Android calculator with button click

Question

I am trying to calculate a field named lblAnswer by adding values txtA + txtB. I am fairly new to the android development world and would like to know what is the best way of going about this. I have already added the necessarily edit fields to the GUI. I am now working in the java file to try and create the method. This method has been named doCalc. Here is what I have thus far.

public void doCalc() 
{
    lblAnswer = txtA + txtB;
}

It has been suggested that I add more code here is the full code. Thank you for that suggestion.

Here is the Java File.

      package com.example.wattsprofessional;

     import android.app.Activity;
     import android.os.Bundle;
     import android.view.Menu;

    public class MainActivity extends Activity {

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

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}


public void doCalc() 
{
    lblAnswer = txtA + txtB;
    Double.parseDouble(txtA.getText().toString());
    lblAnswer.setText"t
}

and here is the xml file.

   <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"
              tools:context=".MainActivity" >

    <EditText
           android:id="@+id/txtA"
           android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
             android:layout_centerHorizontal="true"
             android:layout_marginTop="40dp"
            android:ems="10"
            android:hint="Write Here"
            android:inputType="numberDecimal" >

    <requestFocus />
</EditText>

<EditText
    android:id="@+id/txtB"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/txtA"
    android:layout_below="@+id/txtA"
    android:layout_marginTop="32dp"
    android:ems="10"
    android:hint="Second Here"
    android:inputType="numberDecimal" />

<Button
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_centerVertical="true"
    android:text="@string/calculate" 
    android:onClick="doCalc"/>

<TextView
    android:id="@+id/lblAnswer"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/button1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="50dp"
    android:text="TextView" />

    </RelativeLayout>

Answer 1

Your code is missing a few key components. Review your code, and review the one I have prepared below.

package com.example.wattsprofessional;

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

public class MainActivity extends Activity {

    private EditText txtA, txtB;
    private Button button1;
// ^ we have declared these as fields up here so that we can access them throughout the page, past all the curly brackets

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

        txtA = (EditText) findViewById(R.id.txtA);
        txtB = (EditText) findViewById(R.id.txtB);
        button1 = (Button) findViewById(R.id.button1);
        // ^ this is where we initialize these. You did the xml correctly, but you still need to hook the java to it.
        // it allows us to use any names and locations we like not just same ones.
        // basically you say what it is (Button) and then use the following method to look for the id that you wrote in the xml

        initButton();
        // i made this listener so we'd have time. this is the oncreate method and is called instantly.
        // if we called doCalc here, we'd have no time to put numbers in.
    }

    private void initButton() {
        button1.setOnClickListener(new OnClickListener() {
            // this one performs an action when our button is clicked. it performs whatever is below
            @Override
            public void onClick(View v) {
                String strA = txtA.getText().toString();
                String strB = txtB.getText().toString();
                // we get our strings from our editexts. i think you know how to do this well.

                Double dblAnswer = doCalc(strA, strB);              
                // ^we pass them to our method, it does all the heavy lifting for us. and spits an answer for us.
                TextView lblAnswer = (TextView) findViewById(R.id.lblAnswer);
                // this is a local variable, as opposed to a field. i made so you know that you can do it like this - with the whole line here
                // the disadvantage is that we can't do anything to it outside of this curly bracket. but there are performs gains.
                // in general it's wasteful to use fields when you can suffice with local variable
                String answer = String.valueOf(dblAnswer);
                // we get our answer and turn it to a string.
                lblAnswer.setText(answer);
                // finally we set our result to the textView.
            }
        });
    }

    public double doCalc(String a, String b) {
        // a and b are both variables. they refer to the stuff we put in
        double dblA = Double.parseDouble(a);
        double dblB = Double.parseDouble(b);
        // we're gonna make both of these numbers so we can add them. right now they're just text.
        return dblA + dblB;
        // ^ this statement means that this method will spit a number out when it's done which we can use however.
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

}

Answer 2

In order to get a Double value from an EditText, you'll need to use Double.parseDouble(txtA.getText().toString()). To set the text, you can use lblAnswer.setText("text").

In addition, the easiest way to call this from a button would be to set its android:onClick attribute in the XML, such as android:onClick="doCalc".

EDIT: You also need to create references to your objects. Before your onCreate(), put:

EditText txtA;
EditText txtB;
TextView lblAnswer;

Then inside your onCreate() you need to initialize the objects:

txtA = new (EditText)findViewById(R.Id.txtA);
txtB = new (EditText)findViewById(R.Id.txtB);
lblAnswer = new (TextView)findViewById(R.Id.lblAnswer);