Step 1. in activity_main.xml file copy the following code.

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

    <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Enter First number"
      android:id="@+id/t1" />

    <EditText
        android:layout_width="103dp"
        android:layout_height="wrap_content"
        android:id="@+id/n1"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Enter second number"
        android:id="@+id/t2"
        android:layout_gravity="center_vertical"
        android:layout_below="@+id/n1"
        android:layout_toStartOf="@+id/n1"
        android:layout_marginTop="39dp" />

    <EditText
        android:layout_width="92dp"
        android:layout_height="wrap_content"
        android:id="@+id/n2"
        android:layout_gravity="center_vertical"
        android:layout_alignTop="@+id/t2"
        android:layout_centerHorizontal="true" />

    <EditText
        android:layout_width="209dp"
        android:layout_height="wrap_content"
        android:id="@+id/n3"
        android:layout_gravity="center_vertical"
        android:layout_below="@+id/n2"
        android:layout_alignRight="@+id/n2"
        android:layout_alignEnd="@+id/n2"
        android:layout_marginTop="36dp" />

    <Button
        android:text="Click here"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/btn"
        android:layout_gravity="bottom"
        android:layout_centerVertical="true"
        android:layout_toRightOf="@+id/t1"
        android:layout_toEndOf="@+id/t1" />


</RelativeLayout>

step2: in the MainActivity.java enter following code

 package app.mitindia.com.aop;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;


public class MainActivity extends ActionBarActivity {

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


        final EditText ed1=(EditText)findViewById(R.id.n1);
        final EditText ed2=(EditText)findViewById(R.id.n2);
        final EditText ed3=(EditText)findViewById(R.id.n3);

        Button btn=(Button)findViewById(R.id.btn);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                int x=new Integer(ed1.getText().toString());
                int y=new Integer(ed2.getText().toString());
                int sum=x+y;
                ed3.setText(String.valueOf(sum));

            }
        });


    }

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

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Step 3: Now, Run the application see the following output.

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