1. week51 APCS-A: Java Assignments & Syntax October 3, 2005

2. week52 Checkpoint 80 point Quiz #3 Car Homework  Any questions?  Visual check of everybody’s assignment

3. week53 Making Mistakes What happens if you forget a semi-colon? Forget parenthesis? Forget opening or closing brackets? Don’t use the “right” capitalization? Programming languages are very picky!  Why do you think that is?

4. week54 Syntax Syntax is…  The rules governing the structure of a language  The rules that the compiler understands  Defines meaning of the various symbols used in a language Alice didn’t let you make syntax mistakes  It only allowed you to drag-and-drop code statements that were syntactically correct Java isn’t like this  If you get the syntax wrong, the compiler won’t know what you are trying to do So it won’t do anything (it gives you an error)

5. week55 System.out System.out represents an output device or file, which by default is the monitor screen The print and println methods take one parameter  The string of characters to be printed println - prints the information sent, then moves to the beginning of the next line print - does not advance to the next line

6. week56 Concatenation Joining together or appending one string to the end of another String literals cannot extend across multiple lines in your program  You must use the plus sign to concatenate two string literals together System.out.println(“This is a bad program because it extends across multiple lines without concatenating the strings together. This will not compile.”); System.out.println(“This is a good program” + “ because it uses the + sign to extend” + “ the program across multiple lines.”);

7. week57 Variables A variable is a name for a location in memory that stores data When you declare a variable, you tell the compiler to reserve a certain size piece of memory to store data in (based on the type of data)  We will talk more about the types of data tomorrow Example:  String name;  int number;

8. week58 Variables Variables need a data type and a identifier. Think of a variable as a cup. A container. It holds something.  The data type tells Java the size of the container.  The identifier is there so you know which container is yours. name number

9. week59 Assignments When you declare a variable, you just set aside the memory - but that space is “blank” To actually place something in the variable, you need to assign a value to it:  name = “Nicole”;  number = 123; number “Nicole” 123 name

10. week510 Assignment Statements More examples:  String anotherName = “Philip”;  int num = 5 + 4*23; In Java, here are the basic rules:  On the left side is always the variable name, the “container” in which we are going to store data  On the right side could be a single piece of data, or an expression that has to be evaluated The single piece of data, or the result of whatever is evaluated is then “assigned” to the variable (ie - placed into the memory location)  You always end each statement with a semi-colon

11. week511 A program example public class Example { String name; int number; //make the containers public void doAssignment(){ name = “Nicole”; number = 123; printStuff(); } private void printStuff(){ System.out.println(“The name in this program is: “ + name); System.out.println(“The number is” + number); }

12. week512 Additional assignments If we reassign a value to a variable, the new value is written to memory.. What happens to the old value? int myVar = 123; myVar = 34343435345; If we just reference (use) a variable, the value is not changed System.out.print( “myVar is: “ + myVar); // after this call, myVar still has 34343435345 stored in it

13. week513 Strongly Typed Java doesn’t let us store the wrong kind of data in our variables  Unlike Alice, it doesn’t stop you from trying to assign the wrong kind of data  The compiler will cause an error if you try to use the wrong kind of data for a variable. If we declare a variable of type String: String s; Then we can’t put an integer into that String s = 1232; //Compile error: incompatible types - found int but expected java.lang.String

14. week514 Constants Sometimes we want to store a piece of data that will never change  For example, we may want to say that a table at lunch can hold no more than 12 people. If we just use the number 12 in our program, people looking at our code may not know where it came from So we give it a name like MAX_PEOPLE to help explain what the piece of data is doing in a program Also, lets say that we suddenly get new lunch tables that have a maximum of 15 people…  We only have to change the data in one place, instead of trying to find all the places that the number 12 was used in our program Constants are not variables (the value can never change)  they hold one value for the durations of the program

15. week515 Constants in Java To declare a constant in Java, you use the reserved word final By convention, we use uppercase letters to name constants so that we can tell them about from regular variables final int MAX_PEOPLE = 12; The compiler will then prevent anybody from changing the value once it has been set to the initial value

16. week516 Arithmetic Operators What if we want to have a counter variable?  Something that starts at 0 and then counts up, one-by- one depending on what else we may be doing in a program int x = 0; So we could do: x = x + 1;  So what would x equal? How does this all work? We do this a lot in programming, so there’s shortcut for this (called the increment operator)  x++;// is the same as x = x+1;

17. week517 Other Arithmetic Operators Minus  x = x -1; Multiplication  x = x * 5; Division:  x = x/2; What do you think will happen if we try to divide by zero? You have the special operator for increment, do you think there are any similar operators for these functions?

18. week518 APCS-A: Java Data Types October 4, 2005

19. week519 Checkpoint Car Homework  Any questions?  Visual check of everybody’s assignment

20. week520 Lab Today… but First there are a couple more things to cover… There’s another mathematical operator…  The Modulo operator  Modulo - what the heck is that???  %%%%%%%  Oops -- just one: %

21. week521 Modulo The modulo operator ( % ) returns the remainder from dividing one integer by another integer

22. week522 Elements of a Method Signature of the method  public void makePixel(int x, int y)  Public/private/protected is optional  The return type  The name of the method Good programming practice: Make it descriptive  Parameter declarations (if any)  ==> Signature tells the user and the compiler more about what to expect from the method Body Comment  Good Programming Practice: Place comments before methods that describe what the method does

23. week523 Lab Build a simple calculator Learn how Java deals with ints…  This will lead us into our discussion of data types tomorrow. Wahoo :)

24. week524 APCS-A: Java Data Types October 5, 2005

25. week525 Today Cover some details from yesterday’s lab Some additional information about variables and methods  Return statements  Scope  Programming practices Data Types

26. week526 Lab Redux What is the difference between ‘String’ and ‘string’? What is the difference between ‘System’ and ‘system’? Any other questions from lab yesterday?  What did we get when we divided 5/2?  5%2?

27. week527 Return Statements In methods, you can define the “return type” to be “void” (returns nothing) Return type to be any of the built-in data types we will talk about later today (like int) Return type can also be an object type  Like Dog, Person, or String Note: String is actually an object

28. week528 Return Statements public void add (int x, int y) { int z = x+y; System.out.println(“answer is: “ + z); } public int add (int x, int y) { return (x+y); } The first function is called with the statement: add(5, 10); The second statement can be called in many ways, but we probably want to do something with the answer returned back (we want to catch the answer somehow

29. week529 Catching return types public int add(int x, int y) In BlueJ, we could just get the answer back in an method result window Or we could use the answer in another part of our program  In another method  In a statement or some sort (assignment statement, print statement, etc) System.out.println(“answer is: “ + add(5,10)); int answer = add(5, 10); //notice I “catch” the value in an int variable divide(answer, 2);

30. week530 Scope Parameters and variables have scope  This means that they are only valid in the block (between the curly braces) in which they were defined Class scope  Variables defined at the top of the program (not inside any constructors or methods) -- these variables can be used anywhere in the class Constructor/Method scope  parameters passed in to a constructor/method can only be referred to in that constructor/method  Variables declared in a method can only be referred to in that method What happens when you refer to a parameter/variable outside the block it was defined in?

31. week531 Scope public class ScopeClass { int x = -1; String s = “gook ”; public void scopeFunc1() { String s = “Gobble-D ”; System.out.println(s + x); scopeFunc2(42); } public void scopeFunc2(int x) { System.out.println(s+x); } 1) What do you get if you make a call to scopeFunc1()? 2) What do you get if you make a call directly to scopeFunc2()? Question #1 Question #2

32. week532 Why does scope matter? In the last example, s wasn’t even a particularly good variable name… so we could have come up with even more descriptive names and not had the problem of trying to refer to two different variables with the same name But what about in the Car class we defined for homework:  We could have had an instance variable: String color, which had the car’s color  We could have had a paint method: public void paint (String color) {} in which we wanted to reset the Car’s color (the instance variable) to the new color (passed in with the parameter to the method)

33. week533 How do we solve this scope problem? The beginner’s way -- name your parameters something different than your class (instance) variables In Car class, we could have the instance variable, String color In the paint method, we could instead call the parameter String newColor Or, if you love your name… then when the class variable interferes with the scope of a parameter or method variable, you can use the advanced programmers’ solution: the this keyword to refer to the instance variable (which we will talk about later)  this.color = color;

34. week534 More about variables… The burning question: What can I name my variables?  Almost anything… But not Java reserved words…  abstract do if package synchronized boolean double implements private this break else import protected throw byte extends instanceof public throws case false int return transient catch final interface short true char finally long static try class float native strictfp void const for new super volatile continue goto null switch while default Luckily, BlueJ colors these words a different color so you will know there is something special about them before you know what they all mean

35. week535 Naming Practices in Programming Class names start with a capital letter  public class Dog { } Object names start with a lowercase letter  Dog fido = new Dog(); Method names and variables start with lowercase, but use capital letters for subsequent words  public void changeMyOil() {}  int myAge; Constants use all caps  MAX_INT_SIZE

36. week536 Primitive Data Types What is an int ? (Think back to math…) What might other primitive data types be? What other kinds of information might an object need to know?

37. week537 Data Types A variable's data type determines the values that the variable can contain and the operations that can be performed on it Primitive Data Types are built-in data types (in Java, there are 8 of them)  boolean, char, byte, short, int, long, float, double TypeStorageMin ValueMax Value boolean1 bit0 (false)1 (true) byte8 bits-128128 int32 bits-2,147,483,6482,147,483,647 double64 bitsApprox -1.7E+308 with 15 significant digits Approx 1.7E+308 with 15 significant digits

38. week538 Pre-Defined Data Types Pre-defined data types are just objects that Java has provided you that store information What is a String ?  Stores a section of text  Always in double quotes Java has several other data structures to store data in useful ways  We will be learning many of them this semester and next

39. week539 Converting Data Types What do we do if we want to compare an int variable to a double variable?  Or do int + double?  int / double? There are three ways that Java deals with this  Assignment conversion  Promotion  Casting

40. week540 Assignment Conversion Occurs when one type is assigned to a variable of another type during which the value is converted to the new type (only widening conversions can happen this way) double dollars = 5; Here the 5 will get automatically converted to a double (5.0) to be stored in the variable dollars

41. week541 Promotion If we divide a floating point number (float or double) by an int, then the int will be promoted to a floating point number before the division takes place When we concatenate a number with a string, promotion also happen - the number is converted (promoted) to a string, and then the strings are joined

42. week542 Casting The most general form of conversion in Java  If a conversion is possible, you can make it happen with casting You cast a variable by automatically applying a data type to it int x = 5; double dollars = 5.234; x = (int) dollars; //forces the double to become an int This will truncate (not round) the original value This can also be used to make sure we get the answer we expect --> if we want to divide 5/2 and get 2.5, then we want to force the result to be a double: double answer = (double) x / y; OR double answer = (double) x / (double) y;

43. week543 Tomorrow Quiz - we want to make sure that we are all comfortable with Java and the beginning Java syntax before moving further along Question: Programming Homework or Abstract Homework for 4 day weekend?

44. week544 Lab Redo/Adjust Calculator  Use return statements (change your methods to have non-void return types)  Change your calculator to support decimals (doubles) Add functions to make your calculator “Smarter”  Write a method that will return the average of 3 numbers  Write a Fahrenheit to Celsius conversion function And a Celsius to Fahrenheit converter  Write a method that will determine the value of coins in a “jar” and print the total in dollars and cents. The method will take in 4 integer parameters, representing the numbers of each kind of coin