generics - What does <T> (angle brackets) mean in Java?

Question

I am currently studying Java and have recently been stumped by angle brackets(<>). What exactly do they mean?

public class Pool<T>{
    public interface PoolFactory<T>{
        public T createObject();
    }
    this.freeObjects = new ArrayList<T>(maxsize)
}

What does the <T> mean? Does it means that I can create an object of type T?

Answer 1

<T> is a generic and can usually be read as "of type T". It depends on the type to the left of the <> what it actually means.

I don't know what a Pool or PoolFactory is, but you also mention ArrayList<T>, which is a standard Java class, so I'll talk to that.

Usually, you won't see "T" in there, you'll see another type. So if you see ArrayList<Integer> for example, that means "An ArrayList of Integers." Many classes use generics to constrain the type of the elements in a container, for example. Another example is HashMap<String, Integer>, which means "a map with String keys and Integer values."

Your Pool example is a bit different, because there you are defining a class. So in that case, you are creating a class that somebody else could instantiate with a particular type in place of T. For example, I could create an object of type Pool<String> using your class definition. That would mean two things:

• My Pool<String> would have an interface PoolFactory<String> with a createObject method that returns Strings.
• Internally, the Pool<String> would contain an ArrayList of Strings.

This is great news, because at another time, I could come along and create a Pool<Integer> which would use the same code, but have Integer wherever you see T in the source.