Chapter 5: Methods. by Tony Gaddis. Starting Out with Java: From Control Structures through Objects. Fourth Edition

Transcription

1 Chapter 5: Methods Starting Out with Java: From Control Structures through Objects Fourth Edition by Tony Gaddis Addison Wesley is an imprint of 2010 Pearson Addison-Wesley. All rights reserved.

2 Reading Quiz 1. In Java a named block of code that accomplishes a specific task is called a: a) Function b) Subroutine c) Loop d) Method 2. A method is one that does not return a value when finished. a) boolean b) String c) void d) class

3 Chapter Topics Chapter 5 discusses the following main topics: Introduction to Methods Passing Arguments to a Method More About Local Variables Returning a Value from a Method Problem Solving with Methods 5-3

4 What is a Method? A named group of Java commands.

5 Using Methods to Build A Shakespearean Insult Generator Insult me! How it works

6 PseudoCode for Insult Generator 1. Clear the screen 2. Display Shakespearean Insult Generator 3. Begin the insult with Thou 4. Add adjectives 5. Add noun 6. Add insult ending

7 clearscreen() Method call Method header Method body Method declaration

8 The Method Call Call public static void main(string[] args) clearscreen(); 5-8

9 Two Parts of Method Declaration Header public static void clearscreen() System.out.println( \f"); Body 5-9

10 What is the output of this program? public class Checkpoint public static void main(string[] args) method1(); method1(); method2(); What is the output? public static void method1() System.out.println("abra"); public static void method2() System.out.print("cad"); method1(); Circle method headers. Box method calls

11 What is the output of this program? public class Checkpoint public static void main(string[] args) method1(); method1(); method2(); circle method headers box method calls public static void method1() System.out.println("abra"); public static void method2() System.out.print("cad"); method1(); output: abra abra cadabra

12 Let s write some methods begininsult() adjective() - use if-else-if loop in main() for many insults 2 adjectives noun() - use switch endinsult()

13 Why Write Methods? Methods are commonly used to break a problem down into small manageable pieces. This is called divide and conquer. Methods simplify programs. If a specific task is performed in several places in the program, a method can be written once to perform that task, and then be executed anytime it is needed. This is known as code reuse. 5-13

14 Better and Deeper generateinsults() generateinsults(500) Arguments and parameters Data types must match Can pass many arguments like Math.pow() Pass by reference; scope

15 Checkpoint

16 Checkpoint argument is given to method call parameter receives argument in method body

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19 User Input and Passing Data Ask user for # of insults. getnuminsults() Returns an int

20 Methods we know already. Do they receive and/or return data? int i = keyboard.nextint(); System.out.println( Hello World ); String upper = name.touppercase(); clearscreen(); System.out.println( Math.sqrt(49) );

21 Defining a Value-Returning Method public static int sum(int num1, int num2) int result; result = num1 + num2; return result; This expression must be of the same data type as the return type Return type The return statement causes the method to end execution and it returns a value back to the statement that called the method. 5-21

22 Calling a Value-Returning Method total = sum(value1, value2); public static int sum(int num1, int num2) int result; result = num1 + num2; return result; 5-22

23 Checkpoint is it void or value-returning? 5.3b Is the following line of code a method header or method call? void or value-returning?

24 Checkpoint a void method has no return value when invoked it appears on a line by itself a value returning method comes back with a value when complete must appear in an assignment statement or output line. is it void or value-returning? 5.3b Is the following line of code a method header or method call? void or value-returning?

25 What data types can be passed or returned? Any: int, double, boolean, String, Any primitive or object Parameter type must match Argument type. Return type must match the use in the call.

26 Returning a Reference to a String Object customername = fullname("john", "Martin"); address public static String fullname(string first, String last) John Martin See example: String name; name = first + " " + last; return name; ReturnString.java Local variable name holds the reference to the object. The return statement sends a copy of the reference back to the call statement and it is stored in customername. 5-26

27 String Returning Method Example public class ReturnString public static void main(string[] args) String customername; customername = fullname("john", "Martin"); System.out.println(customerName); public static String fullname(string first, String last) String name; name = first + " " + last; return name;

28 Checkpoint

29 Checkpoint public static int days( int years, int months, int weeks) public static double distance(double rate, double time)