Java Generics and Collections, 2nd Edition

Chapter 4. Declarations

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.

Note

The code examples for this chapter can be found at:

https://github.com/MauriceNaftalin/JGC_2e_Book_Code/blob/main/src/main/java/org/jgcbook/chapter04

Constructors

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

org/jgcbook/chapter04/A_constructors/Pair

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, but actual type arguments are passed to the constructor whenever it is invoked:

org/jgcbook/chapter04/A_constructors/Pair

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

A common mistake is to forget the type parameters when invoking the constructor:

org/jgcbook/chapter04/A_constructors/Pair

Pair pair2 = new Pair<String, Integer>("one", 2);

This mistake produces a warning when compiled with the option -Xlint, which enables detailed warnings:

% javac -Xlint Pair.java Pair.java:18: warning: [rawtypes] found raw type: Pair Pair pair2 = new Pair<String, Integer>("one", ...