Variables and Primitive Types

Transcription

1 CS159

2 Variables and Primitive Types Java Primitive Types: byte, short, int, long, float, double, boolean, char

3 Example int a; int b; boolean aisless; a = 3; b = 4; aisless = a < b; if (aisless && b < a) System.out.println("Hello"); else System.out.println("Goodbye"); What will be printed?

4 Example int a; int b; boolean aisless; a - a = 3; b = 4; aisless = a < b; if (aisless && b < a) System.out.println("Hello"); else System.out.println("Goodbye"); b - aisless - Note: - denotes an uninitialized variable. It is a syntax error to access an uninitialized local variable.

5 Example int a; int b; boolean aisless; a 3 a = 3; b = 4; aisless = a < b; if (aisless && b < a) System.out.println("Hello"); else System.out.println("Goodbye"); b - aisless -

6 Example int a; int b; boolean aisless; a 3 a = 3; b = 4; aisless = a < b; if (aisless && b < a) System.out.println("Hello"); else System.out.println("Goodbye"); b 4 aisless -

7 Example int a; int b; boolean aisless; a 3 a = 3; b = 4; aisless = a < b; if (aisless && b < a) System.out.println("Hello"); else System.out.println("Goodbye"); b 4 aisless true

8 Reference Types String, Arrays, all Classes.

9 Reference Example Person jane; jane = ; jane.setname("jane"); What will be printed? (Assume that setname and getname perform the expected operations.)

10 Reference Example Person jane; jane = ; jane.setname("jane"); - jane -

11 Reference Example Person jane; jane = ; jane.setname("jane"); - Person name "Bob" jane -

12 Reference Example Person jane; jane = ; jane.setname("jane"); Person name "Bob" jane -

13 Reference Example Person jane; jane = ; jane.setname("jane"); Person name "Bob" jane -

14 Reference Example Person jane; jane = ; jane.setname("jane"); Person name "Bob" jane

15 Reference Example Person jane; jane = ; jane.setname("jane"); Person name "Bob" jane The variables and jane are now aliased.

16 Reference Example Person jane; jane = ; jane.setname("jane"); Person name "Bob" jane "Jane"

17 Reference Example Person jane; jane = ; jane.setname("jane"); Quiz: What happens to this? Person name "Bob" jane "Jane"

18 Reference Example Person jane; jane = ; jane.setname("jane"); Quiz: What happens to this? Answer: garbage collection. Person name "Bob" jane "Jane"

19 What is a Reference... Really? We can print the contents of : System.out.println(); Result is: Person@6422b8ff

20 What is a Reference... Really? We can print the contents of : System.out.println(); Result is: Person@6422b8ff Java is telling us that contains the memory location 6422b8f*, where a Person object is stored. *Not quite true in Java.

21 Reference Example Revisited Person jane; jane = ; jane.setname("jane"); 6422b8f jane 6422b8f Memory "Bob" 210ace8d Person name 724eda4a 6422b8ff "Jane" 724eda4a

22 The Moral... Reference variables aren't so diferent from primitive variables. Both store one smallish piece of information (8-64 bits) Assignment (=) copies contents. Equality operator (==) compares contents.

23 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; public static void argdemo() int sage; What will be printed? sage = 21; makechanges(, sage); System.out.println(sAge);

24 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; public static void argdemo() int sage; sage = 21; - makechanges(, sage); System.out.println(sAge); sage -

25 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; public static void argdemo() int sage; Person "Bob" sage = 21; name makechanges(, sage); System.out.println(sAge); sage -

26 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; public static void argdemo() int sage; Person "Bob" sage = 21; name makechanges(, sage); System.out.println(sAge); sage 21

27 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; person age public static void argdemo() int sage; 21 Person "Bob" sage = 21; name makechanges(, sage); System.out.println(sAge); sage 21

28 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; person age public static void argdemo() int sage; 21 Person "Bob" sage = 21; name makechanges(, sage); System.out.println(sAge); sage 21 "Thor"

29 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; person age public static void argdemo() int sage; 31 Person "Bob" sage = 21; name makechanges(, sage); System.out.println(sAge); sage 21 "Thor"

30 Passing Arguments public static void makechanges(person person, int age) person.setname("thor"); age = age + 10; public static void argdemo() int sage; Person "Bob" sage = 21; name makechanges(, sage); System.out.println(sAge); sage 21 "Thor"