Iterating over a collection works in just about the same way as iterating over an array. The main difference is that you're going to have to add some type-casting, as the objects within a collection aren't compile-time determinable, except when using generics (see the next section on Avoiding Unnecessary Typecasts for details on generics and for/in).
Example 7-1 is a simple program that shows several types of collection iteration.
package com.oreilly.tiger.ch07;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class ForInDemo {
public static void main(String[] args) {
// These are collections we'll iterate over below.
List wordlist = new ArrayList( );
Set wordset = new HashSet( );
// We start with a basic loop over the elements of an array.
// The body of the loop is executed once for each element of args[].
// Each time through one element is assigned to the variable word.
System.out.println("Assigning arguments to lists...");
for(String word : args) {
System.out.print(word + " ");
wordlist.add(word);
wordset.add(word);
}
System.out.println( );
// Iterate through the elements of the List now.
// Since lists have an order, these words should appear as above
System.out.println("Printing words from wordlist " +
"(ordered, with duplicates)...");
for(Object word : wordlist) {
System.out.print((String)word + " ");
}
System.out.println( );
// Do the same for the Set. The loop looks the same but by virtue of
// using a Set, we lose the word order and also discard duplicates.
System.out.println("Printing words from wordset " +
"(unordered, no duplicates)...");
for(Object word : wordset) {
System.out.print((String)word + " ");
}
}
}
NOTE
This example is culled from Java in a Nutshell, Fifth Edition (O'Reilly).
NOTE
When compiling this class, you'll get several warnings from the compiler because the code doesn't use a typed list, like List<String>. I'll cover using generics with for/in later in the chapter.
This is pretty basic stuff, so I'll save a tree and cut out repetitive discussion. Here's the output from the program, which should look exactly as expected:
NOTE
If you're using the build.xml supplied with the examples, just type "ant runch07" to see this output.
run-ch07: [echo] Running Chapter 7 examples from Java 1.5 Tiger: A Developer's Notebook\n [echo] Running ForInDemo... [java] Assigning arguments to lists... [java] word1 word2 word3 word4 word1 [java] Printing words from wordlist (ordered, with duplicates)... [java] word1 word2 word3 word4 word1 [java] Printing words from wordset (unordered, no duplicates)... [java] word4 word1 word3 word2
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