CSE115: Introduction to Computer Science I

1. CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu

2. Agenda • Announcements • Cell phones off & name signs out • Web-CAT: back up, lab 1 • Next week • No new lab, but… • hand in lab 1 and do exercises • Monday: review and Q&A • Wednesday: first exam • Today • variables • terminology • maybe class definitions

3. Last time we saw… • how to instantiate a class to create an object new example1.Terrarium() • how to call a method on a newly created object: new example1.Terrarium().add(new example1.Caterpillar())

4. How do we make the Caterpillar move? • We call “start()” on the Caterpillar. • How do we do this? • Can we write the following? new example1.Terrarium().add(new example1.Caterpillar()) new example1.Caterpillar().start() • We can, but it won’t do what we want. • Q: what happens?

5. A: Two example1.Caterpillar objects are created. • One is placed in the example1.Terrarium, and is NOT “started”. • The other is NOT put into the example1.Terrarium, but is “started”. • We cannot refer to the Caterpillar we added to the Terrarium, because: • we have no reference to it after adding it. • The value of an expression is lost if it is not • used right away, or • remembered.

6. We want to write something like this: t . add( c ) c . start() • (notice ‘c’ used in two places) • To do this, we need a variable

7. A variable is:(at its most basic) • a storage location in memory • for example, location 120:

8. A variable has: • a name • a location • a type • a value • a scope • a lifetime •  in the HLL (Java) •  in memory •  representation scheme/size •  contents • We’ll discuss these later

9. Why are types important? • Recall the discussion from early in the semester about how to interpret a string like “01001101” – the proper interpretation depends on the encoding scheme (e.g. two’s complement or IEEE754) • The type of a variable tells the compiler, among other things, which encoding schemed to use to read/write data from/to the variable. • It also tells us what methods we can call.

10. Why are names important? • The location of a variable in memory is not known until runtime. • The name of a variable allows us to refer to the variable without knowing where in memory the variable might end up. • Names make programs easier to write and to read/understand.

11. Variable declaration • A variable declaration consists of two basic things: • the type of the variable • the name of the variable • Java insists that all variables be declared before use (not all languages require this).

12. The variable declaration • Consists minimally of: type & name • What is a type? A class is a type. • Remember: objects are instantiated from classes. • Examples of variable declarations: example1.Terrarium t; example1.Caterpillar c;

13. Naming conventions • CamelCase (camel hump at start of each word) • lower case letter to start • letter or digit follows • each new word, except first, capitalized • Examples: t myTerrarium theVeryHungryCaterpillar

14. Syntax (a little more formal) • variable declarations appear in many places in Java programs • a variable declaration in Java • ALWAYS has: type identifer • In our example • type identifier ; • semicolon is a terminator

15. We have a variable… • … so now what? • After we declare a variable, we need to assign a value to it. • We do that using an assignment statement. • Unlike an expression, a statement has no value.

16. assignment statement • variable = expression ; • ‘=’ is the ASSIGNMENT OPERATOR • Example: t = new example1.Terrarium();

17. Syntax example1.Terrarium t; t = new example1.Terrarium(); example1.Caterpillar c; c = new example1.Caterpillar(); t.add(c); c.start();

18. Live demo in DrJava • Created two example1.Caterpillar objects: • each has the same: • set of properties (e.g. location, direction, color) • set of capabilities (we only saw “start()”) • each has its own: • values for its properties; Caterpillars… • …are at distinct locations • …move independently of each other • …happen to have the same color (green)

20. Object diagram(corresponding to memory diagram on previous slide) t Boxes denote variables c example1.Terrarium Arrows denote references example1.Caterpillar This diagram is an abstraction of the one on the previous slide: it ignores irrelevant details, such as the addresses and sizes of the two objects being shown. An abstraction is thus a simplification. Ovals denote objects