learn how to set a clicklistener on a button in android

As we want to perform an event of click listener on a button so first of all you should declare a widget of button in your xml file such as
name as ist_activity

<?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/ist_activity"

 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.stck.fun.contactsofvipsIstActivity">
    <Button 

  android:layout_width="wrap_content"

  android:layout_height="wrap_content"

  android:id="@+id/btn1"
 android:text="Hello World!" 

/>
</RelativeLayout>

then you should go to your java file.and your java file should be like this

named as IstJava

package com.stck.fun.contactsofvips;

import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

public class IstActivityextends AppCompatActivity  {

    Button btn1;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.ist_activity);
        btn1=(Button) findViewById(R.id.btn1);

    btn1.setOnClickListener(new View.onClickListener){

   pulic void onClick(View v)

{

////perform your desired work

});

}

}

Should Mention tour activity name in android studio's xml file

mine is following

<?xml version="1.0" encoding="utf-8"?><manifest 
xmlns:android="http://schemas.android.com/apk/res/android"
package="contctsofvips.fun.stack.com.myapp">
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".IstActivity">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
   </intent-filter>
        </activity>
    </application>

</manifest>