Objects which go out of scope will be deleted by the garbage collector. If you reassign a variable holding an object, if that's the only reference to the object, it will be garbage collected as well. So at worse, to get rid of an object held by a global variable, assign it a null value.
Note that there is no guarantee on the process of garbage collecting, when it will happen. It always happens when you are near maximum allocated memory.

Note that there is no guarantee on the process of garbage collecting, when it will happen. It always happens when you are near maximum allocated memory.

wow, that sounds really bad.

Can't the garbage collector jump in at once / immediately? Or is it hard for the compiler to determine whether it's really possible / wise to get rid of the object?

Thanks!

Chrisir

Well, it is rarely an issue. After all, as long as you are below the maximum allocated memory, there is no problem.
And I was a bit wrong, garbage collecting also happen at regular intervals. You just have little to no control over this process. Little in Java, none (or almost) in Processing, unless you run the jar yourself (you can pass GC parameters to the Java runtime, but it is a kind of voodoo).

If you really need to recollect memory immediately after clearing the reference to a large image, for example (or large text, etc.), you can call System.gc().
But real Java programs rarely, if ever, call this function.

Side note: in the Java SDK, you have a program called JVisualVM (that can be also downloaded directly at the given link, if you don't have it or if you want a more recent version).
It allows you to see, among other things, the process of garbage collection. It can be interesting to monitor the memory consumption of your sketch.