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Java Programs + Algorithms

Java Programs + Algorithms. “Think twice, code once.” -- Anonymous “Weeks of programming can save you hours of planning.” -- Anonymous Plan for Today More Simple Java Programs Comments in Java Algorithms Problem Decomposition Static Methods

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Java Programs + Algorithms

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  1. Java Programs + Algorithms “Think twice, code once.” -- Anonymous “Weeks of programming can save you hours of planning.” -- Anonymous Plan for Today • More Simple Java Programs • Comments in Java • Algorithms • Problem Decomposition • Static Methods Based on slides by Reges and Stepp at buildingjavaprograms.com

  2. Another Simple Program //*************************************** // File: Camus.java // Author: Mary Eberlein // // A simple first Java program //*************************************** public class Camus { // print a Camus quote public static void main(String[] args) { System.out.println("Camus said: "); System.out.print("Some people talk in their sleep. \nLecturers talk”); System.out.println(“ while other people sleep.”); } }

  3. Camus Program Output Camus said: Some people talk in their sleep. Lecturers talk while other people sleep. Exercise: Re-write the Camus program so that the quote is contained in double quotes: Camus said: “Some people talk in their sleep. Lecturers talk while other people sleep.”

  4. Practice Questions • What is the output of the following println statements? System.out.println("\ta\tb\tc"); System.out.println("\\\\"); System.out.println("'"); System.out.println("\"\"\""); System.out.println("C:\nin\the downward spiral"); • Write a println statement to produce this output: / \ // \\ /// \\\

  5. Answers • Output of each println statement: a b c \\ ' """ C: in he downward spiral • println statement to produce the line of output: System.out.println("/ \\ // \\\\ /// \\\\\\");

  6. More Practice Programs • What println statements will generate this output? This program prints a quote from the Gettysburg Address. "Four score and seven years ago, our 'fore fathers' brought forth on this continent a new nation." • What println statements will generate this output? A "quoted" String is 'much' better if you learn the rules of "escape sequences." Also, "" represents an empty String. Don't forget: use \" instead of " ! '' is not the same as "

  7. Answers • println statements to generate the output: System.out.println("This program prints a"); System.out.println("quote from the Gettysburg Address."); System.out.println(); System.out.println("\"Four score and seven years ago,"); System.out.println("our 'fore fathers' brought forth on"); System.out.println("this continent a new nation.\""); • println statements to generate the output: System.out.println("A \"quoted\" String is"); System.out.println("'much' better if you learn"); System.out.println("the rules of \"escape sequences.\""); System.out.println(); System.out.println("Also, \"\" represents an empty String."); System.out.println("Don't forget: use \\\" instead of \" !"); System.out.println("'' is not the same as \"");

  8. Comments • comment: A note written in source code by the programmer to describe the code and make it easier to understand. • Comments are not executed when your program runs. • Syntax: //comment text, on one lineor,/*comment text; may span multiple lines*/ • Examples: // This is a one-line comment. /* This is a very long multi-line comment. */

  9. Using comments • Where to place comments: • at the top of each file (a "comment header") - identifies the author and describes what the program does • at the start of every method (seen later) - describes the method’s behavior • to explain complex pieces of code inside methods • Comments are useful for: • Understanding larger, more complex programs. • Multiple programmers working together, who must understand each other's code.

  10. Comments example /* Suzy Student, CS 305j, Fall 2099 This program prints the Powerpuff Girls theme song. */ public class PowerpuffSong { public static void main(String[] args) { // first verse System.out.println(”Blossom, she is commander and leader,"); System.out.println(”Bubbles, she is a joy and a laughter,"); System.out.println(“Buttercup, she’s the toughest fighter”); System.out.println(“Powerpuffs save the day.”); System.out.println(); //second verse System.out.println(”Fighting crime,trying to save the world"); System.out.println(”Here they come just in time, the Powerpuff Girls"); } }

  11. Algorithms • algorithm: A list of steps for solving a problem. • Example algorithm: "Bake sugar cookies" • Mix the dry ingredients. • Cream the butter and sugar. • Beat in the eggs. • Stir in the dry ingredients. • Set the oven temperature. • Set the timer. • Place the cookies into the oven. • Allow the cookies to bake. • Spread frosting and sprinkles onto the cookies. • ...

  12. Problems with algorithms • lack of structure: Many tiny steps; tough to remember. • redundancy: Consider making a double batch... • Mix the dry ingredients. • Cream the butter and sugar. • Beat in the eggs. • Stir in the dry ingredients. • Set the oven temperature. • Set the timer. • Place the first batch of cookies into the oven. • Allow the cookies to bake. • Set the timer. • Place the second batch of cookies into the oven. • Allow the cookies to bake. • Mix ingredients for frosting. • ...

  13. Structured algorithms • structured algorithm: a list of steps to solve a problem, broken down into coherent tasks. 1Make the cookie batter. • Mix the dry ingredients. • Cream the butter and sugar. • Beat in the eggs. • Stir in the dry ingredients. 2 Bake the cookies. • Set the oven temperature. • Set the timer. • Place the cookies into the oven. • Allow the cookies to bake. 3 Add frosting and sprinkles. • Mix the ingredients for the frosting. • Spread frosting and sprinkles onto the cookies. ...

  14. Removing redundancy • A well-structured algorithm can describe repeated tasks with less redundancy. 1 Make the cookie batter. • Mix the dry ingredients. • ... 2a Bake the cookies (first batch). • Set the oven temperature. • Set the timer. • ... 2b Bake the cookies (second batch). 3 Decorate the cookies. • ...

  15. A program with redundancy public class BakeCookies { public static void main(String[] args) { System.out.println("Mix the dry ingredients."); System.out.println("Cream the butter and sugar."); System.out.println("Beat in the eggs."); System.out.println("Stir in the dry ingredients."); System.out.println("Set the oven temperature."); System.out.println("Set the timer."); System.out.println("Place a batch of cookies into the oven."); System.out.println("Allow the cookies to bake."); System.out.println("Set the oven temperature."); System.out.println("Set the timer."); System.out.println("Place a batch of cookies into the oven."); System.out.println("Allow the cookies to bake."); System.out.println("Mix ingredients for frosting."); System.out.println("Spread frosting and sprinkles."); } }

  16. class method A • statement • statement • statement method B • statement • statement method C • statement • statement • statement Static methods • static method: A named group of statements. • denotes the structure of a program • eliminates redundancy by code reuse • procedural decomposition:dividing a problem into methods • Writing a static method is likeadding a new command to Java.

  17. Using static methods 1. Design the algorithm. • Look at the structure, and which commands are repeated. • Decide what are the important overall tasks. 2. Declare (write down) the methods. • Arrange statements into groups and give each group a name. 3. Call (run) the methods. • The program's main method executes the other methods to perform the overall task.

  18. Design of an algorithm // This program displays a delicious recipe for baking cookies. public class BakeCookies2 { public static void main(String[] args) { // Step 1: Make the cake batter. System.out.println("Mix the dry ingredients."); System.out.println("Cream the butter and sugar."); System.out.println("Beat in the eggs."); System.out.println("Stir in the dry ingredients."); // Step 2a: Bake cookies (first batch). System.out.println("Set the oven temperature."); System.out.println("Set the timer."); System.out.println("Place a batch of cookies into the oven."); System.out.println("Allow the cookies to bake."); // Step 2b: Bake cookies (second batch). System.out.println("Set the oven temperature."); System.out.println("Set the timer."); System.out.println("Place a batch of cookies into the oven."); System.out.println("Allow the cookies to bake."); // Step 3: Decorate the cookies. System.out.println("Mix ingredients for frosting."); System.out.println("Spread frosting and sprinkles."); } }

  19. Static Methods • static method: a set of statements which is given a name, so it can be executed in a program. • Static methods simplify our programs by eliminating redundancy and breaking our program into manageable, cohesive pieces. • Breaking a problem down into static methods is called procedural decomposition. • Using a static method requires: • Write the method - write down a set of statements and give them a name. This is like writing down the recipe. • Call the method - execute the statements in our method. This is like using the recipe to bake cookies.

  20. Declaring a static method Gives your method a name so it can be executed public class <Class Name> { public static void <Method Name>() {statement;statement;...statement; } } • Example:public static void printWarning() { System.out.println("This product causes cancer"); System.out.println("in lab rats and humans.");}

  21. Calling a method • Calling method = executing method’s code • Syntax: <Method Name>(); • You can call the same method as many times as you like. • Example: printWarning(); System.out.println(); printWarning(); • Output: This product causes cancer in lab rats and humans. This product causes cancer in lab rats and humans.

  22. Program with static method public class FreshPrince { public static void main(String[] args) { rap(); // Calling (running) the rap method System.out.println(); rap(); // Calling the rap method again } // This method prints the lyrics to my favorite song. public static void rap() { System.out.println("Now this is the story all about how"); System.out.println("My life got flipped turned upside-down"); } } Output: Now this is the story all about how My life got flipped turned upside-down Now this is the story all about how My life got flipped turned upside-down

  23. Final cookie program // This program displays a delicious recipe for baking cookies. public class BakeCookies3 { public static void main(String[] args) { makeBatter(); bake(); // 1st batch bake(); // 2nd batch decorate(); } // Step 1: Make the cake batter. public static void makeBatter() { System.out.println("Mix the dry ingredients."); System.out.println("Cream the butter and sugar."); System.out.println("Beat in the eggs."); System.out.println("Stir in the dry ingredients."); } // Step 2: Bake a batch of cookies. public static void bake() { System.out.println("Set the oven temperature."); System.out.println("Set the timer."); System.out.println("Place a batch of cookies into the oven."); System.out.println("Allow the cookies to bake."); } // Step 3: Decorate the cookies. public static void decorate() { System.out.println("Mix ingredients for frosting."); System.out.println("Spread frosting and sprinkles."); } }

  24. Methods calling other methods 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 message 1."); } public static void message2() { System.out.println("This is message 2."); message1(); System.out.println("Done with message 2."); } } • Output: This is message 1. This is message 2. This is message 1. Done with message 2. Done with main.

  25. 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."); } Control flow • When a method is called, the program's execution... • "jumps" into that method, executing its statements, then • "jumps" back to the point where the method was called. public class MethodsExample { public static void main(String[] args) { message1(); message2(); System.out.println("Done with main."); } ... }

  26. When to use methods • Place statements into a static method if: • The statements are related structurally, and/or • The statements are repeated. • You should not create static methods for: • An individual println statement. • Only blank lines. (Put blank printlns in main.) • Unrelated or weakly related statements.(Consider splitting them into two smaller methods.)

  27. Static methods question • Write a program to print these figures using methods. ______ / \ / \ \ / \______/ \ / \______/ +--------+ ______ / \ / \ | STOP | \ / \______/ ______ / \ / \ +--------+

  28. Development strategy ______ / \ / \ \ / \______/ \ / \______/ +--------+ ______ / \ / \ | STOP | \ / \______/ ______ / \ / \ +--------+ First version (unstructured): • Create an empty program and main method. • Copy the expected output into it, surrounding each line with System.out.println syntax. • Run it to verify the output.

  29. Program version 1 - anything wrong with this? public class Figures1 { public static void main(String[] args) { System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); System.out.println("\\ /"); System.out.println(" \\______/"); System.out.println(); System.out.println("\\ /"); System.out.println(" \\______/"); System.out.println("+--------+"); System.out.println(); System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); System.out.println("| STOP |"); System.out.println("\\ /"); System.out.println(" \\______/"); System.out.println(); System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); System.out.println("+--------+"); } }

  30. Development strategy 2 ______ / \ / \ \ / \______/ \ / \______/ +--------+ ______ / \ / \ | STOP | \ / \______/ ______ / \ / \ +--------+ Second version (structured, with redundancy): • Identify the structure of the output. • Divide the main method into static methods based on this structure.

  31. Output structure ______ / \ / \ \ / \______/ \ / \______/ +--------+ ______ / \ / \ | STOP | \ / \______/ ______ / \ / \ +--------+ The structure of the output: • initial "egg" figure • second "teacup" figure • third "stop sign" figure • fourth "hat" figure This structure can be represented by methods: • egg • teaCup • stopSign • hat

  32. Program version 2 public class Figures2 { public static void main(String[] args) { egg(); teaCup(); stopSign(); hat(); } public static void egg() { System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); System.out.println("\\ /"); System.out.println(" \\______/"); System.out.println(); } public static void teaCup() { System.out.println("\\ /"); System.out.println(" \\______/"); System.out.println("+--------+"); System.out.println(); } ...

  33. Program version 2, cont'd. ... public static void stopSign() { System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); System.out.println("| STOP |"); System.out.println("\\ /"); System.out.println(" \\______/"); System.out.println(); } public static void hat() { System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); System.out.println("+--------+"); } }

  34. Development strategy 3 ______ / \ / \ \ / \______/ \ / \______/ +--------+ ______ / \ / \ | STOP | \ / \______/ ______ / \ / \ +--------+ Third version (structured, without redundancy): • Identify redundancy in the output, and create methods to eliminate as much as possible. • Add comments to the program.

  35. Output redundancy ______ / \ / \ \ / \______/ \ / \______/ +--------+ ______ / \ / \ | STOP | \ / \______/ ______ / \ / \ +--------+ The redundancy in the output: • egg top: reused on stop sign, hat • egg bottom: reused on teacup, stop sign • divider line: used on teacup, hat This redundancy can be fixed by methods: • eggTop • eggBottom • line

  36. Program version 3 // Suzy Student, CS 305j, Fall 2099 // Prints several figures, with methods for structure and redundancy. public class Figures3 { public static void main(String[] args) { egg(); teaCup(); stopSign(); hat(); } // Draws the top half of an an egg figure. public static void eggTop() { System.out.println(" ______"); System.out.println(" / \\"); System.out.println("/ \\"); } // Draws the bottom half of an egg figure. public static void eggBottom() { System.out.println("\\ /"); System.out.println(" \\______/"); } // Draws a complete egg figure. public static void egg() { eggTop(); eggBottom(); System.out.println(); } ...

  37. Program version 3, cont'd. ... // Draws a teacup figure. public static void teaCup() { eggBottom(); line(); System.out.println(); } // Draws a stop sign figure. public static void stopSign() { eggTop(); System.out.println("| STOP |"); eggBottom(); System.out.println(); } // Draws a figure that looks sort of like a hat. public static void hat() { eggTop(); line(); } // Draws a line of dashes. public static void line() { System.out.println("+--------+"); } }

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