motoblog reuben: 4 Ways to Traverse a Map

Usually I don't need to traverse a java.util.Map, which is meant to be looked up, not iterated through. I traverse a java.util.Map primarily for debugging purposem, to dump its content. In case you do need to, here are 4 ways I've tried, just as examples:

import static java.net.HttpURLConnection.*;
import java.util.*;

public class MapTest {
    public static void main(String... args) {
        traverseMap();
    }

    private static void traverseMap() {
        Map<Integer, String> data = new HashMap<Integer, String>();
        data.put(HTTP_OK, "HTTP_OK");
        data.put(HTTP_FORBIDDEN, "HTTP_FORBIDDEN");
        data.put(HTTP_NOT_FOUND, "HTTP_NOT_FOUND");

        System.out.printf("%nUsing JDK 5 foreach and entry set:%n");
        Set<Map.Entry<Integer, String>> entries = data.entrySet();
        for(Map.Entry<Integer, String> entry : entries) {
            Integer key = entry.getKey();
            String value = entry.getValue();
            System.out.printf("%s = %s%n", key, value);
        }

        System.out.printf("%nUsing Iterator<Map.Entry> and entry set:%n");
        for(Iterator<Map.Entry<Integer, String>> it = entries.iterator(); it.hasNext();) {
            Map.Entry<Integer, String> entry = it.next();
            Integer key = entry.getKey();
            String value = entry.getValue();
            System.out.printf("%s = %s%n", key, value);
        }

        System.out.printf("%nUsing JDK 5 foreach and key set:%n");
        for(Integer key : data.keySet()) {
            String value = data.get(key);
            System.out.printf("%s = %s%n", key, value);
        }

        System.out.printf("%nUsing traditional Iterator and key set%n");
        for(Iterator<Integer> it = data.keySet().iterator(); it.hasNext();) {
            Integer key = it.next();
            String value = data.get(key);
            System.out.printf("%s = %s%n", key, value);
        }
    }
}

You may have noticed I'm using primitive int numbers (200, 403, 404) as the map key. Thanks to auto-boxing in JDK 5, I can use primitive s and their wrapper types interchangeably. Another useful feature in JDK 5 used here is using static import to import all HTTP status constants from java.net.HttpURLConnection.

To compile and run the test:


$ javac MapTest.java
$ java MapTest

Using JDK 5 foreach and entry set:
200 = HTTP_OK
403 = HTTP_FORBIDDEN
404 = HTTP_NOT_FOUND

Using Iterator<Map.Entry> and entry set:
200 = HTTP_OK
403 = HTTP_FORBIDDEN
404 = HTTP_NOT_FOUND

Using JDK 5 foreach and key set:
200 = HTTP_OK
403 = HTTP_FORBIDDEN
404 = HTTP_NOT_FOUND

Using traditional Iterator and key set
200 = HTTP_OK
403 = HTTP_FORBIDDEN
404 = HTTP_NOT_FOUND

There is some discussion on entrySet vs keySet in the comment section of a previous post: Unnecessary Cast in Map. Basically, using entry set is faster than using key set in traversing a Map. I also learned to use entrySet from there.

motoblog reuben

4 Ways to Traverse a Map

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