If you just have simple, axis-aligned walls, then just compare the distance between the enemy and wall with the size of the enemy. If the distance becomes less, then it has hit the wall. After that, just reverse the direction of the enemy. That's the most simple of collision methods. Others involve swept collisions, which work regardless of an object's speed.

problem solved
just stored the values like
GTC[0]=x;
GTC[1]=y;
before the collsion and where it collides
if (a instanceof Monster ) {
x = GTC[0];
y = GTC[1];
}