Programmer-defined classes, part 2

1. Programmer-defined classes, part 2 More on identifier scope & Parameter passing

2. Categories of data storage • As we’re aware, we store data in memory, either in variables or constants • Each of these storage units must be declared; the location of the declaration determines the unit identifier’s scope: • Class-wide variables and constants are declared within the class but outside any method • Local variables and constants are declared within the body of a method • The scope of an identifier is defined by the boundaries of the block in which it is declared

3. One more category • A local variable is one that is declared within the body of a method • Another kind of local variable can be declared in the method’s parameter list • A part of the method’s heading, the parameter list specifies what, how many, and in what order arguments must be passed in a message that calls the method

4. General syntax for a method • The general syntax for a method definition consists of a heading and body, as follows: • The generic syntax for a method, as we’ve seen before, is: Modifier(s) returnType identifier (parameter(s)) { method body } • The parameter list looks like a list of variable declarations, separated by comma • The scope of a parameter is local to its method

5. Mutator methods and parameters • Data that is stored in variables that belong to the class are automatically accessible to member methods, as we have already seen • Member methods that are used to change these data values are called mutators; it is this type of method that almost always requires parameters

6. Setter methods for the BetterTherm class • Consider the example class we looked at in the last lecture, the BetterTherm class • This class featured a constructor that set a default value for the lone class variable, kelvinTemp, and getter methods that gave us access to this value in various forms • What is missing from the class are mutator methods that allow us to change the temperature value

7. Setter methods for the BetterTherm class • The purpose of a “setter” method is to assign a new value to a class variable • Such a method usually specifies a parameter; the value passed to the parameter is then assigned to the class variable • The methods on the next slide illustrate this concept for the BetterTherm class

8. Setter methods for the BetterTherm class public void setKtemp (int k) { kelvinTemp = Math.abs(k); } public void setCtemp (int c) { kelvinTemp = c + CFACTOR; } public void setFtemp (int f) { int cTemp = (int)((f - FCONVI) / FCONVD); kelvinTemp = cTemp + CFACTOR; }

9. General pattern for a setter method • Most setter methods follow this pattern: public void setValue (dataType param) { classVariable = param; } Where: • “dataType” is the same type as the class variable to be set • “classVariable” is the name of the previously-declared class variable • As we have seen, there are variations on the pattern; the three setter methods for BetterTherm use expressions that incorporate the parameter, rather than just the parameter itself, to set the value of variable kelvinTemp

10. Calling setter methods • A method that includes one or more parameters in its heading must be called with arguments that match its parameters • For example, consider the code below: BetterTherm bt = new BetterTherm(); bt.setCtemp(32); // sets kelvinTemp to 32° Celsius Random rg = new Random(); bt.setCtemp (Math.abs(rg.nextInt()) % 100); // sets kelvinTemp to a random value between 0° and 100° Celsius

11. Parameter matching • Parameters specify the number, ordering, and data type(s) of the argument(s) that can be passed to a method. • When a parameter is one of the simple numeric types, we can pass as an argument to the parameter any expression that is assignment-compatible with the parameter’s specified data type.

12. Parameter matching • Recall that expressions can include: • literal values, e.g. -4, 13.02 • initialized variables or constants • messages to value-returning methods (e.g. Integer.parseInt(), Math.sqrt(a)) • arithmetic operators & parentheses • explicit type casts • Assignment compatibility means that the value being passed is only valid if it can be assigned to a variable of the parameter’s type

13. Examples • The following are valid examples of messages requesting BetterTherm’s setFtemp method: bt.setFtemp(rg.nextInt()); bt.setFtemp(bt.getCtemp() + 5); bt.setFtemp((int)Math.sqrt(rg.nextDouble()));

14. Examples • The following are invalid examples of calls to BetterTherm’s setFtemp method – what is the matter with each? bt.setFtemp(); bt.setFtemp(“new temperature”); bt.setFtemp(Math.PI); bt.setFtemp(Math.sqrt(rg.nextDouble()));

15. Parameter matching and identifiers • Parameters, are variables local to the method they are declared in. As we already know, all variables have two properties: a data type and an identifier. • Arguments can have identifiers too. If we pass a variable or constant as an argument, we are passing a named argument. • There is no relationship between the name of a parameter and the name of its corresponding argument

16. Parameter matching and identifiers • Parameters and arguments can have different names (and usually do) • A parameter and its corresponding argument can have the same name, but: • The two variables (parameter and argument) belong to different methods • They do not have the same scope

17. Yet another analogy • I had a grandfather named Joe; he had a son named Joe, who was my uncle • My mother had a son named Joe, who is my brother • The three Joes are not the same person; and we can easily distinguish between them, because they exist(ed) in different locations – hence, different scope

18. Example • Consider the following code: public class SimpleClass { private int x; public void setVar (int y) { x = y; } int getVar () { return x; } } Consider variables x and y; what is the scope of each one? The code on the next slide contains a tester class for the class shown here. What happens when a new variable named x is declared there?

19. import java.util.*; public class TestSimp { public static void main (String [] args) { SimpleClass simp = new SimpleClass(); int x; Scanner kb = new Scanner (System.in); System.out.print ("Enter a whole number: "); x = kb.nextInt(); simp.setVar(x); System.out.println ("Value of SimpleClass’s x = " + simp.getVar()); x += 10; System.out.println ("Value of TestSimp’s x = " + x); System.out.println ("Value of SimpleClass’s x = " + simp.getVar()); } }