1. 1 Chapter 7 User-Defined Methods Java Programming from Thomson Course Tech, adopted by kcluk

2. 2 Predefined Classes Methods already written and provided by Java Organized as a collection of classes (class libraries) To use: import package Method type: data type of value returned by method

3. 3 class Math (Package: java.lang)

4. 4

5. 5 class Character (Package: java.lang)

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7. 7

8. 8 Syntax: Value-Returning Method modifier(s) returnType methodName(formal parameter list) { //body of method }

9. 9 User-Defined Methods Value-returning methods –Used in expressions –Calculate and return a value –Can save value for later calculation or print value Modifiers: public, private, protected, static, abstract, final returnType: type of value that the method calculates and returns (using return statement) methodName: Java identifier; name of method

10. 10 Syntax Syntax: Formal Parameter List The syntax of the formal parameter list is: (dataType identifier, dataType identifier,…) Method Call The syntax to call a value-returning method is: methodName(actual parameter list)

11. 11 Syntax Syntax: return Statement The return statement has the following syntax: return expr; Syntax: Actual Parameter List The syntax of the actual parameter list is: (expression or variable, expression or variable, …)

12. 12 Equivalent Method Definitions public static double larger(double x, double y) { double max; if(x >= y) max = x; else max = y; return max; }

13. 13 Equivalent Method Definitions public static double larger(double x, double y) { if(x >= y) return x; else return y; }  Activities, ref to Supple: Download and run the class AdditionExample.

14. 14 Flow of Execution Execution always begins with the first statement in the method main User-defined methods execute only when called Call to method transfers control from caller to called method In method call statement, specify only actual parameters, not data type or method type Control goes back to caller when method exits

15. 15 Programming Example: Largest Number Input: Set of 10 numbers Output: Largest of 10 numbers Solution: –Get numbers one at a time –Method largest number: returns the larger of 2 numbers –For loop: calls method largest number on each number received and compares to current largest number  pls refer to book pg 332-334

16. 16 Solution: Largest Number

17. 17 Sample Run: Largest Number Sample Run: (When you execute this program, enter all numbers in the same line.) Enter 10 numbers in the same line. 10.5 56.34 73.3 42 22 67 88.55 26 62 11 The largest number is 88.55

18. 18 Void Methods Similar in structure to value-returning methods No method type Call to method is always stand-alone statement Can use return statement to exit method early

19. 19 Void Methods: Syntax Method Definition The general form (syntax) of a void method without parameters is as follows: modifier(s) void methodName() { statements } Method Call (Within the Class) The method call has the following syntax: methodName();

20. 20 Void Methods with Parameters: Syntax Method Definition The definition of a void method with parameters has the following syntax: modifier(s) void methodName(formal parameter list) { statements } Formal Parameter List The formal parameter list has the following syntax: (dataType variable, dataType variable, …)

21. 21 Void Methods with Parameters: Syntax Method Call The method call has the following syntax: methodName(actual parameter list); Actual Parameter List The actual parameter list has the following syntax: (expression or variable, expression or variable, …)  Activities, ref to Supple: Download and run the class AdditionExample2

22. 22 Primitive Data Type Variables as Parameters A formal parameter receives a copy of its corresponding actual parameter If a formal parameter is a variable of a primitive data type –Value of actual parameter is directly stored –Cannot pass information outside the method –Provides only a one-way link between actual parameters and formal parameters  Download & run class PrimitivePass

23. 23 Reference Variables as Parameters: Created by new operater If a formal parameter is a reference variable –Copies value of corresponding actual parameter –Value of actual parameter is address of object where actual data is stored –Both formal and actual parameter refer to same object  Download & run class ReferencePass

24. 24 Uses of Reference Variables as Parameters Adv: Can return more than one value from a method Can change the value of the actual object When passing address, would save memory space and time, relative to copying large amount of data

25. 25 Reference Variables as Parameters: type String  Download & run Supplement 7C: prog A & B

26. 26 Scope of an Identifier Within a Class Scope (of an identifier): refers to those parts of a program where the identifier is accessible Local variables: variables declared within a method (or block) Within a class –Any method can call any other method –Exception: static method cannot call a nonstatic method

27. 27 Scope Rules Identifier declared within a block is accessible –Only within the block from the point at which it is declared until the end of the block –By those blocks nested within that block if the nested block does not have an identifier with the same name as the identifier in the outside block *Outside block: block that encloses nested block

28. 28 Scope Rules: Demonstrated

29. 29 Scope Rules: Demonstrated public class ScopeRules { static final double rate = 10.50; static int z; static double t; public static void main(String[] args) { int num; double x, z; char ch; //... } public static void one(int x, char y) { //... }

30. 30 public static int w; public static void two(int one, int z) { char ch; int a; //block three { int x = 12; //... }//end block three //... }

31. 31 Scope Rules: Demonstrated

32. 32 Method Overloading: An Introduction Method overloading: more than one method can have the same name Overloading Rules –Every method must have a different number of formal parameters OR –If the number of formal parameters is the same, then the data type of the formal parameter (in the order listed), must differ in at least one position –Types of parameters determine which method executes

33. 33 Programming Example: Data Comparison Input: Data from 2 different files Data format: Course Number followed by scores Output: Course Number, Group Number, Course Average Solution: –Read from more than one file, write output to file –Generate bar graphs –User-defined methods and re-use (calculateAverage and printResult) –Parameter passing

34. 34 Programming Example: Data Comparison

35. 35 Chapter Summary Predefined methods User-defined methods –Value-returning methods –Void methods –Formal parameters –Actual parameters Flow of Execution

36. 36 Chapter Summary Primitive data type variables as parameters –One-way link between actual parameters and formal parameters (limitations caused) Reference variables as parameters –Can pass one or more variables from a method –Can change value of actual parameter Scope of an identifier within a class Method overloading