There should be more to the Key class than what you are showing. You should have an 'act' method that will check for the key stored in the 'key' field and play the sound stored in the 'sound' field if that key is down. It will at the same time set the 'isDown' field to true. Another check will look for the key to NOT be down and reset the 'isDown' field back to false. The world does not need to know when these events occur; only the Key objects do. The argument values brought into the constructor ('keyName' and 'soundFile') are only local variables (only the constructor is aware of them). The Key object being created needs to save those values in its fields ('key' and 'sound') so that it can use them later (after the Key object is added into the world). The two String objects, 'keyName' and 'soundFile', must be declared as parameters to the constructor call in order for those values to be passed from your world object to the Key object being created. Having them in the "( )" informs the compiler as to how many variables and of what type are to be passed when using that construct. Please refer to Java tutorial page on Passing arguments to methods and constructors for more detailed information. You may want to check out other pages within the tutorial by selecting them from the list on the left side of the window.

Dear Danpost, first of all, thanks for your helpful reply, I begin to understand the concept of passing arguments to constructors. I've tried this concept with the piano scenario with a third string array ("test") just for testing purposes, and it went well: However, whenever I try to add a different argument in the Key key = new Key ...)-line, like a private int I declared on top of the world class, Greenfoot won't compile, stating "constructor Key in class Key cannot be applied to given types; required: java.lang.String.java.lang.String.java.lang.String.int; found: java.lang.String.java.lang.String.jaa.lang.string; int; int; reason: actual formal argument lists differ in length" My world class looks like this: in my Key class, it looks like this: numberKey doesn't serve any useful purpose; it is just a trial to implement an int in the loop/constructor. It seems the loop in the world class only accepts Strings as arguments and no int. But why? Thanks in advance!

The problem is that line 5, 'public Key(String keyName, String soundFile, String testKey, int numberKey)', tells the compiler that three Strings and one int are to be passed to create a new Key. If you pass anything other than that, like only two Strings and one int, it will complain.