conn! The difference between =null and conn.isclosed

Tell me the difference between these two pieces of code:
1, Conn! = NULL This is not sure whether the above connection is initialized and judged, because a lot of time to write code, using lazy loading mode, usually first define a connection conn=null, use the time to initialize its object, This can reduce the waste of memory. This time if you are not sure if Conn is initialized, it is best to add conn! = NULL before closing, otherwise it is possible to close conn with a null pointer error.

2, conn.isclosed () This is to determine whether the current connection is closed, the first to ensure that connection has generated an instance.

From a perfect point of view of the program, when closing the connection, it is best to judge two of them:
if (conn!=null&&!conn.isclosed ()) {
Conn.close ();
}

conn! The difference between =null and conn.isclosed