1. Objects vs. Primitives Primitive Primitive byte, short, int, long byte, short, int, long float, double float, double char char boolean boolean Object (instance of a class) Object (instance of a class) String, Integer, Character, ArrayList, … String, Integer, Character, ArrayList, … Animal, Student, DVD, Room,.... Animal, Student, DVD, Room,....

2. Primitives private int x; private int x; x x = 17; Objects private Integer y; // y stores a null reference ! y [null] 0 17

3. Primitives private int x; x x = 17; Objects Integer y [null] y = new Integer (17); // y stores a reference to an object ! 17 0x00e12c ? private Integer y;

4. ArrayList Example ArrayList names; ArrayList names; names = new ArrayList(); names = new ArrayList(); names.add (new String (“joe”)); names.add (new String (“joe”)); names.add(new String (“pat”)); names.add(new String (“pat”)); names.add (new String (“androgynous”)); names.add (new String (“androgynous”)); names size: 0 joe pat androgynous 012 [null] 0x20410c ? size: 1size: 2size: 3

5. Assignment of References ArrayList kids; kids = names; kids.remove (1); names size: 0size: 2size: 1 joe pat androgynous size: 3 012 [null] 0x20410c ? kids [null] 0x20410c ?

6. Assignment of References ArrayList kids; kids = names; kids.remove (1); names size: 0size: 2size: 1 joe androgynous size: 3 01 [null] 0x20410c ? kids [null] 0x20410c ? size: 2

7. “Deep” Copy ArrayList kids = new ArrayList(); for (String aName : names) kids.add (aName); names size: 0size: 2size: 1 joe pat androgynous size: 3 012 0x20410c ? kids 0x204240 ? size: 0size: 1 0 aName

8. “Deep” Copy ArrayList kids = new ArrayList(); for (String aName : names) kids.add (aName); names size: 0size: 2size: 1 joe pat androgynous size: 3 012 0x20410c ? kids 0x204240 ? size: 2 01 aName

9. “Deep” Copy ArrayList kids = new ArrayList(); for (String aName : names) kids.add (aName); names size: 0size: 2size: 1 joe pat androgynous size: 3 012 0x20410c ? kids 0x204240 ? size: 3 012 aName

10. size: 2 size: 3 Automatic Garbage Collection names.remove (1); kids.remove (1); names size: 0size: 3size: 2 joe pat androgynous 012 0x20410c ? kids 0x204240 ? size: 3 012

11. References as Parameters // calling method [client] aMethod (names); names size: 3 joe pat androgynous 012 0x20410c ? kids 0x20410c ? size: 2 // called method [server] public void aMethod (List kids) { kids.remove (1); }

12. Comparison of Primitives int x, y; … if (x == y) … if (x != y) …

13. size: 3 Comparison of Objects if (names == kids) => false if (names.equals (kids)) => true names size: 3 joe pat androgynous 012 0x20410c ? kids 0x204240 ? size: 3 012 Moral: Always use.equals to compare objects.