Building Java Programs Chapter 3 Lecture 3 1 Parameters reading 3 1 2 Promoting reuse Programmers build increasingly complex applications Enabled by existing building blocks e g methods The more general a building block the easier to reuse Abstraction focusing on essential properties rather than implementation details Algebra is all about abstraction Functions solve an entire class of similar problems 3 Redundant recipes Recipe for baking 20 cookies Mix the following ingredients in a bowl 4 cups flour 1 cup butter 1 cup sugar 2 eggs 40 oz chocolate chips Place on sheet and Bake for about 10 minutes Recipe for baking 40 cookies Mix the following ingredients in a bowl 8 cups flour 2 cups butter 2 cups sugar 4 eggs 80 oz chocolate chips Place on sheet and Bake for about 10 minutes 4 Parameterized recipe Recipe for baking 20 cookies Mix the following ingredients in a bowl 4 cups flour 1 cup sugar 2 eggs Recipe for baking N cookies Mix the following ingredients in a bowl N 5 cups flour N 20 cups butter N 20 cups sugar N 10 eggs 2N oz chocolate chips Place on sheet and Bake for about 10 minutes parameter A value that distinguishes similar tasks 5 Redundant figures Consider the task of printing the following lines boxes 6 A redundant solution public class Stars1 public static void main String args lineOf13 lineOf7 lineOf35 box10x3 box5x4 public static void lineOf13 for int i 1 i 13 i System out print System out println public static void lineOf7 for int i 1 i 7 i System out print System out println public static void lineOf35 for int i 1 i 35 i System out print System out println This code is redundant Would variables help Would constants help What is a better solution line A method to draw a line of any number of stars box A method to draw a box of any size 7 Parameterization parameter A value passed to a method by its caller Instead of lineOf7 lineOf13 write line to draw any length When declaring the method we will state that it requires a parameter for the number of stars When calling the method we will specify how many stars to draw main 7 13 line line 8 Declaring a parameter Stating that a method requires a parameter in order to run public static void name type name statement s Example public static void sayPassword int code System out println The password is code When sayPassword is called the caller must specify the integer code to print 9 Passing a parameter Calling a method and specifying values for its parameters name expression Example public static void main String args sayPassword 42 sayPassword 12345 Output The password is 42 The password is 12345 10 Parameters and loops A parameter can guide the number of repetitions of a loop public static void main String args chant 3 public static void chant int times for int i 1 i times i System out println Just a salad Output Just a salad Just a salad Just a salad 11 How parameters are passed When the method is called The value is stored into the parameter variable The method s code executes using that value public static void main String args chant 3 chant 7 3 7 public static void chant int times for int i 1 i times i System out println Just a salad 12 Common errors If a method accepts a parameter it is illegal to call it without passing any value for that parameter chant ERROR parameter value required The value passed to a method must be of the correct type chant 3 7 ERROR must be of type int Exercise Change the Stars program to use a parameterized method for drawing lines of stars 13 Stars solution Prints several lines of stars Uses a parameterized method to remove redundancy public class Stars2 public static void main String args line 13 line 7 line 35 Prints the given number of stars plus a line break public static void line int count for int i 1 i count i System out print System out println 14 Multiple parameters A method can accept multiple parameters separate by When calling it you must pass values for each parameter Declaration public static void name type name type name statement s Call name exp exp exp 15 Multiple parameters example public static void main String args printNumber 4 9 printNumber 17 6 printNumber 8 0 printNumber 0 8 public static void printNumber int number int count for int i 1 i count i System out print number System out println Output 444444444 171717171717 00000000 Modify the Stars program to draw boxes with parameters 16 Stars solution Prints several lines and boxes made of stars Third version with multiple parameterized methods public class Stars3 public static void main String args line 13 line 7 line 35 System out println box 10 3 box 5 4 box 20 7 Prints the given number of stars plus a line break public static void line int count for int i 1 i count i System out print System out println 17 Stars solution cont d Prints a box of stars of the given size public static void box int width int height line width for int line 1 line height 2 line System out print for int space 1 space width 2 space System out print System out println line width 18 Value semantics value semantics When primitive variables int double are passed as parameters their values are copied Modifying the parameter will not affect the variable passed in public static void strange int x x x 1 System out println 1 x x public static void main String args int x 23 strange x System out println 2 x x Output 1 x 24 2 x 23 19 A Parameter Mystery problem public class ParameterMystery public static void main String args int x 9 int y 2 int z 5 mystery z y x mystery y x z public static void mystery int x int z int y System out println z and y x 20 Strings string A sequence of text characters String name text String name expression resulting in String Examples String name Marla Singer int x 3 int y 5 String point x y 21 Strings as parameters public class StringParameters public static void main String args sayHello Marty String teacher Bictolia sayHello teacher public static void sayHello String name System out println Welcome name Output Welcome Marty Welcome Bictolia Modify the Stars program to use string parameters Use a method named repeat that prints a string many times 22 Stars solution Prints several lines and boxes made of stars Fourth version with String parameters public class Stars4 public static void main String args line 13 line 7 line 35 System out println box 10 3 box 5 4 box 20 7 Prints the given number of stars plus a line break public static void line int count repeat count System out println 23 Stars solution cont d Prints a
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