Button

public class MyActivity extends Activity {
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);

         setContentView(R.layout.activity_main);

         final Button button = (Button) findViewById(R.id.button_id);
         button.setOnClickListener(new View.OnClickListener() {
             public void onClick(View v) {
                 //statements to perform task on button click
             }
         });
     }
 }

1. Create a project in eclipse as android application 

Create a Project

2. Now Drag and drop the Button widget in activity_main.xml

3.MainActivity.java

Go to src folder and open MainActivity.java and Put the following code into that java file :

package com.example.buttonui;

import android.app.Activity;

import android.os.Bundle;

import android.view.View;

import android.widget.Button;

import android.widget.Toast;

public class MainActivity extends Activity {

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

 

Button btn ;

btn=(Button) findViewById(R.id.button1);

btn.setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

Toast.makeText(getApplicationContext(), "I am Click", Toast.LENGTH_LONG).show();

}

});

}

}

3. Run the Application as Android Application on your Emulator or on device.

Explanation 

1. Button btn ; 

This line of code create the button object which you are going to use into your application.

2. btn.(Button)findViewById(R.id.button1);

This line Demonstrate that btn is a Button type object and button1 is the id by which btn is going to used in our application.

3.btn.setOnclickListner(new View.onClickListener(){

public void onClick(View v)

{

here you can put your code which is going to perform when click

 event occur 

}

);

4. View.onClickListener 

Interface definition for callback to perform some action when user clicked that view.

Button