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.

Exercise City temperature table

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

Show example solution

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

Test yourself

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
Show answer

Two equal keys are kept — Because keys are unique, the old value is lost.
Match method to purpose

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

Fill in the blank: get the value

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

Show answer

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

Next lesson: put and get

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