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Summary - Collections

Combine ArrayList, HashMap and HashSet to write a phone book.

Summary: Collections

In this group you met three core collections: ArrayList (ordered list), HashMap (key-value), and HashSet (unique set).

They all live in the java.util package and provide type safety with generics. They are managed through common methods like add, get, put, contains and size.

Now you will combine the three to write a small phone book: names in an ArrayList, numbers in a HashMap, and tags in a HashSet.

Practice Phone book skeleton

Create a HashMap<String, String>; set "Ayşe" to "555-1000" and "Mehmet" to "555-2000". Then print the number of "Ayşe".

Starter code

Main.java
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap<String, String> (name -> number)
    // Add Ayşe and Mehmet
    // Print Ayşe's number
  }
}

Sample solution

Main.java
HashMap<String, String> book = new HashMap<>();
book.put("Ayşe", "555-1000");
book.put("Mehmet", "555-2000");
System.out.println(book.get("Ayşe"));

Expected output

555-1000

Questions and answers

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

1 Which collection stores key-value pairs?

  • HashMap
  • HashSet
  • ArrayList Correct answer
  • int[]

Explanation: HashMap maps a key to a value.

2 Collection and typical use

Term Meaning
ArrayList Ordered element list
HashMap Key-value table
HashSet Unique tag set
int[] Fixed-length number array

3 Fill in the blank: import the collection

Question 
import java.util.____;
Answer 
import java.util.HashMap;

Explanation: java.util.HashMap is imported.

All lessons in Beginner

1 Java basics

  1. What is Java?
  2. The main method
  3. Comments
  4. Class skeleton
  5. javac and java
  6. Summary - Java basics

2 Variables

  1. int and double
  2. boolean
  3. String
  4. Naming variables
  5. final variables
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Type casting
  3. String concatenation
  4. Math class
  5. BigInteger and BigDecimal
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Assignment and increment
  6. Summary - Operators

6 String basics

  1. length and charAt
  2. equals and equalsIgnoreCase
  3. indexOf and substring
  4. toUpperCase and trim
  5. String.format
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. else and else if
  3. Nested if
  4. switch
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. do-while
  4. break and continue
  5. for-each
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Filling arrays
  4. Multi-dimensional arrays
  5. Copying arrays
  6. Summary - Arrays

11 Array methods

  1. Arrays.sort
  2. Arrays.toString
  3. Arrays.binarySearch
  4. Arrays.copyOf
  5. Arrays.fill
  6. Summary - Array methods

12 Collections

  1. ArrayList
  2. add and get
  3. HashMap
  4. put and get
  5. HashSet
  6. Summary - Collections

13 Methods

  1. Defining methods
  2. return
  3. void methods
  4. Method overloading
  5. Scope
  6. Summary - Methods

14 Method parameters

  1. Primitive parameters
  2. Object parameters
  3. varargs
  4. Multiple values via array
  5. final parameters
  6. Summary - Method parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String builder

  1. StringBuilder
  2. append
  3. insert and delete
  4. reverse
  5. toString
  6. Summary - String builder

17 Input and output

  1. Scanner
  2. nextInt and nextLine
  3. System.out.printf
  4. BufferedReader
  5. try-with-resources
  6. Summary - Input and output

18 Enum

  1. Declaring enums
  2. values and valueOf
  3. Enum fields
  4. Enum methods
  5. switch with enum
  6. Summary - Enum

19 Exception basics

  1. try/catch
  2. finally
  3. throw
  4. Exception hierarchy
  5. Multiple catch
  6. Summary - Exception basics

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together