//when getting powerup
Wolf.slow = true;
//use some variable as a timer and when the power up time is over (unless it should be for ever)
Wolf.slow = false;

public static boolean slow = false;
private int slowTimer = 0;

//beginning of act method
if (slow)
{
    if (slowTimer > 0)
    {
        slowTimer--;
        return;
    }
    slowTimer = 1; //the higher the number, the slower the wolves
}
//rest of the act method

//collected power up
Wolf.speed *= 0.8; //set the speed to 80 %

//power up over
Wolf.speed *= 1.25; //set it back to 100 %

public static double speed = 5; //current speed ("100 %")

//when moving
move(speed);

/**  in Sheep class act method (or method it calls) */
if (<collected slow powerup>)
{
    Wolf.slowDown();
}

/** in Wolf class */
public static boolean slow;
public static int slowTimer;

public static void slowdown()
{
    if (!slow)
    {
        slow = true;
        slowTimer = 200; // adjust value as needed
    }
}

/** in World class act method (or a method it calls) */
if (Wolf.slow)
{
    Wolf.slowTimer--;
    if (Wolf.slowTimer == 0) Wolf.slow = false;
}

/** in Wolf class act method (or a method it calls) */
if (slow) move(3); else move(5);

/** in Sheep class act method (or method it calls) */
if (<collected slow powerup>)
{
    Wolf.slowTimer = 200; // adjust value as needed
}

/** in Wolf class */
public static int slowTimer;

/** in Wolf class act method (or a method it calls) */
if (slowTimer > 0) move(3); else move(5);

/** in World class act method (or a method it calls) */
if (Wolf.slowTimer > 0) Wolf.slowTimer--;