1. Introduction to Methods

2. Introduction Modules Small pieces of a problem
e.g., divide and conquer
Facilitate design, implementation, operation and maintenance of large programs

3. Program Modules in Java
Methods (procedural programming)
Classes (object oriented programming)
Java API provides several modules
Programmers can also create modules
e.g., programmer-defined methods
Methods
Invoked by a method call
Returns a result to calling method (caller)
Similar to a boss (caller) asking a worker (called method) to complete a task.
The boss method delegates certain jobs to specific methods.
Modified by Evan Korth

4. Examples of methods
A method that determines the maximum of two numbers.
A method that sorts a list of names
A method that opens a file from the file system.
A method that reads from the open file.
A method that opens a new socket to the internet.
A method that reads from that socket.
A method that parses an integer value from a String.
A method that gets a String from the user.

5. Important concept #1
Divide and Conquer: Break large programs into a series of smaller modules
Helps manage complexity
Makes it easier to build large programs
Makes it easier to debug programs

6. Important concept #2
Abstraction: Most of the time, you need to know what a method does, but not how it actually does it.
Also helps manage complexity
You use other people’s code without knowing how it does it’s job.

7. Using Static methods in the Java API

8. Math-Class Methods Class java.lang.Math
Provides common mathematical calculations
Calculate the square root of 900.0:
Math.sqrt( )
Method sqrt belongs to class Math
Dot (.) allows access to method sqrt
The argument is located inside parentheses

9. Often we want our programs to generate random numbers.
games of chance
testing without user interaction
java.lang.Math provides a method which can be used to generate random numbers

10. Random-Number Generation
Java random-number generators
Math.random()
Returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0.
What if we want to generate random integers?

11. Random-Number Generation
( int ) ( Math.random() * 6 )
Produces integers from 0 - 5

12. Produce integers in range 1-6
// Fig. 6.8: RollDie.java
// Roll a six-sided die 6000 times.
import javax.swing.*; 4
public class RollDie { 6
public static void main( String args[] ) {
int frequency1 = 0, frequency2 = 0, frequency3 = 0,
frequency4 = 0, frequency5 = 0, frequency6 = 0, face; 11
// summarize results
for ( int roll = 1; roll <= 6000; roll++ ) {
face = 1 + ( int ) ( Math.random() * 6 ); 15
// determine roll value and increment appropriate counter
switch ( face ) { 18
case 1:
frequency1;
break; 22
case 2:
frequency2;
break; 26
case 3:
frequency3;
break; 30
RollDie.java Line 14 Produce integers in range 1-6 Lines Increment appropriate frequency counter, depending on randomly generated number
Produce integers in range 1-6
Increment appropriate frequency counter, depending on randomly generated number

13. You do not need to know JTextArea for this class!
case 4:
frequency4;
break; 34
case 5:
frequency5;
break; 38
case 6:
frequency6;
break; 42
} // end switch 44
} // end for 46
JTextArea outputArea = new JTextArea(); 48
outputArea.setText( "Face\tFrequency" + "\n1\t" + frequency1 +
"\n2\t" + frequency2 + "\n3\t" + frequency3 +
"\n4\t" + frequency4 + "\n5\t" + frequency5 +
"\n6\t" + frequency6 ); 53
JOptionPane.showMessageDialog( null, outputArea,
"Rolling a Die 6000 Times", JOptionPane.INFORMATION_MESSAGE ); 56
System.exit( 0 ); // terminate application 58
} // end main 60
61 } // end class RollDie
RollDie.java
This is different from the example I showed in the compiler. I left it this way to show you that showMessageDialog() can take other objects besides String for the message. Remember the API.
You do not need to know JTextArea for this class!

14. User Defined Methods Declarations
Allow programmers to modularize programs
Makes program development more manageable
Software reusability
Avoid repeating code
Local variables
Declared in method declaration
Parameters
Communicates information between methods via method calls
Programmers can write customized methods

15. Method square returns int that result stores
1 // Fig. 6.3: SquareIntegers.java
2 // Creating and using a programmer-defined method.
3 public class SquareIntegers {
4 public static void main (String args[])
5 {
6 int result; // store result of call to method square
7 // loop 10 times
8 for ( int counter = 1; counter <= 10; counter++ ) {
9 result = square( counter ); // method call
10 // print the result of one call to the method
11 System.out.println ("The square of " + counter + " is " +
result );
13 } // end for
14 } // end method main()
15 // square method declaration
16 public static int square( int y )
17 {
18 return y * y; // return square of y
19 } // end method square
20 } // end class SquareIntegers
Line 9: method call to square line 16: header for method square. States that we have a method that accepts one integer as a parameter and returns one integer line 18: returns the value y * y
Method square returns int that result stores
y is the parameter of method square
Method square returns the square of y
 2003 Prentice Hall, Inc. All rights reserved.
Modified by Evan Korth

16. User defined Method Declarations
General format of method declaration: modifiers return-value-type method-name( parameter1, …, parameterN ) { declarations and statements }
Method can also return values:
return expression;

17. As with variables naming methods is important
Naming your methods
As with variables naming methods is important
You should give your methods names which clearly describe what the function is doing
helps debugging
helps others read your code
Same rules as naming variables
E.g. public static double calculateTax( int sale )
When you write about a method in an explanation use the parenthesis to indicate you are referencing a method (as opposed to a regular variable):
E.g. //call squareInteger() to calculate the square

18. Good programming with methods
A method should do one and only one useful action
If you see names for your method that suggest multiple actions then it’s time to break it up into separate functions; for example,
calculateTaxAndPrintReturnAndSaveFile(); -ugh
If you do something more than once in a program, you should write a method for that action.

19. More Good Programming
If you have written a method to do something in one project, and you need to do the same action in another project, you should reuse the method.
In Java this is usually accomplished by using classes which we will cover later this semester.

20. You can only return one value from a method. Returning void
Return Value Types
You can only return one value from a method.
Returning void
void: means nothing
A method that returns void therefore returns nothing.
Hence, there is no need for the optional return statement. But using one can force early exit from the method.
Example:
public static void printIntro (int n);

21. Unlike return values, you can pass as many parameters as you like.
Parameter Data Types
Unlike return values, you can pass as many parameters as you like.
To pass more than one parameter, you need to separate the parameters with commas.
public static int max (int x, int y) {
/*body*/ }

22. Warning
Unlike declaring variables, you must specifically state the type for multiple variables
For example
takeTwoInts( int x, y )
is incorrect
Instead you must write
takeTwoInts(int x, int y)

23. No parameters
You can also have a method that accepts no parameters. In such case, you would just have an empty parameter list.
E.g.
public static int rollDie ()
public static void printIntro ()

24. Argument Promotion Coercion of arguments Promotion rules
Forcing arguments to appropriate type to pass to method
e.g., System.out.println( Math.sqrt( 4 ) );
Evaluates Math.sqrt( 4 )
Then evaluates System.out.println()
Promotion rules
Specify how to convert types without data loss

26.  2000 Prentice Hall, Inc. All rights reserved.
Exercise 1
Implement the following methods
Method convertToCelsius returns the Celsius equivalent of a Fahrenheit temperature (( 5.0 / 9.0 * ( fTemp - 32 ) )
Method convertToFahrenheit returns the Fehrenheit equivalent of a Celsius temperature ( 9.0 / 5.0 * cTemp + 32 )
Use these methods to write a program that prints charts showing the Fahrenheit equivalent of all Celsius temperatures from 0 to 100 degrees, and the Celsius equivalents of all Fahrenheit temperatures from 32 to 212 degrees. Print the output in a neat tabular format.
 2000 Prentice Hall, Inc. All rights reserved.

27. Exercise 2
Write a method that will print a line of stars. The method should take one parameter which will determine the number of stars to print. Write a main() method which calls the star printing method 5 times using random numbers from

28. When is the return statement required?
review
When is the return statement required?
What do the following method headers tell us?
public static int max (int a, int b)
public static void nPrintln (String s, int n)
public static void main (String args[])
public static int max (int a, n)
public static String pickLine ()

29. review
Given the following method headers:
public static int max (int x, int y)
public static double min (double x, double y)
Which of the following method calls are legal?
max (1,2);
min (1,2);
max (1.0, 2.0);
min (1.0, 2.0);

30. Method Overloading Method overloading Several methods of the same name
Different parameter set for each method
Number of parameters
Parameter types
The Java compiler determines which method to use based on the parameters.
Can also be used in conjunction with argument coercion.
The combination can lead to ambiguous invocation which is an error

31. Understanding Scope

32. Determines where the variable can be referenced in a program.
Scope
Determines where the variable can be referenced in a program.
By understanding scope, you can make your programs:
more modular
easier to debug

33. Scope of Declarations Scope
Portion of the program that can reference an entity by its name
Basic scope rules
Scope of a parameter declaration
The entire method
Scope of a local-variable declaration
From where it is declared until the end of the block it is declared in
Scope of a local-variable declaration that appears in the initialization section of a for statement’s header
The entire code block of the for loop
Modified by Evan Korth

34. pass by value
In Java when a parameter is passed to a method and the variable is modified by that method, the value does not change upon return to the calling method (there are exceptions to this including passing arrays and passing object references)
This is not true in all programming languages.

35. Good Programming Habits
Pick a method name that strongly, completely and clearly describes what the function does or returns
E.g. verb-plus-object or description of returned value (getters, setters, predicates are object orientated naming conventions)
Make sure your actual parameters (i.e. the actual data you pass to a method) matches the data type the method expects.
Make sure you use all the parameters in your method; if not, lose the ones you don’t use.
Document any assumptions about your parameters!