Shield

Sword

Create an Actor subclass for the Shield. Give it an instance 'int' field for the timer and assign some value (maybe 800) to it. Have the act method decrement the timer and check for zero for removal of object from world. That should give it a lifespan of about 15 seconds.

The powerup for a shield should be a different actor (from a different class). Your world class should control when and where it should be placed in the world. The class of the powerup should control its lifespan similar to how the shield does it. Again, 'Greenfoot.getRandomNumber(int)' should be used to set the timer that regulates the spawning of the powerup (and which powerup, if you have more than one kind).

Thanks but im have to create a game for an assingment and our teacher gave us no advice on how to make a powerup and it's my first year doing i.s.t so i don't really get this advanced stuff, do you mind writing out the code or explain it in simpler words

Sounds like you need to look over the first three section (skip over what you are already familiar with) of the Java tutorials. What I suggested above was fairly basic and if you are having trouble understanding it, you should review those sections.