[Android] Basic Calculator

///////////////////////
// MainActivity.java //
//////////////////////

package com.example.basicoperations;

import android.support.v7.app.ActionBarActivity;
import android.widget.*;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;


public class MainActivity extends ActionBarActivity {

    EditText a, b;
    Button add, sub, mul, div;
    TextView op;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        a = (EditText) findViewById(R.id.a);
        b = (EditText) findViewById(R.id.b);
        add = (Button) findViewById(R.id.add);
        sub = (Button) findViewById(R.id.sub);
        mul = (Button) findViewById(R.id.mul);
        div = (Button) findViewById(R.id.div);
        op = (TextView) findViewById(R.id.op);

        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                float n1=Float.parseFloat(a.getText().toString());
                float n2=Float.parseFloat(b.getText().toString());

                op.setText(String.valueOf(n1+n2));
            }
        });

        sub.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                float n1=Float.parseFloat(a.getText().toString());
                float n2=Float.parseFloat(b.getText().toString());

                op.setText(String.valueOf(n1-n2));
            }
        });

        mul.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                float n1=Float.parseFloat(a.getText().toString());
                float n2=Float.parseFloat(b.getText().toString());

                op.setText(String.valueOf(n1*n2));
            }
        });

        div.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                float n1=Float.parseFloat(a.getText().toString());
                float n2=Float.parseFloat(b.getText().toString());

                op.setText(String.valueOf(n1/n2));
            }
        });
    }


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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}


///////////////////////
// activity_main.xml //
//////////////////////

<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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.basicoperations.MainActivity" >

    <EditText
        android:id="@+id/a"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="164dp"
        android:ems="10"
        android:hint="@string/input1"
        android:inputType="numberDecimal" >

        <requestFocus />
    </EditText>

    <EditText
        android:id="@+id/b"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/a"
        android:layout_below="@+id/a"
        android:layout_marginTop="44dp"
        android:ems="10"
        android:hint="@string/input2"
        android:inputType="numberDecimal" />

    <Button
        android:id="@+id/add"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/b"
        android:layout_below="@+id/b"
        android:layout_marginTop="39dp"
        android:text="@string/add" />

    <Button
        android:id="@+id/mul"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/add"
        android:layout_alignParentBottom="true"
        android:text="@string/mul" />

    <Button
        android:id="@+id/div"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/mul"
        android:layout_alignBottom="@+id/mul"
        android:layout_toRightOf="@+id/b"
        android:text="@string/divide" />

    <Button
        android:id="@+id/sub"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/add"
        android:layout_alignBottom="@+id/add"
        android:layout_alignRight="@+id/div"
        android:text="@string/sub" />

    <TextView
        android:id="@+id/op"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/mul"
        android:layout_toRightOf="@+id/mul"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</RelativeLayout>