Understanding Implementation vs. Interface in Java: A Guide to Variables, Strings, and HashMaps

1. Coming up • Implementation vs. Interface • The Truth about variables • Comparing strings • HashMaps

2. Lecture 9 Shhh, no talking in the library OO

3. Coming up • Implementation vs. Interface • The Truth about variables • Comparing strings • HashMaps

4. Implementation and interface • The API describes • what a class does (interface of the class) • not how a class does it (implementation of a class) • When you write classes, you write the implementation of those classes

5. Library • The java library is full of helpful classes • Like ArrayList • What does the inside of an ArrayList look like? • How does it handle the resizing? • How does it know when the throw an error? • How does it handle renumbering when removing elements? Implementation

6. OO Interface • All we need to know to use the ArrayList class, and the other library classes is what their interface is • Interface is how we interact with a class • what methods to call • what they will return • etc

7. Importing • Library classes must be imported using an import statement • except classes from java.lang. • i.e. String – look it up on API import java.util.ArrayList; public class myClass{ ArrayList<String> arrl; arrl = new ArrayList<String>; public static void main(String[] args){ } }

8. Importing packages • Classes are organised in packages. • Single classes may be imported:import java.util.ArrayList; • Whole packages can be imported:import java.util.*;

9. Strings • Strings are actually objects • Did you notice we always use a capital S like other classes? • You don’t need to import them – they are from the automatically imported from java.lang.*; • As is System as in System.out.println()

10. Coming up • Implementation vs. Interface • The Truth about variables • Well, an abstract version of the truth anyway • Comparing strings • HashMaps

11. OO Have you ever.. • Accidently tried to output (print) an object? int[] array = new int[10]; System.out.println(array); You’ll get something like: [I@955cd5

12. OO What’s that? • This is the address in memory that the computer uses to store objects • in some other langauges (i.e. C++) you can manipulate those addresses and their contents directly • It’s time to look into the “variables as cups” anaology in a bit more detail

13. So far... • We’ve said • A variable is a cup • A primitive fits into a cup • A remote control to an object fits into a cup

14. The Truth • The “cup” is a bit of storage on a silicone chip in the computer somewhere • By giving the storage cups names, the computer associates the name of the cup with the address in memory where the storage is. • int age = 7; really means • “when the user types ‘age’ they really mean the contents of memory area 010111011010001010111001110101010”

15. The Truth • A bit pattern goes into the storage • 0100100111110101110101000011011000010011 • This bit pattern can either represent: • A number, character or other primitive value • The address in memory of something else, an object

16. OO So what? • Why is this important? • It’s important to know for comparing Strings.

17. Coming up • Implementation vs. Interface • The Truth about variables • Comparing strings • HashMaps

18. A word about comparing Strings Tests Identity if(input == “hello") { //code here } if(input.equals(“hello")) { //code here } • Strings should always be compared with.equals Tests Equality OO

19. OO identity vs equality • name1==name2 • False, different addresses • name1.equals(name2) • True, same value • name1==name2 • True, same address • name1.equals(name2) • True, same characters • name1 == name2 • False, different addresses • name1.equals(name2) • False, different value Memory Harry Ron Harry name1 name2 String String

20. Coming up • Implementation vs. Interface • The Truth about variables • Comparing strings • HashMaps

21. HashMap • One of the most useful classes in the library is the HashMap • It: • takes an object (The key) • looks at the internal properties within it • uses algorithms to compute a number based on those properties. This is the object’s hash code • stores the object in something like an array in the index given by the hash code

22. Then... • Then the HashMap associates the second object (the value) with the key object • This is useful

23. Maps Alfie 407351 • Maps are a collection type that map a key to a value • add(“Alfie”,“407351”); • and so on Memory Map: Name -> Phone number Jenny 763412

24. Lookup • Lookup is the act of supplying the key and having the value returned String num = myHashMap.get(“Alfie”); Memory myHashMap<String, Integer> Alfie 407351

25. HashMaps are used in upcoming labs... • you will come across Sets. They are a collection like any other, you can use the for-each loop with them etc • What happens if you add a key and a value, and then add the same key again, but with a different value? • Check out HashMaps in the API

26. Covered today • Implementation vs. Interface • The Truth about variables • Comparing strings • HashMaps