Android Button OnClick Tutorial

activity_main.xml

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp">

    <Button
        android:id="@+id/button1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:text="Button1"
        android:onClick="showMessage"/>

    <Button
        android:id="@+id/button2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button2"
        android:layout_below="@+id/button1"
        android:layout_marginTop="32dp"/>

    <Button
        android:id="@+id/button3"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button3"
        android:layout_below="@+id/button2"
        android:layout_marginTop="32dp"/>

    <Button
        android:id="@+id/button4"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button4"
        android:layout_below="@+id/button3"
        android:layout_marginTop="32dp"/>

</RelativeLayout>

MainActivity.java

package com.devahoy.android.sample.button;

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends ActionBarActivity implements View.OnClickListener {

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

        Button button2 = (Button) findViewById(R.id.button2);
        button2.setOnClickListener(this);

        Button button3 = (Button) findViewById(R.id.button3);
        button3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(getApplicationContext(),
                        "Button3 Clicked!", Toast.LENGTH_SHORT).show();
            }
        });

        Button button4 = (Button) findViewById(R.id.button4);
        button4.setOnClickListener(Button4OnClick);

    }

    public void showMessage(View view) {
        Toast.makeText(this, "Button1 Clicked!", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onClick(View v) {
        Toast.makeText(this, "Button2 Clicked!", Toast.LENGTH_SHORT).show();
    }

    View.OnClickListener Button4OnClick = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(getApplicationContext(),
                    "Button4 Clicked!", Toast.LENGTH_SHORT).show();
        }
    };
}

Hey, i want to create a Java class (none activity) so how can i make it in the class (method)