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10. Java Collections: Lists, Maps, Sets

• BeginnersBook: Java Collections Framework Tutorials.
• Jenkov Aps: Java Collections Tutorial.
• Vogella: Java Collections - Tutorial.
• Oracle: The Java Tutorials - Collections.
• w3schools: Java ArrayList.
• w3schools: Java HashMap.

1) Make a new Eclipse project called task10 or similar.

2) Make a List of 10 Circle objects. Use a random radius. Keep adding circles to the list until Math.random() returns less than 0.5. Then, iterate over the list and print each area.

3) Make a Map that associates the following employee IDs with names. Make test cases where you test several valid and invalid ID’s and print the associated name.

ID	Name
a1234	Steve Jobs
a1235	Scott McNealy
a1236	Jeff Bezos
a1237	Larry Ellison
a1238	Bill Gates

Hint: Keys and values of Maps can be any Object type, so in real life you would probably have the key be a String and the associated value be a Person or Employee object.

Hint: To make things simpler on this exercise, you can use String for both the ID and the name, rather than bothering to create a Person or Employee class. The point here is to associate keys with values, then retrieve values later based on keys.

4) Go back to the previous problem and make your lookup method work with keys in any case. For example, both a1234 and A1234 should match Steve Jobs.

5) Make a Map that associates numbers (e.g., 2) with words (e.g, two or dos). Test the Map out by passing in a few ints and printing out the corresponding words.

Extra

6) Create a comparator to sort a List of Circles in descending order of their area.

class MyComparator implements Comparator<Circle> {
	public int compare(Circle o1, Circle o2) {
		// TODO: Implement comparison algorithm here.
		return 0;
	}
}

Show the sorting result.

public static void main(String[] args) {
	List<Circle> arrayList = new ArrayList<Circle>();
	arrayList.add(new Circle(3));
	arrayList.add(new Circle(2));
	arrayList.add(new Circle(1));
	System.out.println(arrayList);
	MyComparator myComparator = new MyComparator();
	arrayList.sort(myComparator);
	System.out.println(arrayList);
}

Expected result:

[Circle: radius = 3.0, Circle: radius = 2.0, Circle: radius = 1.0]
[Circle: radius = 1.0, Circle: radius = 2.0, Circle: radius = 3.0]

7) Do some timing tests to verify the performance claims regarding the difference between ArrayList and LinkedList for accessing the middle element.

Hint: Use System.currentTimeMillis or System.nanoTime to lookup the current time. Compute a delta and divide to get an elapsed time in seconds.

Hint: To ensure meaningful results, use very long lists and access the middle element many times.

Hint: Run several tests, each with larger and larger lists.

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