Create an application with three buttons vertically aligned, on selecting a button color of the screen will change.

/*Create an application with three buttons vertically aligned, on selecting a button color of the screen will change*/

File Name: activity_main.xml

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:id="@+id/RL1"

    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">

    <Button

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:id="@+id/btn1"

        android:text="Red"

        android:layout_centerHorizontal="true"/>

    <Button

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:layout_below="@+id/btn1"

        android:layout_centerHorizontal="true"

        android:id="@+id/btn2"

        android:layout_marginTop="30dp"

        android:text="Blue"/>

    <Button

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:id="@+id/btn3"

        android:text="Green"

        android:layout_below="@+id/btn2"

        android:layout_centerHorizontal="true"

        android:layout_marginTop="30dp"/>

</RelativeLayout>    

File Name: MainActivity.java

package com.ducskecode.color_changer_vertical;

import android.graphics.Color;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.view.Menu;

import android.view.MenuItem;

import android.view.View;

import android.widget.Button;

import android.widget.RelativeLayout;

import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    Button b1,b2,b3;

    RelativeLayout r1;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        b1=(Button)findViewById(R.id.btn1);

        b2=(Button)findViewById(R.id.btn2);

        b3=(Button)findViewById(R.id.btn3);

        r1=(RelativeLayout)findViewById(R.id.RL1);

        b1.setOnClickListener(new View.OnClickListener(){

            public void onClick(View v)

            {

                r1.setBackgroundColor(Color.RED);

            }

        });

        b2.setOnClickListener(new View.OnClickListener(){

            public void onClick(View v)

            {

                r1.setBackgroundColor(Color.BLUE);

            }

        });

        b3.setOnClickListener(new View.OnClickListener(){

            public void onClick(View v)

            {

                r1.setBackgroundColor(Color.GREEN);

            }

        });

    }

        public void send()

        {

            Toast.makeText(this,"color will change now",Toast.LENGTH_LONG).show();

        }

    @Override

    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.menu_main, menu);

        return true;

    }

    @Override

    public boolean onOptionsItemSelected(MenuItem item) {

        int id = item.getItemId();

        if (id == R.id.action_settings) {

            return true;

        }

        return super.onOptionsItemSelected(item);

    }

}