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Hash map lookup
Hash the key, jump to a bucket, then compare entries there.
Ada hashes to bucket 2; lookup only scans entries in that bucket.
Flash cards
Review the key moves
What is the main idea behind Java LinkedHashMap?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
import java.util.LinkedHashMap; // Import the LinkedHashMap ___Put the learning moves in the order that makes the concept easiest to apply.
A LinkedHashMap stores keys and values, and keeps them in the same order you put them in.
It is part of the java.util package and implements the Map interface.
Tip
Use LinkedHashMap when you want predictable iteration order (insertion order).
Create a LinkedHashMap
Create a LinkedHashMap object called capitalCities that will store String keys and String values:
Example
import java.util.LinkedHashMap; // Import the LinkedHashMap class
LinkedHashMap<String, String> capitalCities = new LinkedHashMap<>();Now you can use methods like put() to add key/value pairs, get() to retrieve a value by key, and remove() to delete an entry - all while maintaining insertion order.
Add Items
Use the put() method to add items to the LinkedHashMap :
Example
// Import the LinkedHashMap class import java.util.LinkedHashMap; public class Main { public static void main(String[] args) { LinkedHashMap<String, String> capitalCities = new LinkedHashMap<>(); capitalCities.put("England", "London"); capitalCities.put("India", "New Dehli"); capitalCities.put("Austria", "Wien"); capitalCities.put("Norway", "Oslo"); capitalCities.put("Norway", "Oslo"); // Duplicate capitalCities.put("USA", "Washington DC"); System.out.println(capitalCities);
}
}Output: The items will appear in the order they were added (e.g., {England=London, India=New Dehli, Austria=Wien, Norway=Oslo, USA=Washington DC}).
Note
Duplicates like "Norway" are ignored.
Access an Item
Use get() with a key to get its associated value:
capitalCities.get("England");Remove an Item
Use remove() to remove an item by key:
capitalCities.remove("England");Use clear() to remove all items
capitalCities.clear();LinkedHashMap Size
Use size() to check how many key/value pairs are in the map:
capitalCities.size();Note
The size only counts unique keys. If a key is added more than once, only the latest value is kept.
Loop Through a LinkedHashMap
You can loop through a LinkedHashMap using a for-each loop. Use:
- keySet() to get all keys
- values() to get all values
Example
// Print keys
for (String key : capitalCities.keySet()) {
System.out.println(key);
}Example
// Print values
for (String value : capitalCities.values()) {
System.out.println(value);
}Example
// Print keys and values
for (String key : capitalCities.keySet()) {
System.out.println("Key: " + key + ", Value: " + capitalCities.get(key));
}HashMap vs LinkedHashMap
| Feature | HashMap | LinkedHashMap |
|---|---|---|
| Order | No guaranteed order | Insertion order preserved |
| Performance | Faster for random access | Slightly slower due to ordering |
| Duplicates | Keys must be unique | Keys must be unique |
Tip
Use LinkedHashMap when you want the map to remember the order in which entries were added.
The var Keyword
From Java 10, you can use the var keyword to declare a LinkedHashMap variable without writing the type twice. The compiler figures out the type from the value you assign.
This makes code shorter, but many developers still use the full type for clarity . Since var is valid Java, you may see it in other code, so it's good to know that it exists:
// Without var LinkedHashMap<String, String> capitalCities = new LinkedHashMap<String, String>();
// With var var capitalCities = new LinkedHashMap<String, String>();The Map Interface
Note
Sometimes you will see both Map and LinkedHashMap in Java code, like this:
import java.util.Map;
import java.util.LinkedHashMap;
Map<String, String> capitalCities = new LinkedHashMap<>();This means the variable (capitalCities) is declared as a Map (the interface), but it stores a LinkedHashMap object (the actual map). Since LinkedHashMap implements the Map interface, this is possible.
It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.