CISC124, Reminders

1. CISC124, Reminders • Course Web Site: http://research.cs.queensu.ca/home/cisc124w • Last night we had 69 out of 110 students having filled out the lab section survey. CISC124 - Prof. McLeod

2. Today… • Start OOP Syntax in Java. CISC124 - Prof. McLeod

3. Questions from Last Lecture: • Suppose we have two kinds of errors: • Syntax errors • Runtime errors • What is the difference? • Considering the two step process of running a Java program, at what stages will these errors be caught and who does the catching? • Who does the fixing??? CISC124 - Prof. McLeod

4. OOP in Java • A class or “object definition” or an “object”, consists of instance variables and/or methods. • By convention, instance variables are all declared before the methods: public class ShowStructure { // instance variables or “attributes” here // methods here } // end class ShowStructure CISC124 - Prof. McLeod

5. OOP in Java – Cont. • In Java, a class is an Object, and an Object is a class (ratherZenis it not!) • Code and attributes cannot be defined outside of a class. • The only code that can exist outside a method are attributes or other (“inner”) class definitions. CISC124 - Prof. McLeod

6. Using Methods Defined in Objects • Methods that are declared static can be invoked from a class without instantiation of that class. (System.out.println(), for example). • Non-static methods can only be invoked from instances of a class. (The Scanner class methods like .nextLine(), for example.) • Instantiation is through the use of the new keyword and usually involves the “customization” of a class by supplying particular values for class attributes. (Supplying System.in when instantiating the Scanner class, for example.) CISC124 - Prof. McLeod

7. Attributes • Also called “class variables” or “instance variables” or “fields”. • Declared within a class at the same level as the method declarations. • These variables are known to all methods within a class (their “scope”). • You can control their privacy and the way they are stored in memory (using public/private/protected and static). • (We’ll talk more about these keywords later…) CISC124 - Prof. McLeod

8. Attribute Declaration • Syntax: [private|public] [static] [final] type attributeName [= literalValue]; • Note that the type part is not optional – this is why java is a “declarative” language. • And, a variable cannot change its type later (“static typing”). • You cannot use a variable unless you have declared it first. CISC124 - Prof. McLeod

9. Variable Declaration • Declaring a variable inside a method gives that variable the scope of just inside the method, not outside the method. • Generally a variable is only available inside the block ({…}) in which it is declared. • The syntax for declaration inside a method is the same except you don’t need the [private|public] [static] [final] parts. CISC124 - Prof. McLeod

10. Method Declaration (a “Header”) • The syntax for simple method declaration: [private|public] [static] [final] returnTypemethodName ([parameterList]) {…} • If main invokes methods or uses attributes in the same class as itself then those attributes and methods must also be declared static. CISC124 - Prof. McLeod

11. Method Declaration - Cont. • A method must have a returnType. • The returnType can be any single Object or a primitive type. (For example: an int, double, String, an array (like int[], or any other pre-defined Object.) • If the method does not return anything, then the keyword void is used instead. • The main method does not return any value, so that’s why it is declared as in: public static void main (String[] args) {…} CISC124 - Prof. McLeod

12. Aside - The main Method • For the JVM to run an application, it must know where to start. • By design, the starting point is always the execution of the main method. • The JVM expects the main method to be declared exactly as shown – the only thing you can change is the name of the String array, called argsabove. CISC124 - Prof. McLeod

13. Method Declaration - Cont. • parameterListprovides a means of passing items, or parameters, into a method. • It is optional. • It can consist of one or many parameters, separated by commas. • Each parameter type must be declared in the parameter list, as in type variableName, type variableName, … • Java does not have named or optional parameters as does Python. CISC124 - Prof. McLeod

14. Aside - varargs • Use can use the ellipsis operator (three dots - …) to create a parameter that can take any number of arguments of that type. For example, suppose you have an object of type Bling: public void seeBling (Bling... blingers) {// code } • Within seeBling, you get at the individual arguments of type Bling by pretending that blingers is an array (use [ ] along with an index value.) CISC124 - Prof. McLeod

15. Aside – varargs, Cont. • We’ll need to look at an example of this later. • It is not used too often, but • See the declaration of the printf method for example: public PrintStreamprintf(String format, Object... args) CISC124 - Prof. McLeod

16. Methods - the return Statement • Unless the return type is void, the method must contain at least one return statement. A void method can also use a returnstatement without anything after it, as a means of terminating the method. • A method always stops (terminates) when a return statement is encountered. • Syntax: return [literal|expression]; CISC124 - Prof. McLeod

17. The return Statement - Cont. • The type of “literal|expression” must match the return type specified in the method declaration statement. CISC124 - Prof. McLeod

18. Method Examples public void printHello() { System.out.println(“Hello”); } // end printHello public void printHelloName(String yourName) { System.out.println(“Hello “ + yourName); } // end printHelloName public void printAvg(int a, int b) { System.out.println((a + b) / 2.0); } // end printAvg CISC124 - Prof. McLeod

19. Method Examples - Cont. public double average(double a, double b) { return (a + b) / 2; } // end average public int lowest(int a, int b) { if (a <= b) return a; else return b; } // end lowest Does this have to be here? CISC124 - Prof. McLeod

20. Attribute Examples public static double aVar; public static intaNum = 100; private static String hello = “Hello”; CISC124 - Prof. McLeod

21. Example: A Simple Class public class Simple { public static intaNum = 100; public static intsumNums(int num1, int num2) { return num1 + num2; } public static void main(String[] args) { intanotherNum = 200; System.out.println(sumNums(aNum, anotherNum)); } } CISC124 - Prof. McLeod

22. Another Simple Class // An example of the use of varargs (the ellipse...) // Also a for each loop! public class AnotherSimple { public static intsumNums(int... nums) { intsum = 0; for (intnum : nums) sum += num; return sum; } public static void main(String[] args) { System.out.println(sumNums(2, 5, 7, 10, 3)); } } // end AnotherSimple class CISC124 - Prof. McLeod

23. Python versus Java, so Far • You can see some major syntactical differences already: • Variable type declaration in mandatory. Return type declaration is mandatory. Parameter type declaration is mandatory. • Variable types are not dynamic! • A class header is required. • public static, The ; • Indentation in Java is for style only. { } are used to indicate code blocks or code containment. • println() must be invoked as a member of the System.out object. CISC124 - Prof. McLeod