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put and get

Learn map operations with put() and get().

Map operations: put and get

You use put to add a pair to a HashMap and get to read a value by key. get returns null if the key is missing.

HashMap<String, Integer> stock = new HashMap<>();
stock.put("Kalem", 12);
stock.put("Defter", 5);
int n = stock.get("Kalem");
// n = 12

You can check whether a key exists with containsKey. This is safer when null is a possible value.

Safe lookup with containsKey

To check whether a key exists in the map, use containsKey. This is a lifesaver when you need to distinguish a missing key from an actual null value that get might return.

if (stock.containsKey("Silgi")) {
  int n = stock.get("Silgi");
  System.out.println(n);
} else {
  System.out.println("Not in stock");
}

Similarly, containsValue checks whether a value exists; but it is slower because the whole map must be scanned.

Practice Print the product stock

Print the value of the "Kalem" key in the given map; then check with containsKey whether "Silgi" exists, and print its value if so, or 0 otherwise.

Starter code

Main.java
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    HashMap<String, Integer> stock = new HashMap<>();
    stock.put("Kalem", 12);
    // Print the value of Kalem
    // If Silgi exists print its value, otherwise print 0
  }
}

Sample solution

Main.java
System.out.println(stock.get("Kalem"));
if (stock.containsKey("Silgi")) {
  System.out.println(stock.get("Silgi"));
} else {
  System.out.println(0);
}

Expected output

12
0

Questions and answers

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

1 Fill in the blank: update the stock

Question 
HashMap<String, Integer> stock = new HashMap<>();
stock.____("Kalem", 20);
Answer 
HashMap<String, Integer> stock = new HashMap<>();
stock.put("Kalem", 20);

Explanation: put("Kalem", 20) updates the stock.

2 What does stock.get("Silgi") return when the key is missing?

  • It throws an error Correct answer
  • 0
  • An empty String
  • null

Explanation: A missing key returns null.

3 Match scenario to method

Term Meaning
Add or update a key put
Get the value of a key get
Does the key exist? containsKey
Does the value exist? containsValue

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