Java Generics and Collections

Chapter 1. Introduction

Generics and collections work well with a number of other new features introduced in the latest versions of Java, including boxing and unboxing, a new form of loop, and functions that accept a variable number of arguments. We begin with an example that illustrates all of these. As we shall see, combining them is synergistic: the whole is greater than the sum of its parts.

Taking that as our motto, let’s do something simple with sums: put three numbers into a list and add them together. Here is how to do it in Java with generics:

List<Integer> ints = Arrays.asList(1,2,3);
int s = 0;
for (int n : ints) { s += n; }
assert s == 6;

You can probably read this code without much explanation, but let’s touch on the key features. The interface List and the class Arrays are part of the Collections Framework (both are found in the package java.util). The type List is now generic; you write List<E> to indicate a list with elements of type E. Here we write List<Integer> to indicate that the elements of the list belong to the class Integer, the wrapper class that corresponds to the primitive type int. Boxing and unboxing operations, used to convert from the primitive type to the wrapper class, are automatically inserted. The static method asList takes any number of arguments, places them into an array, and returns a new list backed by the array. The new loop form, foreach, is used to bind a variable successively to each element of the list, and the loop body adds these into ...