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Introduction to Methods in Computer Science

This article provides an introduction to methods in computer science, including declaring a method, calling a method, methods calling methods, control flow of method call, generalizing method, parameterization, return values, overloading, and recursion.

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Introduction to Methods in Computer Science

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  1. CSC240 Computer Science III Zhen Jiang Dept. of Computer Science West Chester University West Chester, PA 19383 zjiang@wcupa.edu

  2. Table of Contents • Introduction (use of method) • Declaring a method • Calling a method • Methods calling methods • Control flow of method call • Generalizing method • Parameterization • Multiple parameters • Parameter mystery • Return values (vs. void) • Overloading • Recursion

  3. Procedural decomposition using methods

  4. public class FraggleRock { public static void main(String[] args) { System.out.println("Dance your cares away,"); System.out.println("Worry's for another day."); System.out.println("Let the music play,"); System.out.println("Down at Fraggle Rock."); System.out.println(); System.out.println("Dance your cares away,"); System.out.println("Worry's for another day."); System.out.println("Let the music play,"); System.out.println("Down at Fraggle Rock."); } } • Redundancy in programs

  5. This is a space & timing problem: to locate the redundancy of any part of program running at any time. Compared with the timing problem in loop: 1st, 2nd, 3rd … iterations Compared with the space problem in arrays: 1st, 2nd, 3rd … elements 1st, 2nd, 3rd … appearances of the same/similar procedure in your program!

  6. public class FraggleRock { public static void singChorus() { System.out.println("Dance your cares away,"); System.out.println("Worry's for another day."); System.out.println("Let the music play,"); System.out.println("Down at Fraggle Rock."); } public static void main(String[] args) { singChorus(); System.out.println(); singChorus(); } } finclaration, definition, body First time, four print lines repeated The second time, four prints repeated

  7. static method: A group of statements given a name. To use a static method: 1. define it (write down the recipe)‏ Write a group of statements and give it a name. 2. call it (cook using the recipe)‏ Tell our program to execute the method.

  8. public class FraggleRock { public static void singChorus() { System.out.println("Dance your cares away,"); System.out.println("Worry's for another day."); System.out.println("Let the music play,"); System.out.println("Down at Fraggle Rock."); } public static void main(String[] args) { singChorus(); System.out.println(); singChorus(); } } Method signature Method body Method call, 1st time Method call, 2nd time

  9. Declaring a static method The syntax for declaring a static method: public class <class name> { ... public static void <method name>() { <statement>; <statement>; … <statement>; } }

  10. Calling a static method The syntax for calling a static method (cooking using the recipe): <method name>();

  11. Declaring a static method public static void printAffirmation() { System.out.println("I am good enough!"); System.out.println("I am smart enough!"); System.out.println("People like me!"); } Calling a static method (possibly multiple times)‏ printAffirmation(); printAffirmation(); Output I am good enough! I am smart enough! People like me! I am good enough! I am smart enough! People like me!

  12. Methods calling methods One static method can call another (p310): public class MethodsExample { public static void main(String[] args) { message1(); message2(); System.out.println("Done with main."); } public static void message1() { System.out.println("This is message1."); } public static void message2() { System.out.println("This is message2."); message1(); System.out.println("Done with message2."); } } Output: This is message1. This is message2. This is message1. Done with message2. Done with main.

  13. When a method is called, the execution "jumps" into that method executes all of the method’s statements "jumps" back to the statement after the method call Control flow of methods

  14. public class MethodsExample { public static void main(String[] args) { message1(); message2(); ... } } public static void message1() { System.out.println("This is message1."); } public static void message2() { System.out.println("This is message2"); message1(); System.out.println("Done with message2."); } public static void message1() { System.out.println("This is message1."); } Output: This is message1. This is message2. This is message1. Done with message2. Done with main.

  15. Generalizing Method Consider the task of drawing the following figures: ************* ******* *********************************** ********** * * ********** ***** * * * * ***** The lines and figures are similar, but not exactly the same.

  16. public class Stars { public static void main(String[] args) { drawLineOf13Stars(); drawLineOf7Stars(); drawLineOf35Stars(); draw10x3Box(); draw5x4Box(); } public static void drawLineOf13Stars() { for (int i = 1; i <= 13; i++) { System.out.print("*"); } System.out.println(); } public static void drawLineOf7Stars() { for (int i = 1; i <= 7; i++) { System.out.print("*"); } System.out.println(); } public static void drawLineOf35Stars() { for (int i = 1; i <= 35; i++) { System.out.print("*"); } System.out.println(); } ... Observation: Methods are redundant. Would constants help us solve this problem? Other ideas?

  17. What if we had the following? drawLine - A method to draw a line of any number of stars. drawBox - A method to draw a box of any size. 7 drawLine main ******* 13 drawLine ************* Character of each repetition General format of repetition

  18. Parameterization parameterized method: A method that is given extra information (e.g. number of stars to draw) when it is called. The common part of all calls in a generalization: 1st, 2nd, 3rd, … parameter: A value passed to a method by its caller, mapping the difference of each call. Examples: System.out.println(“*”) or System.out.println(“**”) random.nextInt(10)

  19. Parameterized method declaration syntax: public static void <name> (<type><name>) { <statement(s)>; } The scope of the parameter is the entire method. P284 Example: public static void printSpaces(int count) { for (int i = 1; i <= count; i++) { System.out.print(" "); } } Whenever printSpaces is called, the caller must specify how many spaces to print. count’s scope

  20. passing an argument for parameter: Calling a parameterized method and specifying a value for its parameter(s). P290 Parameterized method call syntax: <name>(<expression>); Example: System.out.print("*"); printSpaces(7); System.out.print("**"); int x = 3 * 5; printSpaces(x + 2); System.out.println("***"); Output: * ** ***

  21. When the parameterized method call executes: the value passed to the method is copied into the parameter variable the method's code executes using that value public static void main(String[] args) { printSpaces(7); printSpaces(13); } public static void printSpaces(int count) { for (int i = 1; i <= count; i++) { System.out.print(" "); } } 7 13 count:

  22. value semantics: When primitive variables (such as int or double) are passed as parameters, their values are copied into the method's parameter variable. P285-291 Modifying the method’s parameter variable will NOT affect the the variable which was passed to the method. ... int x = 23; strange(x); System.out.println("2. x = " + x); // this x unchanged ... } public static void strange(int x) { x = x + 1; // modifies my x System.out.println("1. x = " + x); } Output: 1. x = 24 2. x = 23

  23. ERROR: Not passing a parameter to a method that accepts parameters. printSpaces(); // ERROR: parameter value required ERROR: Passing a parameter of the wrong type. printSpaces(3.7); // ERROR: must be of type int The parameter must satisfy the domain of the method.

  24. Change the Stars program to use parameterized methods. public class Stars { public static void main(String[] args) { drawLineOf13Stars(); drawLineOf7Stars(); drawLineOf35Stars(); draw10x3Box(); draw5x4Box(); } public static void drawLineOf13Stars() { for (int i = 1; i <= 13; i++) { System.out.print("*"); } System.out.println(); } public static void drawLineOf7Stars() { for (int i = 1; i <= 7; i++) { System.out.print("*"); } System.out.println(); } ...

  25. // Prints several lines of stars. // Uses a parameterized method to remove redundancy. public class Stars2 { public static void main(String[] args) { drawLine(13); drawLine(7); drawLine(35); } // Prints the given number of stars plus a line break. public static void drawLine(int count) { for (int i = 1; i <= count; i++) { System.out.print("*"); } System.out.println(); } }

  26. Multiple parameters Methods can accept as many parameters as you like. When the method is called, it must be passed values for each of its parameters. Multiple parameters declaration syntax: public static void <name> (<type><name>, <type><name>, ..., <type><name>) { <statement(s)>; } Multiple parameters call syntax: <name>(<expression>, <expression>, ..., <expression>);

  27. // Prints several lines and boxes made of stars. // Third version with multiple parameterized methods. public class Stars3 { public static void main(String[] args) { drawLine(13); drawLine(7); drawLine(35); System.out.println(); drawBox(10, 3); drawBox(5, 4); drawBox(20, 7); } // Prints the given number of stars plus a line break. public static void drawLine(int count) { for (int i = 1; i <= count; i++) { System.out.print("*"); } System.out.println(); }

  28. // Prints a box of stars of the given size. public static void drawBox(int width, int height) { drawLine(width); for (int i = 1; i <= height - 2; i++) { System.out.print("*"); printSpaces(width - 2); System.out.println("*"); } drawLine(width); } // Prints the given number of spaces. public static void printSpaces(int count) { for (int i = 1; i <= count; i++) { System.out.print(" "); } } }

  29. Exercise public static void main(String[] args) { printNumber(4, 9); printNumber(17, 6); printNumber(8, 0); printNumber(0, 8); } Output: 444444444 171717171717 00000000

  30. public static void main(String[] args) { printNumber(4, 9); printNumber(17, 6); printNumber(8, 0); printNumber(0, 8); } Output: 444444444 171717171717 00000000 public static void printNumber(int number, int count) { for(int i=0; i<count; i++) { System.out.print(number); } System.out.println(); }

  31. Write a method named printDiamond that accepts a height as a parameter and prints a diamond figure. * *** ***** *** * Write a method named multiplicationTable that accepts a maximum integer as a parameter and prints a table of multiplication from 1 x 1 up to that integer times itself.

  32. Rewrite the following program to use parameterized methods: // Draws triangular figures of stars. public class Loops { public static void main(String[] args) { for (int i = 1; i <= 5; i++) { for (int j = 1; j <= i - 1; j++) { System.out.print(" "); } for (int j = 1; j <= 10 - 2 * i + 1; j++) { System.out.print("*"); } System.out.println(); } for (int i = 1; i <= 12; i++) { for (int j = 1; j <= i - 1; j++) { System.out.print(" "); } for (int j = 1; j <= 25 - 2 * i; j++) { System.out.print("*"); } System.out.println(); } } }

  33. // Draws triangular figures using parameterized methods. public class Loops { public static void main(String[] args) { triangle(5); triangle(12); } // Draws a triangle figure of the given size. public static void triangle(int height) { for (int i = 1; i <= height; i++) { printSpaces(i - 1); drawLine(2 * height + 1 - 2 * i); } } }

  34. Parameter mystery (value vs. object) • What does this code do? public static void main(String[] args) { int a = 7; int b = 35; System.out.println(a + " " + b); int temp = a; a = b; b = temp; System.out.println(a + " " + b); }

  35. public static void main(String[] args) { int a = 7; int b = 35; System.out.println(a + " " + b); // swap a with b swap(a, b); System.out.println(a + " " + b); } public static void swap(int a, int b) { int temp = a; a = b; b = temp; } • Does this work? Why or why not?

  36. When a primitive type of argument is passed for parameter, the value is copied. A method cannot change the value of an argument. • When an object (i.e., class variable) is passed as a parameter, the object is not copied. The same object is referred to by both the original argument and the method's parameter. • If a method is called on the parameter, it will affect the original object that was passed to the method.

  37. Object are reference value like this: Dog balto = new Dog(); balto.name = “Balto”; name: breed: age: balto: “Balto” ? ? Variable, with slot in memory Reference, a pointer to the object’s data

  38. p • Example: public static void main(String[] args) { Point p1 = new Point(2, 3); move(p1); } public static void move(Point p) { p.setLocation(-1, -2); } 3 x: 2 y: -2 -1 p1

  39. What is the output of the following program? public class Mystery { public static void main(String[] args) { int x = 5, y = 9, z = 2; mystery(z, y, x); System.out.println(x + " " + y + " " + z); mystery(y, x, z); System.out.println(x + " " + y + " " + z); } } public static void mystery(int x, int z, int y) { x++; y = x - z * 2; x = z + 1; System.out.println(x + " " + y + " " + z); } } x: y: z: x: z: y:

  40. Return Values (vs. Void) return: To send a value out as the result of a method, which can be used in an expression. A return value is like the opposite of a parameter. Parameters pass information in from the caller to the method. Return values pass information out from a method to its caller. How would this be useful?

  41. Java has a class called Math that has several useful static methods to perform mathematical calculations. Method name Description abs(value)‏ absolute value cos(value)‏ cosine, in radians log(value)‏ logarithm base e log10(value)‏ logarithm base 10 max(value1, value2)‏ larger of two values min(value1, value2)‏ smaller of two values pow(base, exponent)‏ base to the exponent power random()‏ random double between 0 and 1 round(value)‏ nearest whole number sqrt(value)‏ square root

  42. Math method call syntax: Math.<method name>(<parameter(s)>)‏ Examples: double squareRoot = Math.sqrt(121.0); System.out.println(squareRoot); // 11.0 int absoluteValue = Math.abs(-50); System.out.println(absoluteValue); // 50 System.out.println(Math.min(3, 7) + 2); // 5 Notice that the preceding calls are used in expressions; they can be printed, stored into a variable, etc…

  43. The Math methods do not print results to the console. Instead, each method evaluates to produce (or return) a numeric result, which can be used in an expression. Math.abs -42 42 main 2.71 3 Math.round

  44. Evaluate the following expressions: Math.abs(-1.23)‏ Math.pow(3, 2)‏ Math.pow(10, -2)‏ Math.sqrt(121.0) - Math.sqrt(256.0)‏ Math.ceil(6.022) + Math.floor(15.9994)‏ Math.abs(Math.min(-3, -5))‏ Math.max and Math.min can be used to bound numbers. Consider an int variable named age. What statement would replace negative ages with 0? What statement would cap the maximum age to 40?

  45. Declaring a method that returns a value (p297): public static <type><name>(<parameters>) { <statement(s)>; } Returning a value from a method: return <expression>; Example: // Returns the given number cubed (to the third power). public static int cube(int number) { return number * number * number; }

  46. ERROR: Writing statements after a return statement. public static int increment(int x) { return (x + 1); x = x + 1; // ERROR: statement unreachable! } ERROR: Confusing the return variable with a variable in the calling method, AKA ignoring the return value. public class ReturnExample { public static void main(String[] args) { int x = 1; addOne(x); System.out.println("x = " + x); } public static int addOne(int x) { x = x + 1; return x; } }

  47. Write a method called isFactor that accepts two integers as parameters, and returns true if the first is a factor of the second.

  48. Methods can return different values under different conditions: public static int min(int a, int b) { if (a > b) { return b; } else { return a; } } public static String message(int place) { if (place == 1) { return "You won!"; } else { return "If you're not first, you're last!"; } }

  49. public static int min(int a, int b) { if (a > b) { return b; } } The compiler will complain about a "missing return statement". Why? ERROR: Not returning a value in every path. In the above example, what if a <= b?

  50. public static int min(int a, int b) { if (a > b) { return b; } else if (a <= b) { return a; } } It still produces the "missing return statement" error. Why? To our eyes, it is clear that all paths (greater, equal, less) do return a value. But the compiler thinks that if/else if code might choose not to execute any branch, so it refuses to accept this code. How can we fix it?

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