Programming: Java. Chapter Objectives. Chapter Objectives (continued) Program Design Including Data Structures. Chapter 7: User-Defined Methods

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1 Chapter 7: User-Defined Methods Java Programming: Program Design Including Data Structures 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: Program Design Including Data Structures 2 Chapter Objectives (continued) 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: Program Design Including Data Structures 3 1

Program Design Including Data Structures 4 Predefined Classes (continued) Java Programming: Program Design

2 Predefined Classes Methods already written and provided by Java Organized as a collection of classes (class libraries) Must import package Method type: The data type of the value returned by the method Java Programming: Program Design Including Data Structures 4 Predefined Classes (continued) Java Programming: Program Design Including Data Structures 5 Predefined Classes (continued) Java Programming: Program Design Including Data Structures 6 2

Predefined Classes (continued) Java Programming:

Data Structures 8 class lang) (continued) Java

3 Predefined Classes (continued) Java Programming: Program Design Including Data Structures 7 class Character (Package: java.lang) Java Programming: Program Design Including Data Structures 8 class Character (Package: java.lang) (continued) Java Programming: Program Design Including Data Structures 9 3

4 Syntax of Value-Returning Method modifier(s) returntype methodname (formal parameter list) statements Java Programming: Program Design Including Data Structures 10 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: Program Design Including Data Structures 11 Syntax Syntax of formal parameter list: datatype identifier, datatype identifier,... Syntax to call a value-returning method: methodname(actual parameter list) Java Programming: Program Design Including Data Structures 12 4

5 Syntax (continued) Syntax of the actual parameter list: expression or variable, expression or variable,... Syntax of the return statement: return expr; Java Programming: Program Design Including Data Structures 13 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: Program Design Including Data Structures 14 Equivalent Method Definitions (continued) public static double larger(double x,double y) if (x >= y) return x; else return y; Java Programming: Program Design Including Data Structures 15 5

6 Equivalent Method Definitions (continued) public static double larger(double x, double y) if (x >= y) return x; return y; Java Programming: Program Design Including Data Structures 16 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: Program Design Including Data Structures 17 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: Program Design Including Data Structures 18 6

methods execute only when called Call to method transfers control from caller to called In the method call statement, specify only actual

7 Sample Runs: Palindrome Java Programming: Program Design Including Data Structures 19 Sample Runs: Palindrome (continued) Java Programming: Program Design Including Data Structures 20 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 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: Program Design Including Data Structures 21 7

8 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 larger of 2 numbers For loop: Calls method largest number on each number received and compares to current largest number Java Programming: Program Design Including Data Structures 22 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++) num = console.nextdouble(); max = larger(max, num); System.out.println("The largest number is " + max); Java Programming: Program Design Including Data Structures 23 Sample Run: Largest Number Sample Run: Enter 10 numbers: The largest number is Java Programming: Program Design Including Data Structures 24 8

9 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: Program Design Including Data Structures 25 Void Methods: Syntax Method definition: syntax of a void method without parameters: modifier(s) void methodname() statements Method call (within the class): use following syntax: methodname(); Java Programming: Program Design Including Data Structures 26 Void Methods with Parameters: Syntax Method definition: definition of a void method with parameters has the following syntax: modifier(s) void methodname (formal parameter list) statements Formal parameter list: following syntax: datatype variable, datatype variable,... Java Programming: Program Design Including Data Structures 27 9

10 Void Methods with Parameters: Syntax (continued) Method call: use the following syntax: methodname(actual parameter list); Actual parameter list: use the following syntax: expression or variable, expression or variable,... Java Programming: Program Design Including Data Structures 28 Primitive Data Type Variables as Parameters A formal parameter receives a copy of its corresponding actual parameter If a formal parameter is 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: Program Design Including Data Structures 29 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: Program Design Including Data Structures 30 10

11 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: Program Design Including Data Structures 31 Reference Variables as Parameters: type String Java Programming: Program Design Including Data Structures 32 Scope of an Identifier within a Class Local identifier: declared in a method or block; visible only in that method or block Cannot nest methods: cannot include the definition of one method in the body of another method An identifier must be declared before it can be used A method s definition can contain several blocks Within a class, outside of every method definition (and block), an identifier can be declared anywhere Java Programming: Program Design Including Data Structures 33 11

12 Scope of an Identifier within a Class In 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. 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: Program Design Including Data Structures 34 Scope Rules Scope rules of an identifier declared within a class and accessed within a method (block) of the class An identifier, say X, 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 Java Programming: Program Design Including Data Structures 35 Scope Rules (continued) Suppose X is declared within a class and outside of every method s definition (block) If X is declared without static (e.g., a named constant or a method name), then it cannot be accessed in a static method If X is declared with static (e.g., 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: Program Design Including Data Structures 36 12

Scope Rules (continued) 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; //.

13 Scope Rules (continued) 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: Program Design Including Data Structures 37 Scope Rules (continued) 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: Program Design Including Data Structures 38 Scope Rules: Demonstrated Java Programming: Program Design Including Data Structures 39 13

14 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 methods have a different number of 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: Program Design Including Data Structures 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: Program Design Including Data Structures 41 Method Overloading (continued) public void methodsix(int x, double y, char ch) public void methodseven(int one, double u, char firstch) methodsix and methodseven both have three formal parameters, and the data type of the corresponding parameters is the same These methods have the same formal parameter lists Java Programming: Program Design Including Data Structures 42 14

15 Method Overloading (continued) Method overloading: Creating several methods within a class with the same name The signature of a method: name and formal parameter list Two methods have different signatures if they have either different names or different formal parameters Signature does not include the return type Java Programming: Program Design Including Data Structures 43 Method Overloading (continued) 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: Program Design Including Data Structures 44 Method Overloading (continued) public void methodabc(int x, double y) public int methodabc(int x, double y) These method headings have the same name and same formal parameter list Incorrect overloading Compiler generates a syntax error, even though return types are different Java Programming: Program Design Including Data Structures 45 15

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

16 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: Program Design Including Data Structures 46 Programming Example: Data Comparison (continued) Sample Output Course No Group No Course Average CSC ENG HIS MTH PHY Avg for group 1: Avg for group 2: Java Programming: Program Design Including Data Structures 47 Programming Example: Data Comparison (continued) Java Programming: Program Design Including Data Structures 48 16

17 Chapter Summary Pre-defined methods User-defined methods: Value-returning methods Void methods Formal parameters Actual parameters Flow of execution Java Programming: Program Design Including Data Structures 49 Chapter Summary (continued) 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: Program Design Including Data Structures 50 17

Chapter 7 User-Defined Methods. Chapter Objectives

Chapter 7 User-Defined Methods 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