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# Practice Exercises: Java Collections Hierarchy

These exercises help reinforce understanding of the **Java Collections Framework hierarchy** and the relationships between its core interfaces and implementations.

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# Multiple Choice

## Question 1

Which interface allows an object to be used in an enhanced for-loop?

A. Collection  
B. Iterable  
C. List  
D. Queue

Answer: **B Iterable**
---

## Question 2

Which interface is the parent of `List`, `Set`, and `Queue`?

A. Iterable  
B. Collection  
C. Map  
D. Deque

Answer: **B Collection**
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## Question 3

Which of the following is **NOT part of the Collection hierarchy**?

A. List  
B. Set  
C. Queue  
D. Map

Answer: **D Map**

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## Question 4

Which collection type allows **duplicate elements and maintains order**?

A. Set  
B. List  
C. Queue  
D. TreeSet

Answer: **B List**
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## Question 5

Which structure stores **key-value pairs**?

A. List  
B. Set  
C. Queue  
D. Map

Answer: **D Map**
---

# True or False

Write **True** or **False**.

1. `ArrayList` implements the `List` interface. TRUE
2. `Set` allows duplicate elements. FALSE
3. `Map` stores elements using keys and values. TRUE 
4. `Queue` typically follows FIFO behavior.  TRUE
5. `Iterable` allows collections to be used in enhanced for-loops. TRUE

---

# Short Answer

## Question 1

What is the difference between an **interface** and a **class** in the Java Collections Framework?

An interface defines behaviors(what can be done), and a class implements the behaviors and does the work.
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## Question 2

Why is it recommended to declare variables using interfaces like `List` instead of concrete classes like `ArrayList`?

It proves better flexibility, code design, and Abstraction.
Example:

```java
List<String> list = new ArrayList<>();

Question 3

Name three interfaces that extend Collection.

List, Set, and Queue

Code Reading Exercise

Look at the code below.

List<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Cherry");

for (String fruit : fruits) {
    System.out.println(fruit);
}

Questions:

  1. Which interface type is used for the variable? List
  2. Which class is used as the implementation? ArrayList
  3. Why can the enhanced for-loop be used here? List extends Collections which extends iterable, so the enhanced for-loop can be used on all collections.

Coding Exercise

Create a small program that demonstrates the following:

  1. A List that stores three names.
  2. A Set that stores unique numbers.
  3. A Queue that stores three tasks.
  4. A Map that stores student names and grades.

Print the contents of each structure.

Challenge Exercise

Write a program that:

  1. Creates a List of student names
  2. Creates a Set of unique course names
  3. Creates a Queue of tasks
  4. Creates a Map that stores student → grade

Then:

  • Print each structure
  • Add one new element to each structure
  • Print them again