Collections

Thinking in Java

Bruce Eckel, Thinking in Java, Chapter 11, Collections of Objects, Choosing an Implementation, Page 550

By now you should understand that there are really only three container components: Map, List and Set, but more than one implementation of each interface.
To convert an old style Array to a List or Set, pass the result of Arrays.asList() to the constructor of a List or Set.

Java 2

Replaced Vector with ArrayList
Replaced Hashtable with HashMap

Collection

Conversion

To convert a collection to an array:

Collection names = getNames();
names.toArray();

I guess the array contains Objects…!?

Iterate

Java 1.5 (and above):

SomeObject obj;
for (Object aContainer : vec) {
    obj = (SomeObject) aContainer;
    // code to use obj
}

List

Sample code

import java.util.ArrayList;
import java.util.List;

List headings = new ArrayList();

To get an item in an ArrayList from a specific index:

searchResults.get(i)

Synchronized

http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collections.html

List list = Collections.synchronizedList(new ArrayList());

Note: It is imperative that the user manually synchronize on the returned list when iterating over it:

Iterator

Iterate through an ArrayList:

for (Iterator i = dataList.iterator(); i.hasNext();) {
    DocumentData data = (DocumentData) i.next();
}

Version 1.5 and above…

void cancelAll(Collection<TimerTask> c) {
    for (TimerTask t : c)
        t.cancel();
}

Append

To append one array list to another:

unitTestResult.addAll(search.getItems());

Sort

import java.util.Collections;
Collections.sort(siteList);

Set

A collection that contains no duplicate elements.

Set mySet = new java.util.HashSet();

To add an integer to a set…

int i = 20;
Set documentIDs = new HashSet();
documentIDs.add(new Integer(i));

To add our own objects to a set we need to create the hashCode() and equals() methods. See the Map, Keys notes below to see how to implement these methods.

To convert a set to an array list:

List<Integer> list = new ArrayList<Integer>(documentIDs);

Map

JavaDoc - Interface - java.util.Map - Java 2 Platform Std. Ed. v1.4.2

Add and Retrieve

Map result = new HashMap();
result.put("Name", "Patrick");

assertTrue(result.get("Name").equals("Patrick"));

if (!result.containsKey("Name")) {

Values

To get the values in a map

Collection c = result.values();

Keys

To get the keys in the map

Set s = result.keySet();

Iterate

To iterate through the keys/values:

Java 1.5…

for(Map.Entry<K, V> e : map.entrySet())
    System.out.println(e.getKey() + ": " + e.getValue());

Java 1.4…

Map names = new HashMap();
names.put("ADK", "Alison");
names.put("PJK", "Peter");

for (Iterator i = names.entrySet().iterator(); i.hasNext();) {
    Map.Entry entry = (Map.Entry) i.next();
    System.out.println((String) entry.getKey());
    System.out.println("  " + (String) entry.getValue());
}

Synchronized

http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collections.html

Map m = Collections.synchronizedMap(new HashMap());

It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:

Keys

To use your own classes as keys in a HashMap, you must override both hashCode() and equals().

The hash code can be generated in (at least) two ways:

Using the HashCodeBuilder utility from Commons Lang: Commons Lang.
Eclipse has an option to Generate hashCode() and equals()… available from the Source menu.

The equals() method will take a similar form to this:

public class Person {
  String name;
  int age;

  public boolean equals(final Object o) {
    return (o instanceof Person)
      && name.equals(((Person) o).name)
      && age == ((Person) o).age;
  }
}

Stack

From Bruce Eckel, Thinking in Java:

You’ll notice that the legacy classes Vector, Stack, and Hashtable are not included, because in all cases there are preferred classes within the Java 2 Containers.

LinkedList, Also has addFirst(), addLast(), getFirst(), getLast(), removeFirst(), and removeLast() (which are not defined in any interfaces or base classes) to allow it to be used as a stack, a queue, and a deque.