I’ve been studying for the SCJP, now Oracle Certified Professional Java SE Programmer exam.
I’ve had a difficult time wrapping my head around all the different collections and when to use them. I’m also fond of flash cards. So I tried to create a set of classes that are essentially the same except which collection they’re using. I’ll have to identify how the output would come out and what the primary “features” are of each collection.
Unfortunately I don’t trust myself. I would like to have someone confirm that all the information is accurate or if any is missing. Then after some feedback/corrections I think it’ll make a great exercise for anyone else trying to understand Java collections.
The collections covered are:
HashMap, Hashtable, TreeMap, LinkedHashMap, HashSet, TreeSet, LinkedHashSet, ArrayList, Vector, LinkedList, PriorityQueue.
I also have all the files separated out, they can be downloaded here: http://www.allgo.com/personal/MyCollections.zip
Thanks in advance
import java.util.*;
import java.lang.*;
class MyItem implements Comparable{
private String name;
MyItem(String n){ name = n; }
public String toString(){return name;}
public String getName(){return name;}
public boolean equals(Object obj){
if(this==obj) return true;
else if(obj==null) return false;
else if(getName() != ((MyItem)obj).getName()) return false;
else return true;
}
public int hashCode(){ return 5; }
public int compareTo(MyItem b){return b.getName().compareTo(getName());}
}
public class MyCollections{
public static void main(String[] args){
MyHashMap.main(args); System.out.println("HashMap: Hash=Unsorted, Unordered. Map=key/value pair\n##\n");
MyHashtable.main(args); System.out.println("Hashtable: Thread Safe. Hash=Unsorted, Unordered. Map=key/value pair\n##\n");
MyTreeMap.main(args); System.out.println("TreeMap: Tree=sorted. Map=key/value.\n##\n");
MyLinkedHashMap.main(args); System.out.println("LinkedHashMap: Linked=Maintains Insertion Order. Hash=unsorted, unordered. Map=key/value pair.\n##\n");
MyHashSet.main(args); System.out.println("HashSet: Hash=Unsorted, Unordered. Set=Unique. Define=equals/hashCode\n##\n");
MyTreeSet.main(args); System.out.println("TreeSet: Tree=Sorted. Set=Unique. Define=Comparable/Comparator\n##\n");
MyLinkedHashSet.main(args); System.out.println("LinkedHashSet: Liniked=Maintains Insertion Order. Hash=Unsorted. Set=Unique. Define=equals/hashCode\n##\n");
MyArrayList.main(args); System.out.println("ArrayList: List=Queue. Maintains insertion order, Allowed duplicates\n##\n");
MyVector.main(args); System.out.println("Vector: Thread Safe. ArrayList. Maintains Insertion Order, Allows duplicates\n##\n");
MyLinkedList.main(args); System.out.println("LinkedList: Linked=Maintaines Insertion Order. List=Queue. Advanced ArrayList with more methods.\n##\n");
MyPriorityQueue.main(args); System.out.println("PriorityQueue: Define=Comparable/comparator\n##\n");
}
}
class MyHashMap{
public static void main(String[] args){
HashMap c = new HashMap();
MyItem Eight = new MyItem("Eight");
c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));
c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));
c.remove(3); c.put(7, new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyHashtable{
public static void main(String[] args){
Hashtable c = new Hashtable();
MyItem Eight = new MyItem("Eight");
c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));
c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));
c.remove(3); c.put(7, new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyTreeMap{
public static void main(String[] args){
TreeMap c = new TreeMap();
MyItem Eight = new MyItem("Eight");
c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));
c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));
c.remove(3); c.put(7, new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyLinkedHashMap{
public static void main(String[] args){
LinkedHashMap c = new LinkedHashMap();
MyItem Eight = new MyItem("Eight");
c.put(5, new MyItem("Five")); c.put(1, new MyItem("One")); c.put(8, Eight); c.put(3, new MyItem("Three"));
c.put(4, new MyItem("Four")); c.put(1, new MyItem("1")); c.put(8, Eight); c.put(9, new MyItem("Nine"));
c.remove(3); c.put(7, new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyHashSet{
public static void main(String[] args){
HashSet c = new HashSet();
MyItem Eight = new MyItem("Eight");
c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
c.remove(3); c.add(new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyTreeSet{
public static void main(String[] args){
TreeSet c = new TreeSet();
MyItem Eight = new MyItem("Eight");
c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
c.remove(Eight); c.add(new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyLinkedHashSet{
public static void main(String[] args){
LinkedHashSet c = new LinkedHashSet();
MyItem Eight = new MyItem("Eight");
c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
c.remove(3); c.add(new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyArrayList{
public static void main(String[] args){
ArrayList c = new ArrayList();
MyItem Eight = new MyItem("Eight");
c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
c.remove(3); c.add(new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyVector{
public static void main(String[] args){
Vector c = new Vector();
MyItem Eight = new MyItem("Eight");
c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
c.remove(3); c.add(new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyLinkedList{
public static void main(String[] args){
LinkedList c = new LinkedList();
MyItem Eight = new MyItem("Eight");
c.add(new MyItem("Five")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Three"));
c.add(new MyItem("Four")); c.add(new MyItem("One")); c.add(Eight); c.add(new MyItem("Nine"));
c.remove(3); c.add(new MyItem("Seven"));
System.out.println(c);//output?
}
}
class MyPriorityQueue{
public static void main(String[] args){
PriorityQueue c = new PriorityQueue();
MyItem Eight = new MyItem("Eight");
c.offer(new MyItem("Five")); c.offer(new MyItem("One")); c.offer(Eight); c.offer(new MyItem("Three"));
c.offer(new MyItem("Four")); c.offer(new MyItem("One")); c.offer(Eight); c.offer(new MyItem("Nine"));
System.out.println(c.peek());
System.out.println(c.poll());
c.offer(new MyItem("Seven"));
System.out.println(c);//output?
}
}
For starters, you should refactor your code. Basically, everywhere you used “copy paste”, don’t.
Create a method like this:
Then instead of the method you have, do this:
And do that for all the collections you have.
Next, do a similar thing for your maps:
Your code will shrink, be readable and may even one day become usable.