single edittext field

Question

I am new to android development and I am trying to develop a calculator with single edit text field. It's not showing any compiler error but it crashes as soon as I press any button. I'm first taking the input for num1 and clearing the edit text field followed by a switch case to detect the operation and collecting the data of the second number.

public class MainActivity extends Activity implements OnClickListener{



    EditText ino;



    Button btnadd;

    Button btnsub;

    Button btnmult;

    Button btndiv;

    Button btneq;



    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);



        ino=(EditText) findViewById(R.id.ino);



        btnadd=(Button) findViewById(R.id.add);

        btndiv=(Button) findViewById(R.id.div);

        btnsub=(Button) findViewById(R.id.sub);

        btnmult=(Button) findViewById(R.id.mult);

        btneq=(Button) findViewById(R.id.eq);



        btnadd.setOnClickListener((OnClickListener) this);

        btnsub.setOnClickListener(this);

        btnmult.setOnClickListener(this);

        btndiv.setOnClickListener(this);



    }



    @Override

    public void onClick(View v)

    {



        float num1=0,num2=0,res=0;

        String xyz="",ans;

        num1=Float.parseFloat(ino.getText().toString());



        ino.setText(xyz);

        switch(v.getId())

        {

            case R.id.add:



                num2=Float.parseFloat(ino.getText().toString());



                res=num2+num1;



                ans=Float.toString(res);



                break;

            case R.id.div:

                num2=Float.parseFloat(ino.getText().toString());

                ino.setText(xyz);

                res=num1/num2;



                ans=Float.toString(res);

                ino.setText(ans);

                break;

            case R.id.mult:

                num2=Float.parseFloat(ino.getText().toString());

                ino.setText(xyz);

                res=num2*num1;



                ans=Float.toString(res);

                ino.setText(ans);

                break;

            case R.id.sub:

                num2=Float.parseFloat(ino.getText().toString());

                ino.setText(xyz);

                res=num1-num2;



                ans=Float.toString(res);

                ino.setText(ans);

                break;

            case R.id.eq:

                ans=Float.toString(res);

                ino.setText(ans);

                break;

            default: break;

        }







    }

}

Here is the copy of stack trace.

 12-28 00:49:52.646 2125-2125/com.example.rishabh.calculator2

 E/AndroidRuntime: FATAL EXCEPTION: main

    Process: com.example.rishabh.calculator2, PID: 2125

    java.lang.NumberFormatException: Invalid float: ""

        at java.lang.StringToReal.invalidReal(StringToReal.java:63)

        at java.lang.StringToReal.parseFloat(StringToReal.java:308)

        at java.lang.Float.parseFloat(Float.java:306)

        at com.example.rishabh.calculator2.MainActivity.onClick(MainActivity.java:56)

        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)

Answer 1

Your code contains several problems. The first is that Float.parseFloat() will throw a java.lang.NumberFormatException if the string is empty or contains letters. This should be handled with try/catch.

The second problem is that the variables num1, num2 and res cannot be declared locally, because they will be initialized to 0.0 every time a button is clicked.

For the calculator to work it is necessary to remember the operation that is going to be performed when the user clicks the "equals"-button. The previous clicked button can be stored using an int.

The following code is tested and works.

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

public class MainActivity extends Activity implements OnClickListener {

    EditText ino;

    Button btnadd;
    Button btnsub;
    Button btnmult;
    Button btndiv;
    Button btneq;

    private float num1 = 0;
    private float num2 = 0;
    private float res = 0;

    private int operation = 0;

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

        ino=(EditText) findViewById(R.id.ino);

        btnadd=(Button) findViewById(R.id.add);
        btndiv=(Button) findViewById(R.id.div);
        btnsub=(Button) findViewById(R.id.sub);
        btnmult=(Button) findViewById(R.id.mult);
        btneq=(Button) findViewById(R.id.eq);

        btnadd.setOnClickListener(this);
        btnsub.setOnClickListener(this);
        btnmult.setOnClickListener(this);
        btndiv.setOnClickListener(this);
        btneq.setOnClickListener(this);
    }

    @Override
    public void onClick(View v)
    {
        String s = ino.getText().toString();
        try
        {
            Float.parseFloat(s);
        }
        catch (NumberFormatException e)
        {
            s = "0.0";
        }

        String xyz="",ans;
        ino.setText(xyz);

        switch(v.getId())
        {
            case R.id.eq:
                num1=Float.parseFloat(s);

                switch (operation) {
                    case R.id.add:
                        res = num2 + num1;
                        break;
                    case R.id.sub:
                        res = num2 - num1;
                        break;
                    case R.id.mult:
                        res = num2 * num1;
                        break;
                    case R.id.div:
                        res = num2 / num1;
                        break;
                    default:
                        res = 0;
                        break;
                }
                ans=Float.toString(res);
                ino.setText(ans);
                break;
        }
        num2 = Float.parseFloat(s);
        operation = v.getId();
    }
}

Calculator with single edittext field

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