1. Chapter 7: User-Defined Methods
Java Programming:
From Problem Analysis to Program Design,
Second Edition

2. Chapter Objectives
Understand how methods are used in Java programming.
Learn about standard (predefined) methods and discover how to use them in a program.
Learn about user-defined methods.
Examine value-returning methods, including actual and formal parameters.
Java Programming: From Problem Analysis to Program Design, Second Edition

3. Chapter Objectives
Explore how to construct and use a value-returning, user-defined method in a program.
Learn how to construct and use user-defined void methods in a program.
Explore variables as parameters.
Learn about the scope of an identifier.
Become aware of method overloading.
Java Programming: From Problem Analysis to Program Design, Second Edition

4. Predefined Classes Methods already written and provided by Java.
Organized as a collection of classes (class libraries).
To use, import package.
Method type: The data type of the value returned by the method.
Java Programming: From Problem Analysis to Program Design, Second Edition

5. Predefined Classes
Java Programming: From Problem Analysis to Program Design, Second Edition

6. Predefined Classes
Java Programming: From Problem Analysis to Program Design, Second Edition

7. Predefined Classes
Java Programming: From Problem Analysis to Program Design, Second Edition

8. class Character (Package: java.lang)
Java Programming: From Problem Analysis to Program Design, Second Edition

9. class Character (Package: java.lang)
Java Programming: From Problem Analysis to Program Design, Second Edition

10. Syntax of Value-Returning Method
modifier(s) returnType methodName
(formal parameter list) {
statements }
Java Programming: From Problem Analysis to Program Design, Second Edition

11. 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 the value that the method calculates and returns (using return statement).
methodName: Java identifier; name of method.
Java Programming: From Problem Analysis to Program Design, Second Edition

12. Syntax Syntax of formal parameter list:
dataType identifier, dataType identifier,...
Syntax to call a value-returning method:
methodName(actual parameter list)
Java Programming: From Problem Analysis to Program Design, Second Edition

13. Syntax Syntax of the actual parameter list:
expression or variable, expression or variable, ...
Syntax of the return statement:
return expr;
Java Programming: From Problem Analysis to Program Design, Second Edition

14. Equivalent Method Definitions
public static double larger(double x, double y) {
double max;
if (x >= y)
max = x;
else
max = y;
return max; }
Java Programming: From Problem Analysis to Program Design, Second Edition

15. Equivalent Method Definitions
public static double larger(double x, double y) {
if (x >= y)
return x;
else
return y; }
Java Programming: From Problem Analysis to Program Design, Second Edition

16. Equivalent Method Definitions
public static double larger(double x, double y) {
if (x >= y)
return x;
return y; }
Java Programming: From Problem Analysis to Program Design, Second Edition

17. Programming Example: Palindrome Number
Palindrome: An integer or string that reads the same forwards and backwards.
Input: Integer or string.
Output: Boolean message indicating whether integer string is a palindrome.
Java Programming: From Problem Analysis to Program Design, Second Edition

18. Solution: isPalindrome Method
public static boolean isPalindrome(String str) {
int len = str.length();
int i, j;
j = len - 1;
for (i = 0; i <= (len - 1) / 2; i++)
if (str.charAt(i) != str.charAt(j))
return false;
j--; }
return true;
Java Programming: From Problem Analysis to Program Design, Second Edition

19. Sample Runs: Palindrome Number
Java Programming: From Problem Analysis to Program Design, Second Edition

20. Sample Runs: Palindrome Number
Java Programming: From Problem Analysis to Program Design, Second Edition

21. 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 the method call statement, specify only actual parameters, not data type or method type.
Control goes back to caller when method exits.
Java Programming: From Problem Analysis to Program Design, Second Edition

22. Programming Example: Largest Number
Input: Set of 10 numbers
Output: Largest of 10 numbers
Solution:
Get numbers one at a time.
Method larger number: Returns the larger of 2 numbers.
For loop: Calls method larger number on each number received and compares to current largest number.
Java Programming: From Problem Analysis to Program Design, Second Edition

23. Solution: Largest Number
static Scanner console = new Scanner(System.in);
public static void main(String[] args) {
double num;
double max;
int count;
System.out.println("Enter 10 numbers.");
num = console.nextDouble();
max = num;
for (count = 1; count < 10; count++)
max = larger(max, num); }
System.out.println("The largest number is "
+ max);
Java Programming: From Problem Analysis to Program Design, Second Edition

24. Sample Run: Largest Number
Enter 10 numbers:
The largest number is 88.55
Java Programming: From Problem Analysis to Program Design, Second Edition

25. Void Methods Similar in structure to value-returning methods.
Call to method is always stand-alone statement.
Can use return statement to exit method early.
Java Programming: From Problem Analysis to Program Design, Second Edition

26. 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();
Java Programming: From Problem Analysis to Program Design, Second Edition

27. 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, ...
Java Programming: From Problem Analysis to Program Design, Second Edition

28. 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, ...
Java Programming: From Problem Analysis to Program Design, Second Edition

29. 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.
Java Programming: From Problem Analysis to Program Design, Second Edition

30. Reference Variables as Parameters
If a formal parameter is a reference variable:
Copies value of corresponding actual parameter.
Value of actual parameter is address of the object where actual data is stored.
Both formal and actual parameters refer to same object.
Java Programming: From Problem Analysis to Program Design, Second Edition

31. Uses of Reference Variables as Parameters
Can return more than one value from a method.
Can change the value of the actual object.
When passing an address, saves memory space and time, relative to copying large amount of data.
Java Programming: From Problem Analysis to Program Design, Second Edition

32. Reference Variables as Parameters: type String
Java Programming: From Problem Analysis to Program Design, Second Edition

33. Scope of an Identifier within a Class
Local identifier: An identifier that is declared within a method or block and that is visible only within that method or block.
Java does not allow the nesting of methods. That is, you cannot include the definition of one method in the body of another method.
Within a method or a block, an identifier must be declared before it can be used. Note that a block is a set of statements enclosed within braces.
A method’s definition can contain several blocks. The body of a loop or an if statement also forms a block.
Within a class, outside of every method definition (and block), an identifier can be declared anywhere.
Java Programming: From Problem Analysis to Program Design, Second Edition

34. Scope of an Identifier within a Class
Within a method, an identifier that is used to name a variable in the outer block of the method cannot be used to name any other variable in an inner block of the method. For example, in the following method definition, the second declaration of the variable x is illegal:
public static void illegalIdentifierDeclaration() {
int x;
//block
double x; //illegal declaration,
//x is already declared
... }
Java Programming: From Problem Analysis to Program Design, Second Edition

35. Scope Rules
Scope rules of an identifier that is declared within a class and accessed within a method (block) of the class.
An identifier, say X, that is declared within a method (block) is accessible:
Only within the block from the point at which it is declared until the end of the block.
By those blocks that are nested within that block.
Suppose X is an identifier that is declared within a class and outside of every method’s definition (block).
If X is declared without the reserved word static (such as a named constant or a method name), then it cannot be accessed in a static method.
If X is declared with the reserved word static (such as a named constant or a method name), then it can be accessed within a method (block) provided the method (block) does not have any other identifier named X.
Java Programming: From Problem Analysis to Program Design, Second Edition

36. Scope Rules Example 7-12 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)
Java Programming: From Problem Analysis to Program Design, Second Edition

37. Scope Rules public static int w;
public static void two(int one, int z) {
char ch;
int a;
//block three
int x = 12;
//...
} //end block three }
Java Programming: From Problem Analysis to Program Design, Second Edition

38. Scope Rules: Demonstrated
Java Programming: From Problem Analysis to Program Design, Second Edition

39. Method Overloading: An Introduction
Method overloading: More than one method can have the same name.
Two methods are said to have different formal parameter lists:
If both methods have a different number of formal parameters.
If the number of formal parameters is the same in both methods, the data type of the formal parameters in the order you list must differ in at least one position.
Java Programming: From Problem Analysis to Program Design, Second Edition

40. Method Overloading
public void methodOne(int x)
public void methodTwo(int x, double y)
public void methodThree(double y, int x)
public int methodFour(char ch, int x,
double y)
public int methodFive(char ch, int x,
String name)
These methods all have different formal parameter lists.
Java Programming: From Problem Analysis to Program Design, Second Edition

41. Method Overloading
public void methodSix(int x, double y,
char ch)
public void methodSeven(int one, double u,
char firstCh)
The methods methodSix and methodSeven both have three formal parameters, and the data type of the corresponding parameters is the same.
These methods all have the same formal parameter lists.
Java Programming: From Problem Analysis to Program Design, Second Edition

42. Method Overloading
Method overloading: Creating several methods within a class with the same name.
The signature of a method consists of the method name and its formal parameter list. Two methods have different signatures if they have either different names or different formal parameter lists. (Note that the signature of a method does not include the return type of the method.)
Java Programming: From Problem Analysis to Program Design, Second Edition

43. Method Overloading
The following method headings correctly overload the method methodXYZ:
public void methodXYZ()
public void methodXYZ(int x, double y)
public void methodXYZ(double one, int y)
public void methodXYZ(int x, double y,
char ch)
Java Programming: From Problem Analysis to Program Design, Second Edition

44. Method Overloading
public void methodABC(int x, double y)
public int methodABC(int x, double y)
Both these method headings have the same name and same formal parameter list.
These method headings to overload the method methodABC are incorrect.
In this case, the compiler will generate a syntax error. (Notice that the return types of these method headings are different.)
Java Programming: From Problem Analysis to Program Design, Second Edition

45. Programming Example: Data Comparison
Input: Data from two 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.
Java Programming: From Problem Analysis to Program Design, Second Edition

46. Programming Example: Data Comparison
Sample Output
Course No Group No Course Average
CSC
ENG
HIS
MTH
PHY
Avg for group 1: 82.04
Avg for group 2: 82.01
Java Programming: From Problem Analysis to Program Design, Second Edition

47. Programming Example: Data Comparison
Java Programming: From Problem Analysis to Program Design, Second Edition

48. Chapter Summary Pre-defined methods User-defined methods:
Value-returning methods
Void methods
Formal parameters
Actual parameters
Flow of execution
Java Programming: From Problem Analysis to Program Design, Second Edition

49. 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
Java Programming: From Problem Analysis to Program Design, Second Edition