Java Generics and Collections, 2nd Edition

Chapter 3. Declarations

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This chapter discusses how to declare a generic class. It describes constructors, static members, and nested classes, and it fills in some details of how erasure works.

Constructors

In a generic class, type parameters appear in the header that declares the class, but not in the constructor:

class Pair<T,U> {
  private final T first;
  private final U second;
  public Pair(T first, U second) {
        this.first = first;
        this.second = second;
  }
  public T getFirst() { return first; }
  public U getSecond() { return second; }
}

The type parameters T and U are declared at the beginning of the class, not in the constructor. However, actual type parameters are passed to the constructor whenever it is invoked:

Pair<String, Integer> pair1 = new Pair<String, Integer>("one",2);
assert pair1.getFirst().equals("one") && pair1.getSecond() == 2;

Look Out for This!   A common mistake is to forget the type parameters when invoking the ...