Question

this morning I came across this code, and I have absolutely no idea what that means. Can anyone explain me what do these <T> represent? For example:

public class MyClass<T>
...

some bits of code then 

private Something<T> so;

private OtherThing<T> to;

private Class<T> c;

Thank you

Answer 1

You have bumped into “generics”. They are explained very nicely in this guide.

In short, they allow you to specify what type that a storage-class, such as a List or Set contains. If you write Set<String>, you have stated that this set must only contain Strings, and will get a compilation error if you try to put something else in there:

Set<String> stringSet = new HashSet<String>();
stringSet.add("hello"); //ok.
stringSet.add(3);
      ^^^^^^^^^^^ //does not compile

Furthermore, another useful example of what generics can do is that they allow you to more closely specify an abstract class:

public abstract class AbstClass<T extends Variable> {

In this way, the extending classes does not have to extend Variable, but they need to extend a class that extends Variable.

Accordingly, a method that handles an AbstClass can be defined like this:

public void doThing(AbstClass<?> abstExtension) {

where ? is a wildcard that means “all classes that extend AbstClass with some Variable“.