HashMaps – Storing Key‐Value Pairs

➡️ Think of a HashMap as a dictionary where each word (key) has a definition (value).
➡️ Instead of just storing a list of items like an ArrayList, a HashMap lets us connect two pieces of data (a key and a value).

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1. Why Use a HashMap?

Feature	ArrayList	HashMap (✅ More Organized)
Stores multiple values?	✅ Yes	✅ Yes
Uses an index (number) to find values?	✅ Yes (0, 1, 2...)	❌ No (Uses keys instead)
Uses meaningful keys?	❌ No	✅ Yes (Can use words, names, etc.)
Fast lookup?	❌ No (Loops through everything)	✅ Yes (Directly finds the value)

Example:

Imagine you have a box of labeled gumballs.

• With an ArrayList, you just store gumballs in order: [Red, Blue, Green].

• With a HashMap, each gumball has a label (key), so you can find it easily:

  "A1" → Red  
  "B2" → Blue  
  "C3" → Green  
  

  Instead of searching for a color in a list, you can just look up the label!

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2. Creating a HashMap

To use a HashMap, we need to import it first.

import java.util.HashMap; // Import HashMap

public class GumballMachine {
    public static void main(String[] args) {
        HashMap<String, String> gumballs = new HashMap<>(); // Create a HashMap

        gumballs.put("A1", "Red");    // Add items
        gumballs.put("B2", "Blue");
        gumballs.put("C3", "Green");

        System.out.println(gumballs); // Print all gumballs
    }
}

Output:

{A1=Red, B2=Blue, C3=Green}

🔹 .put("A1", "Red") → Adds "Red" with the key "A1".
🔹 .put("B2", "Blue") → Adds "Blue" with the key "B2".

➡️ Unlike an ArrayList, we don’t use index numbers. We use meaningful keys instead!

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3. Accessing Items in a HashMap

Instead of using .get(index), we use .get(key) to find a value using its key.

System.out.println(gumballs.get("A1")); // Get the gumball at A1

Example:

public class GumballMachine {
    public static void main(String[] args) {
        HashMap<String, String> gumballs = new HashMap<>();
        gumballs.put("A1", "Red");
        gumballs.put("B2", "Blue");
        gumballs.put("C3", "Green");

        System.out.println("Gumball at A1: " + gumballs.get("A1"));
        System.out.println("Gumball at B2: " + gumballs.get("B2"));
    }
}

Output:

Gumball at A1: Red
Gumball at B2: Blue

🔹 .get("A1") → Finds the value for "A1" ("Red").
🔹 .get("B2") → Finds the value for "B2" ("Blue").

➡️ Unlike an ArrayList, we don’t need to remember index numbers!

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4. Changing an Item

We can update a value using .put(key, newValue).

gumballs.put("A1", "Purple"); // Change "Red" to "Purple"

Example:

public class GumballMachine {
    public static void main(String[] args) {
        HashMap<String, String> gumballs = new HashMap<>();
        gumballs.put("A1", "Red");
        gumballs.put("B2", "Blue");
        gumballs.put("C3", "Green");

        gumballs.put("A1", "Purple"); // Change "Red" to "Purple"

        System.out.println(gumballs);
    }
}

Output:

{A1=Purple, B2=Blue, C3=Green}

🔹 "Red" at key "A1" is now "Purple".

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5. Removing Items

We can remove items using .remove(key).

gumballs.remove("A1"); // Removes "A1"

Example:

public class GumballMachine {
    public static void main(String[] args) {
        HashMap<String, String> gumballs = new HashMap<>();
        gumballs.put("A1", "Red");
        gumballs.put("B2", "Blue");
        gumballs.put("C3", "Green");

        gumballs.remove("A1"); // Removes "Red"

        System.out.println(gumballs);
    }
}

Output:

{B2=Blue, C3=Green}

🔹 The key "A1" and its value "Red" are removed.

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6. Checking if a Key Exists

We can check if a key exists using .containsKey(key).

if (gumballs.containsKey("A1")) {
    System.out.println("A1 exists!");
}

Example:

public class GumballMachine {
    public static void main(String[] args) {
        HashMap<String, String> gumballs = new HashMap<>();
        gumballs.put("A1", "Red");
        gumballs.put("B2", "Blue");

        if (gumballs.containsKey("A1")) {
            System.out.println("A1 exists!");
        }
    }
}

Output:

A1 exists!

🔹 .containsKey("A1") checks if "A1" exists.

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7. Looping Through a HashMap

We can loop through all keys and values using a for loop.

for (String key : gumballs.keySet()) {
    System.out.println(key + " → " + gumballs.get(key));
}

Example:

public class GumballMachine {
    public static void main(String[] args) {
        HashMap<String, String> gumballs = new HashMap<>();
        gumballs.put("A1", "Red");
        gumballs.put("B2", "Blue");
        gumballs.put("C3", "Green");

        for (String key : gumballs.keySet()) {
            System.out.println(key + " → " + gumballs.get(key));
        }
    }
}

Output:

A1 → Red
B2 → Blue
C3 → Green

🔹 .keySet() gets all keys in the HashMap.
🔹 .get(key) gets each value.

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8. Summary of HashMaps

Feature	What It Does	Example
Create a HashMap	Makes a key-value map	HashMap<String, String> map = new HashMap<>();
Add Items	Adds key-value pairs	.put("Key", "Value")
Get Items	Finds a value using a key	.get("Key")
Change Items	Updates a value	.put("Key", "New Value")
Remove Items	Deletes a key-value pair	.remove("Key")
Check if Key Exists	Checks if a key is in the map	.containsKey("Key")
Loop Through Keys	Goes through all keys	for (String key : map.keySet())

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9. What’s Next?

Now that we can store key-value pairs using HashMaps, we’ll move on to Object-Oriented Programming (OOP) to create real-world objects in Java! 🚀