2. PAC Introduction to Methods Professor: Evan Korth New York University

3.  Prentice Hall, Inc. All rights reserved. 6.1 Introduction Modules –Small pieces of a problem e.g., divide and conquer –Facilitate design, implementation, operation and maintenance of large programs

4.  Prentice Hall, Inc. All rights reserved. 6.2 Program Modules in Java 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

5.  Prentice Hall, Inc. All rights reserved. Fig. 6.1 Hierarchical boss-method/worker-method relationship. boss worker1worker2worker3 worker4worker5

6. 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.

7. 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

8. 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.

9. Using Static methods in the Java API

10.  Prentice Hall, Inc. All rights reserved. 6.3 Math -Class Methods Class java.lang.Math –Provides common mathematical calculations –Calculate the square root of 900.0 : Math.sqrt( 900.0 ) –Method sqrt belongs to class Math Dot (. ) allows access to method sqrt –The argument 900.0 is located inside parentheses

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12. 6.4 Methods Declarations Methods –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

13.  Prentice Hall, Inc. All rights reserved. 6.4 Method Declarations (Cont.) Programmers can write customized methods

14. 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 1// Fig. 6.3: SquareIntegers.java 2// Creating and using a programmer-defined method. 3public class SquareIntegers { 4public static void main (String args[]) 5{ 6int result; // store result of call to method square 7// loop 10 times 8for ( int counter = 1; counter <= 10; counter++ ) { 9result = square( counter ); // method call 10// print the result of one call to the method 11System.out.println ("The square of " + counter + " is " + 12result ); 13} // end for 14} // end method main() 15// square method declaration 16public static int square( int y ) 17{ 18return y * y; // return square of y 19} // end method square 20} // end class SquareIntegers Method square returns int that result stores Modified by Evan Korth  2003 Prentice Hall, Inc. All rights reserved. Method square returns the square of y y is the parameter of method square

15.  Prentice Hall, Inc. All rights reserved. 6.4 Method Declarations (cont.) General format of method declaration: modifiers return-value-type method-name ( parameter1, …, parameterN ) { declarations and statements } Method can also return values: return expression ;

16. random numbers 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

17.  Prentice Hall, Inc. All rights reserved. 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?

18.  Prentice Hall, Inc. All rights reserved. Random-Number Generation –( int ) ( Math.random() * 6 ) Produces integers from 0 - 5

19.  Prentice Hall, Inc. All rights reserved. Outline RandomIntegers. java Line 16 Produce integers in range 1-6 Line 16 Math.random returns double s. We cast the double as an int 1 // Fig. 6.7: RandomIntegers.java 2 // Shifted, scaled random integers. 3 import javax.swing.JOptionPane; 4 5 public class RandomIntegers { 6 7 public static void main( String args[] ) 8 { 9 int value; 10 String output = ""; 11 12 // loop 20 times 13 for ( int counter = 1; counter <= 20; counter++ ) { 14 15 // pick random integer between 1 and 6 16 value = 1 + ( int ) ( Math.random() * 6 ); 17 18 output += value + " "; // append value to output 19 20 // if counter divisible by 5, append newline to String output 21 if ( counter % 5 == 0 ) 22 output += "\n"; 23 24 } // end for 25 Produce integers in range 1-6 Math.random returns double s. We cast the double as an int

20.  Prentice Hall, Inc. All rights reserved. Outline RandomIntegers. java 26 JOptionPane.showMessageDialog( null, output, 27 "20 Random Numbers from 1 to 6", 28 JOptionPane.INFORMATION_MESSAGE ); 29 30 System.exit( 0 ); // terminate application 31 32 } // end main 33 34 } // end class RandomIntegers

21.  Prentice Hall, Inc. All rights reserved. Outline RollDie.java Line 14 Produce integers in range 1-6 Lines 17-43 Increment appropriate frequency counter, depending on randomly generated number 1 // Fig. 6.8: RollDie.java 2 // Roll a six-sided die 6000 times. 3 import javax.swing.*; 4 5 public class RollDie { 6 7 public static void main( String args[] ) 8 { 9 int frequency1 = 0, frequency2 = 0, frequency3 = 0, 10 frequency4 = 0, frequency5 = 0, frequency6 = 0, face; 11 12 // summarize results 13 for ( int roll = 1; roll <= 6000; roll++ ) { 14 face = 1 + ( int ) ( Math.random() * 6 ); 15 16 // determine roll value and increment appropriate counter 17 switch ( face ) { 18 19 case 1: 20 ++frequency1; 21 break; 22 23 case 2: 24 ++frequency2; 25 break; 26 27 case 3: 28 ++frequency3; 29 break; 30 Produce integers in range 1-6 Increment appropriate frequency counter, depending on randomly generated number

22.  Prentice Hall, Inc. All rights reserved. Outline RollDie.java 31 case 4: 32 ++frequency4; 33 break; 34 35 case 5: 36 ++frequency5; 37 break; 38 39 case 6: 40 ++frequency6; 41 break; 42 43 } // end switch 44 45 } // end for 46 47 JTextArea outputArea = new JTextArea(); 48 49 outputArea.setText( "Face\tFrequency" + "\n1\t" + frequency1 + 50 "\n2\t" + frequency2 + "\n3\t" + frequency3 + 51 "\n4\t" + frequency4 + "\n5\t" + frequency5 + 52 "\n6\t" + frequency6 ); 53 54 JOptionPane.showMessageDialog( null, outputArea, 55 "Rolling a Die 6000 Times", JOptionPane.INFORMATION_MESSAGE ); 56 57 System.exit( 0 ); // terminate application 58 59 } // end main 60 61 } // end class RollDie 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!

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24. User defined methods

25.  Prentice Hall, Inc. All rights reserved. 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 ;

26. 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

27. 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. 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.

28. 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);

29. 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 maximum (int x, int y) { /*body*/ }

30. Warning Unlike declaring variables, you must specifically state the type for multiple variables –For example takeTwoFloats( float x, y ) is incorrect –Instead you must write takeTwoFloats(float x, float y)

31. 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 ()

32.  Prentice Hall, Inc. All rights reserved. 6.5 Argument Promotion Coercion of arguments –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

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34. 6.15 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

35. Understanding Scope

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

37.  Prentice Hall, Inc. All rights reserved. 6.9 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

38. 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.

39. 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!

40. More Good Programming Habits Limit the number of a method’s parameters to approximately seven –Studies have shown that people can’t keep track of more than seven pieces of info at once –(e.g. 7 digit phone numbers for a reason) If a method is to return a value, make sure it returns something under all possible circumstances. Actually, Java will enforce this for you.

41. Superman program Superman needs a program to print S’s of various size. You should have one method that prints the horizontal lines and one method that prints the vertical lines. Write the whole program. –Vertical lines can be in the first or last column and are width/2 – 1 characters tall. –For example, an S of size 6 would look like : SSSSSS S SSSSSS S SSSSSS

42. RaiseIntToPower Write a method raiseIntToPower that takes two integers, n and k, and returns n k. For example the method call: raiseIntToPower (2, 4); would return 16 Hints: –What value should you use to initialize the return value? –2 4 = 2 X 2 X 2 X 2 –You can assume n and k are non-negative