Summary - Collections

Combine ArrayList, HashMap and HashSet to write a phone book.

Summary: Collections

In this group you met three core collections: ArrayList (ordered list), HashMap (key-value), and HashSet (unique set).

They all live in the java.util package and provide type safety with generics. They are managed through common methods like add, get, put, contains and size.

Now you will combine the three to write a small phone book: names in an ArrayList, numbers in a HashMap, and tags in a HashSet.

Exercise Phone book skeleton

Create a HashMap<String, String>; set "Ayşe" to "555-1000" and "Mehmet" to "555-2000". Then print the number of "Ayşe".

Show example solution

HashMap<String, String> book = new HashMap<>();
book.put("Ayşe", "555-1000");
book.put("Mehmet", "555-2000");
System.out.println(book.get("Ayşe"));

Test yourself

Which collection stores key-value pairs?
- HashMap
- HashSet
- ArrayList
- int[]
Show answer

ArrayList — HashMap maps a key to a value.
Collection and typical use

ArrayList

Ordered element list
HashMap

Key-value table
HashSet

Unique tag set
int[]

Fixed-length number array
Fill in the blank: import the collection
```
import java.util.____;
```
Show answer
```
import java.util.HashMap;
```

Next lesson: Defining methods

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