How To Declare a Button in android source code

YOUR XML FILE SHOULD BE LIKE THIS

named as 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/activity_main" 

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="contctsofvips.fun.stack.com.buttonclick.MainActivity">

    <Button     

android:layout_width="wrap_content"

android:layout_height="wrap_content" 

android:background="@drawable/apps"

android:id="@+id/btn1"         />

</RelativeLayout>

Your Main Activity should be Like this

named as  ActivityMain

package contctsofvips.fun.stack.com.buttonclick;

package contctsofvips.fun.stack.com.buttondesign;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

import com.dd.processbutton.iml.ActionProcessButton;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // get the button view
      
            public void onClick(View view) {
               // Do Your Code
                }
            }
        });
    }
}