Collections Practice

These exercises demonstrate properties of some common collections.

1. Clone the starter code from Github Classroom: https://classroom.github.com/a/0F28sTB7
2. Do the exercises
3. Write your findings in README.md
4. Push the completed code to Github

Set and Implementations

A Set is collection with:

• no duplicates
• order of items is determined by the Set, not by the programmer

In Java, Set is an interface that extends the Collection interface. Set doesn’t add any new methods, but changes the semantics of some methods defined in Collection.

The Java SE defines 2 concrete classes that implement Set:

• HashSet puts objects in “buckets” using the object’s hashCode.
• TreeSet maintains an ordered collection of Objects. The ordering can be either:
  (a) the natural order, for objects that implement Comparable (b) order specified by a Comparator that is given as a parameter to the TreeSet constructor. You used a Comparator in Lab 3.

Set Practice

Code: SetExercise.java

Complete the setExplorer method:

1. Create a HashSet object and add these strings: "dog", "ant", "bird", "elephant", "cat". `Set` and `Collection` interfaces defines an `add` method for this:

   set.add("dog");
   set.add("ant");
   set.add("bird");
   etc.
   

   To eliminate redundant code, you may write a method that let’s you add all the elements at once:

   private static void addAll(Set<String> set, String ... args) {
       for(String arg: args) {
          System.out.print(" "+arg):
          set.add(arg);
       }
       System.out.println();
   }
   // then use:
   addAll(set, "dog", "ant", "bird", etc.);
   

2. Print the items in the set, using an Iterator. Every collection has an iterator() method that creates a new Iterator for the contents of the collection. So you write:

   Iterator<String> iter = set.iterator();
   while( iter.hasNext() ) {
      // get the next element from iterator and print it
      String element = iter.next();
   
   }
   

   • You’ll need to print the set again, so consider writing a method print(set) to do this.
   • Collection defines an iterator() method that returns a new Iterator over the collection. For example:
3. Question: are the items in Set in the same order that you added them? What does this tell you about Sets that is different from a List?
4. Remove all items from the Set.
   • Set (and every Collection) has a method to do this. What is it?
5. Add the same elements as item 1, but in a different order such as: “cat”, “elephant”, “ant”, “dog”, “bird”.
6. Use an Iterator to print the items in the set (again).
   Question: Are the items in the same order as before?
7. Add duplicate items. Add another “cat” and “dog” to the set. To ensure that these are distinct objects use ‘new String(“cat”)’ and ‘new String(“dog”)’.
   Print the set again.
8. Question: Are there 2 cats and 2 dogs? What property of Set does this demonstate?
9. Repeat 1-8 using a TreeSet instead of HashSet.
   Hint: Use Polymorphism. Modify setExplorer so that the Set object is a parameter to the method (instead of creating set in the method), so you can easily call the method with a different Set (TreeSet) as parameter. In main, create a TreeSet as parameter.

Question: What is different when you use TreeSet?

1. Look at the code for these methods:

   private static void addAll(Set<String> set, String ... args)
   private static void print(Set<String> set)
   

   what Set behavior do they use? Could you change the parameter to Collection instead?

   • Collection is a parent of Set, List, and some other collections. So a method that accepts any Collection is more reusable.