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HashMap

Learn key-value mapping with HashMap.

Key-value: HashMap

A HashMap maps a key to a value. You use it when you need fast lookup from key to value, like a dictionary.

HashMap<String, Integer> ages = new HashMap<>();
ages.put("Ayşe", 28);
ages.put("Mehmet", 32);
int age = ages.get("Ayşe");
// age = 28

Each key appears only once inside. If you call put again with the same key, the old value is overwritten. HashMap lives in the java.util package.

Looping over the key set

To walk every key of a HashMap, you use keySet. This method returns a Set view of the keys.

for (String name : ages.keySet()) {
  System.out.println(name + " - " + ages.get(name));
}

To walk pairs (key and value together), entrySet is more efficient because you skip the get call. Each item of entrySet is a Map.Entry.

Practice City temperature table

Create a HashMap<String, Integer>; set "İstanbul" to 14 and "Ankara" to 11. Then print the value of "İstanbul".

Starter code

Main.java
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap<String, Integer>
    // İstanbul -> 14, Ankara -> 11
    // Print the value of İstanbul
  }
}

Sample solution

Main.java
HashMap<String, Integer> temps = new HashMap<>();
temps.put("İstanbul", 14);
temps.put("Ankara", 11);
System.out.println(temps.get("İstanbul"));

Expected output

14

Questions and answers

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

1 What happens if you call put twice with the same key?

  • The old value is overwritten
  • The second value is ignored
  • It throws an error
  • Two equal keys are kept Correct answer

Explanation: Because keys are unique, the old value is lost.

2 Match method to purpose

Term Meaning
put(k, v) Adds a key-value pair
get(k) Returns the value of the key
keySet() Returns the set of keys
size() Returns the number of pairs

3 Fill in the blank: get the value

Question 
HashMap<String, Integer> ages = new HashMap<>();
ages.put("Ayşe", 28);
int age = ages.____("Ayşe");
Answer 
HashMap<String, Integer> ages = new HashMap<>();
ages.put("Ayşe", 28);
int age = ages.get("Ayşe");

Explanation: get("Ayşe") returns the value.

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