ArrayLists - Building Java Programs: A Back to Basics Approach - page 643

Table 10.3 Common Wrapper Classes

Primitive type Wrapper class

int Integer

double Double

char Character

boolean Boolean

Notice that you can write a for-each loop to use a variable of type int even though
the ArrayList stores values of type Integer . Java will unbox the objects and per-
form the appropriate conversions for you.
Because Java has boxing and unboxing, the only place you generally need to use
the wrapper class is when you describe a type like ArrayList<Integer> . You can't
actually declare it to be of type ArrayList<int> , but you can manipulate it as if it is
of type ArrayList<int> .
Table 10.3 lists the major primitive types and their corresponding wrapper classes.

10.2 The Comparable Interface

The method Collections.sort can be used to sort an ArrayList . It is part of the
java.util package. The following short program demonstrates how to use
Collections.sort :

1 // Constructs an ArrayList of Strings and sorts it.
2
3 import java.util.*;
4
5 public class SortExample {
6 public static void main(String[] args) {
7 ArrayList<String> words = new ArrayList<String>();
8 words.add("four");
9 words.add("score");
10 words.add("and");
11 words.add("seven");
12 words.add("years");
13 words.add("ago");
14
15 // show list before and after sorting
16 System.out.println("before sort, words = " + words);
17 Collections.sort(words);
18 System.out.println("after sort, words = " + words);
19 }
20 }