Similar to Dictionaries in literally every other programming language. Java likes being special.
A HashMap is a generic type which, when created, specifies it’s Key and Value data type. It inherits Map, which is an interface and cannot be instanced.
Why does Jeg use Map and HashMap like that?!
// Map here // HashMap here!?
Map<String, Integer> map = new HashMap<String, Integer>();There are many implementations of Maps, so in order to keep your code maintainable, it is best to use the interface type everywhere you can (besides when instancing, of course) so that in the future you can use different types of maps easily.
Map’s are inherited by:
- HashMap (the simplest one)
- TreeMap
- LinkedHashMap
- etc!
That’s why Jeg makes the type for Maps a “Map” but instances it with “HashMap”
Map = Interface (Not instantiable, can mean many classes) HashMap = Class (Instantiable, only has 1 meaning — to be a HashMap)
Creating a Map
// The String is the key type, and the Integer is the value type
Map<String, Integer> ages = new HashMap<String, Integer>();Every key is mapped to 1 value. You cannot have more of the same key, but you can have several of the same value. Observe the table below
| Key | Value |
|---|---|
| Michael | 26 |
| Jodi | 26 |
| Jeremy | 32 |
Methods
.put()
"put()" is like "add()" for Array Lists. Requires a key, then value
ages.put("John", 26);.containsKey()
Returns true if the key exists, or false otherwise
if (ages.containsKey("Jeremy")) {
// Runs if jeremy is a key (it is)
}.get()
Gets the value from a key
int johnsAge = ages.get("John");.keySet()
Returns a "Set" of all the keys, which update when the map updates. NOT an array or arrayList because you cannot add to it, only remove. Weird
for (String name : ages.keySet()) {
System.out.println();
}