Java Slick2D Platformer Collision Help

[thejava]

So far I've gotten this much code but now I am stuck with the left and right movement collisions. I don't even know how I got the jumping collision to work but I really need help.

package state;

import java.util.ArrayList;

import org.newdawn.slick.Color;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.Input;
import org.newdawn.slick.SlickException;
import org.newdawn.slick.geom.Rectangle;
import org.newdawn.slick.geom.Vector2f;
import org.newdawn.slick.state.BasicGameState;
import org.newdawn.slick.state.StateBasedGame;

public class StateMenu extends BasicGameState
{
	private int id;
	private Vector2f playerPos;
	private Vector2f enemyPos;
	private Vector2f enemyPos2;
	private Rectangle playerCol;
	private Rectangle enemyCol;
	private Rectangle enemyCol2;
	private float velocityY;
	private float gravity = 0.365f;
	private boolean isJumping = false;
	private boolean onGround = false;
	
	public StateMenu(int id)
	{
		this.id = id;
	}
	@Override
	public void init(GameContainer gc, StateBasedGame sbg) throws SlickException
	{
		playerPos = new Vector2f(0, 0);
		enemyPos = new Vector2f(0, 150);
		enemyPos2 = new Vector2f(50, 100);
		playerCol = new Rectangle(playerPos.getX(), playerPos.getY(), 50, 50);
		enemyCol = new Rectangle(enemyPos.getX(), enemyPos.getY(), 50, 50);
		enemyCol2 = new Rectangle(enemyPos2.getX(), enemyPos2.getY(), 50, 50);
	}
	@Override
	public void render(GameContainer gc, StateBasedGame sbg, Graphics g) throws SlickException
	{
		g.setColor(Color.green);
		g.fillOval(playerPos.getX(), playerPos.getY(), 50, 50);
		g.setColor(Color.blue);
		g.fillRect(enemyPos.getX(), enemyPos.getY(), 50, 50);
		g.setColor(Color.red);
		g.fillRect(enemyPos2.getX(), enemyPos2.getY(), 50, 50);
	}
	@Override
	public void update(GameContainer gc, StateBasedGame sbg, int delta) throws SlickException
	{
		Input input = gc.getInput();
		if (input.isKeyDown(Input.KEY_SPACE) && isJumping == false)
		{
			velocityY = 7.5f;
			isJumping = true;
		}
		if (input.isKeyDown(Input.KEY_SPACE) && onGround == true)
		{
			isJumping = false;
			onGround = false;
			velocityY = 7.5f;
			playerPos.set(playerPos.getX(), playerPos.getY() - velocityY);
		}
		if (isJumping)
		{
			if (!playerCol.intersects(enemyCol) && !playerCol.intersects(enemyCol2))
			{
				velocityY -= gravity;
				playerPos.set(playerPos.getX(), playerPos.getY() - velocityY);
			}
			else
			{
				onGround = true;
			}
		}
		if (input.isKeyDown(Input.KEY_ESCAPE))
		{
			gc.exit();
		}
		if (input.isKeyDown(Input.KEY_LEFT))
		{
			playerPos.set(playerPos.getX() - 2, playerPos.getY());
		}
		if (input.isKeyDown(Input.KEY_RIGHT))
		{
			playerPos.set(playerPos.getX() + 2, playerPos.getY());
		}
		playerCol.setX(playerPos.getX());
		playerCol.setY(playerPos.getY());
	}
	@Override
	public int getID()
	{
		return id;
	}
}