Calculator

 `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" >

    <TextView

        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:text="Calc"
        android:textSize="45dp"
        android:id="@+id/tvDisplay"
        />

     <Button
         android:id="@+id/itwo"
         android:layout_width="50dp"
         android:layout_height="50dp"
         android:layout_alignParentRight="true"
         android:layout_below="@+id/tvDisplay"
         android:layout_marginRight="42dp"
         android:layout_marginTop="51dp"
         android:text="2"
         android:textSize="25dp" />

     <Button
         android:id="@+id/iplus"
         android:layout_width="50dp"
         android:layout_height="50dp"
         android:layout_alignBaseline="@+id/itwo"
         android:layout_alignBottom="@+id/itwo"
         android:layout_centerHorizontal="true"
         android:text="+"
         android:textSize="25dp" />

     <Button
         android:id="@+id/ione"
         android:layout_width="50dp"
         android:layout_height="50dp"
         android:layout_alignBaseline="@+id/iplus"
         android:layout_alignBottom="@+id/iplus"
         android:layout_alignParentLeft="true"
         android:layout_marginLeft="32dp"
         android:text="1"
         android:textSize="25dp" />

     <Button
         android:id="@+id/iequal"
         android:layout_width="50dp"
         android:layout_height="wrap_content"
         android:layout_alignLeft="@+id/iplus"
         android:layout_below="@+id/iplus"
         android:layout_marginTop="84dp"
         android:text="="
         android:textSize="25dp" />

</RelativeLayout>`







java:
           `package com.example.calc;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import com.example.calc.R;;
public class MainActivity extends Activity {
    int sum=0;
	Button one,two,plus,equal;
	TextView display;
    int counter=0;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        one=(Button) findViewById(R.id.ione);
        two=(Button) findViewById(R.id.itwo)	;
        plus=(Button) findViewById(R.id.iplus);
        equal=(Button) findViewById(R.id.iequal);
        display=(TextView) findViewById(R.id.tvDisplay);

        one.setOnClickListener(new View.OnClickListener() {

			public void onClick(View v) {
				// TODO Auto-generated method stub
				counter=counter*10 +1;
	         //  display.setText(counter);
			}
		});

           two.setOnClickListener(new View.OnClickListener() {

			public void onClick(View v) {
				// TODO Auto-generated method stub
				counter=counter*10 +2;
				//display.setText(counter);
			}
		});
           plus.setOnClickListener(new View.OnClickListener() {

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

   				sum=counter+sum;
   		counter=0;
   			}
   		});
           equal.setOnClickListener(new View.OnClickListener() {

   			public void onClick(View v) {
   				// TODO Auto-generated method stub
   				counter=0;
   				display.setText(sum);

   			}
   		});

    }

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