How to get post thumbnail in wordpress?

<?php 
if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
  the_post_thumbnail();
} 
?>
<?php the_content(); ?>

this is the default method to retrieve post thumbnail

<?php 
if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
  the_post_thumbnail(array(150,150));
} 
?>
<?php the_content(); ?>
this is the method which is used to alter that thumbnail like size in array 150px & 150px