An implementation of a Java connection pool (Confused Coders)

ConnectionPool.java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;



/** A Connection Pool with 5 Available Connections **/
class ConnectionPool {

	private List<Connection>availableConnections = 
								new ArrayList<Connection>();
	private List<Connection>usedConnections = new ArrayList<Connection>();
	private final int MAX_CONNECTIONS = 5;

	private String URL;
	private String USERID;
	private String PASSWORD;


	/** Initialize all 5 Connections and put them in the Pool **/
	public ConnectionPool(String Url, String UserId, String password)
			throws SQLException {
		this.URL = Url;
		this.USERID = UserId;
		this.PASSWORD = password;

		for (int count = 0; count <MAX_CONNECTIONS; count++) {
			availableConnections.add(this.createConnection());
		}

	}






/** Private function, 
	used by the Pool to create new connection internally **/

	private Connection createConnection() throws SQLException {
		return DriverManager
				.getConnection(this.URL, this.USERID, this.PASSWORD);
	}




	/** Public function, used by us to get connection from Pool **/
	public Connection getConnection() {
		if (availableConnections.size() == 0) {
			System.out.println("All connections are Used !!");
			return null;
		} else {
			Connection con = 
			availableConnections.remove(
				availableConnections.size() - 1);
			usedConnections.add(con);
			return con;
		}
	}



	/** Public function, to return connection back to the Pool **/
	public boolean releaseConnection(Connection con) {
		if (null != con) {
			usedConnections.remove(con);
			availableConnections.add(con);
			return true;
		}
		return false;
	}





	/** Utility function to check the number of Available Connections **/
	public int getFreeConnectionCount() {
		return availableConnections.size();
	}
}





/** Test Class for testing Connection Pool **/
public class ConnectionPoolTest {

	public static void main(String[] args) throws SQLException {

		ConnectionPool pool
			 = new ConnectionPool("jdbc:mysql://localhost/mydb",
				"testusr", "somepwd");
		Connection con1 = pool.getConnection();
		Connection con2 = pool.getConnection();
		System.out.println(pool.getFreeConnectionCount());
		Connection con3 = pool.getConnection();
		Connection con4 = pool.getConnection();
		Connection con5 = pool.getConnection();
		Connection con6 = pool.getConnection();
		System.out.println(pool.getFreeConnectionCount());
		pool.releaseConnection(con1);
		pool.releaseConnection(con2);
		pool.releaseConnection(con4);
		System.out.println(pool.getFreeConnectionCount());
	}

}

Below statement has a time complexity of O(n)
usedConnections.remove(con);

We can do this by O(1) by using Double linked list and hashmap.

Please note both releaseConnection() and getConnection() must be of O(1).