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Summary - Generic collections

Combine List, Map and Set to write a word frequency counter.

Summary - Generic collections

This group covered four core collection interfaces: List ordered, Map key-value, Set unique, Queue/Deque FIFO and double-ended. Each has concrete implementations: ArrayList, HashMap, HashSet, ArrayDeque.

The Collections class provides helpers like sort, reverse, max and min. Type parameters (<E>, <K,V>) give compile-time type safety; adding the wrong type is a compile error.

Practice Build a score board

Combine three collections. Declare List<Integer> scores = new ArrayList<>();, add 70, 85, 70, 90 (70 is duplicated). Build Set<Integer> unique = new HashSet<>(scores); to drop duplicates. Print the list size with System.out.println(scores.size()); and the unique count with System.out.println(unique.size());.

Starter code

Main.java
import java.util.List;
import java.util.ArrayList;
import java.util.Set;
import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        // Declare List<Integer>, add 70,85,70,90; build Set to dedupe; print two sizes

    }
}

Sample solution

Main.java
import java.util.List;
import java.util.ArrayList;
import java.util.Set;
import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        List<Integer> scores = new ArrayList<>();
        scores.add(70);
        scores.add(85);
        scores.add(70);
        scores.add(90);
        Set<Integer> unique = new HashSet<>(scores);
        System.out.println(scores.size());
        System.out.println(unique.size());
    }
}

Expected output

4
3

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which collection rejects duplicate elements?

  • List
  • ArrayList
  • Set Correct answer
  • Queue

Explanation: Set holds unique elements; a duplicate add returns false.

2 Match interface to implementation

Term Meaning
List ArrayList
Map HashMap
Set HashSet
Queue ArrayDeque

3 Match collection traits

Term Meaning
List Ordered, allows duplicates
Map Key-value pairs
Set Unique elements
Queue FIFO order

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. new
  5. toString
  6. Summary - Classes

2 Constructor

  1. Constructor basics
  2. Parameterized constructor
  3. Constructor overloading
  4. this()
  5. Default constructor
  6. Summary - Constructor

3 Inheritance

  1. extends
  2. super
  3. Method overriding
  4. Object class
  5. final class and method
  6. Summary - Inheritance

4 Project: Library

  1. Class design
  2. Book model
  3. Member management
  4. Borrow flow
  5. Putting it together

5 Encapsulation

  1. private fields
  2. Getter
  3. Setter
  4. Package-private
  5. Record
  6. Summary - Encapsulation

6 Static

  1. static fields
  2. static methods
  3. static block
  4. Class variables
  5. instanceof
  6. Summary - Static

7 Constants

  1. static final
  2. Uppercase convention
  3. Constants with enum
  4. Interface constants
  5. Math constants
  6. Summary - Constants

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Interface

  1. interface
  2. implements
  3. default methods
  4. static methods
  5. Multiple interfaces
  6. Summary - Interface

10 Abstract class

  1. abstract class
  2. abstract methods
  3. Abstract and concrete
  4. Abstract vs interface
  5. Sealed class
  6. Summary - Abstract class

11 Polymorphism

  1. Polymorphism basics
  2. Dynamic binding
  3. Override rules
  4. Abstract reference
  5. Pattern matching
  6. Summary - Polymorphism

12 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

13 File I/O

  1. File class
  2. FileReader and FileWriter
  3. BufferedReader
  4. PrintWriter
  5. try-with-resources
  6. Summary - File I/O

14 Serialization

  1. Jackson ObjectMapper
  2. readValue
  3. writeValueAsString
  4. JsonProperty
  5. List and Map serialization
  6. Summary - Serialization

15 Generics

  1. Generic class
  2. Generic method
  3. Bounded type
  4. Wildcard
  5. Type erasure
  6. Summary - Generics

16 Generic collections

  1. List<E>
  2. Map<K,V>
  3. Set<E>
  4. Queue and Deque
  5. Collections class
  6. Summary - Generic collections

17 Iterator

  1. Iterator
  2. remove
  3. ListIterator
  4. Iterable
  5. Spliterator
  6. Summary - Iterator

18 Lambda expressions

  1. Lambda basics
  2. Parameterized lambda
  3. Method reference
  4. forEach with lambda
  5. Comparator with lambda
  6. Summary - Lambda expressions

19 Stream API basics

  1. stream()
  2. filter
  3. map
  4. forEach and count
  5. collect toList
  6. Summary - Stream API basics

20 Intermediate final

  1. Project design
  2. OOP model
  3. Stream layer
  4. File I/O
  5. Putting it together