The syntax of Generics in Java

Generics are simple to start with; here is IndexedList ADT with generic base type

public interface IndexedList<T> {

  void put(int index, T value);

  T get(int index);

  int length();
}

Note the three changes:

• The interface name is appended with <T> (notice the angle brackets).
• The data type of value is changed to T.
• The return type of get is changed to T.

The T is a placeholder for any type for which a user might want to use IndexedList. There is no significance in calling it T. You can rename it to anything you want (subject to rules of variable naming in Java). However, in most resources, either T or E is used to declare a generic type.

When declaring IndexedList, you must specify the intended base type:

IndexedList<Integer> numbers;
IndexedList<Apple> apples;

Notice the use of angle brackets <> in the above statements.

When you declare numbers as IndexedList<Integer>, you signal your intention to the compiler. All values stored in numbers must be of type Integer. In turn, the compiler will

1. ensure that (providing type safety), so attempting to store, e.g., a string in numbers will result in a compile-time error.
2. not bother you to (down) cast a value retrieved from the ADT (e.g., through using the get method here).

A note on documenting generics

Generic variables must be documented similar to method parameters.

/**
 * IndexedList ADT.
 * @param <T> the base type of the items in the IndexedList.
 */
public interface IndexedList<T> {

}

In a way, generics enable types (classes and interfaces) to have parameters.