🚀

Collections & Generics

Pick the right data structure and use it correctly.

Choose the right one

Ordered, duplicates, index access
```
List → ArrayList
```
Unique items
```
Set → HashSet
```
Key → value pairs
```
Map → HashMap
```
First-in-first-out
```
Queue → ArrayDeque
```
Sorted
```
TreeSet / TreeMap
```
Insertion order kept
```
LinkedHashSet / LinkedHashMap
```

List

```
List<String> l = new ArrayList<>();
```
```
l.add("a"); l.get(0); l.remove("a");
```
```
l.size(); l.contains("a");
```

Map

Map<String,Integer> m = new HashMap<>();

```
m.put("k", 1); m.get("k");
```
```
m.getOrDefault("x", 0);
```

for (var e : m.entrySet()) { e.getKey(); e.getValue(); }

Generics

```
class Box<T> { T value; }
```
type parameter
```
<T extends Number>
```
bounded type
```
? extends T
```
producer (read out)
```
? super T
```
consumer (write in)
PECS: Producer Extends, Consumer Super