how to change image when click on button in android

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:background="#b43" >
<ImageView
   android:id="@+id/iv"
   android:layout_width="200dp"
   android:layout_height="200dp"
   android:layout_alignParentTop="true"
   android:layout_centerHorizontal="true"
   android:layout_marginTop="72dp"
/>
<Button
   android:id="@+id/bt"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   android:layout_below="@+id/iv"
   android:layout_centerHorizontal="true"
   android:onClick="change_image"
   android:textSize="20sp"
   android:text="Change image" />
</RelativeLayout>

MainActivity.java

import android.os.Bundle;
import android.view.View;
import android.widget.ImageView;
import android.app.Activity;

public class MainActivity extends Activity {
  Boolean flag=false;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
  }

  //mess method is declared in XML file
  //This function will call when we click on button
  //and we have to pass View object in this method
  //which will take id of clicked button

  public void change_image(View v)
  {
    ImageView iv=(ImageView)findViewById(R.id.iv);
    //use flag to change image
    if(flag==false)
  {
      iv.setImageResource(R.drawable.myimage);
      flag=true;
    }
    else
    {
      iv.setImageResource(R.drawable.myimage2);
      flag=false;
    }
  }
}