Lecture 6. Instructor: Craig Duckett

Transcription

1 Lecture 6 Instructor: Craig Duckett

2 Assignment 1, 2, and A1 Revision Assignment 1 I have finished correcting and have already returned the Assignment 1 submissions. If you did not submit an Assignment 1, and want the points, then you can submit it as the Assignment 1 Revision (see below). Make sure and submit all the necessary files as they are each worth 20 points (two.java files, one debug table, two trace tables). The A1 Upload Section in StudentTracker is unavailable and locked. Assignment 2 Due Lecture 8 by midnight PLEASE NOTE: I WILL BE GOING OVER ASSIGNMENT 2 AND OFFERING HINTS FOR SUCCESSFULLY COMPLETING PART 3 IN NEXT MONDAY S CLASS. SEE THE LECTURE 7 POWERPOINT IF YOU WANT A HEAD START (particularly Slides 5 through 11). Assignment 1 Revision due Lecture 10 by midnight

3 Assignment Dates (By Due Date) Assignment 1 (LECTURE 5) GRADED! Section 1: Monday, January 22 nd Assignment 2 (LECTURE 8) Section 1: Wednesday, January 31 st The Fickle Finger of Fate Assignment 1 Revision (LECTURE 10) Section 1: Wednesday, February 7 th Assignment 2 Revision (LECTURE 12) Section 1: Wednesday, February 14 th Assignment 3 (LECTURE 13) Section 1: Wednesday, February 21 st Assignment 3 Revision (LECTURE 16) Section 1: Monday, March 5 th Assignment 4 (LECTURE 18) NO REVISION AVAILABLE! Section 1: Monday, March 12 th 3

4 And Now The Warm-Up Quiz

5 It s a Mish-Mash Lecture 1 Class File vs. Multi Class File Primitive Data Types Review Constants: The final Keyword Temporary Variables (Local Variables) Counters and Counters with Loops Scope

6 REVIEW 1 Class File vs. 2 Class File

7 SINGLE FILE STYLE 1 import becker.robots.*; public class MrRoboto extends Robot { public MrRoboto(City thecity, int avenue, int street, Direction adirection) { super(thecity, avenue, street, adirection); } public void turnaround() { this.turnleft(); this.turnleft(); } public void move3() { this.move(); this.move(); this.move(); } In this style, since there is only one class name, then: the class name MrRoboto, the constructor name MrRoboto, and the file name MrRoboto must all be the same. public void turnright() { this.turnaround(); this.turnleft(); } Also, since there is only the single class here, it is made public class by default. public static void main(string[] args) { City bothell = new City(); MrRoboto lisa = new MrRoboto(bothell, 3, 2, Direction.SOUTH); } } lisa.move3(); lisa.turnright(); lisa.move3(); lisa.turnaround();

8 SINGLE FILE STYLE 2 import becker.robots.*; class MrRoboto extends Robot { // Construct a new MrRoboto public MrRoboto(City thecity, int avenue, int street, Direction adirection) { super(thecity, avenue, street, adirection); } } public void turnaround() { this.turnleft(); this.turnleft(); } public void move3() { this.move(); this.move(); this.move(); } public void turnright() { this.turnaround(); this.turnleft(); } In this style, since there are two class names, then the file name must match the public class name, in this case the class that contains the main method, MrRobotoMain. Also the class that holds the constructor and the new methods only starts with class, and the constructor starts with public. There can only be one public class in a file, and typically the class that holds main will start with public class. Developers often set it up this way by convention to remind them which class is the class that holds the main method, although the reverse will also work as long as the file name matches the public class class name. See the MrRobotoReversedClass.java example. public class MrRobotoMain extends Object { public static void main(string[] args) { City bothell = new City(); MrRoboto lisa = new MrRoboto(bothell, 3, 2, Direction.SOUTH); } } lisa.move3(); lisa.turnright(); lisa.move3(); lisa.turnaround();

9 REVIEW Primitive Data Types

10 Primitive Data Types byte short int long float double 1 byte 8 bits 2 bytes 16 bits 4 bytes 32bits 8 bytes 64 bits 4 bytes 32 bits 8 bytes 64 bits Integers in the range -128 to +127 Integers in the range of -32,768 to +32,767 Integers in the range of -2,147,483,648 to +2,147,483,647 Integers in the range of -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807 Floating-point numbers in the range of ± to ± , with 7 digits of accuracy Floating-point numbers in the range of ± to ± , with 15 digits of accuracy

11 Integer Data Types byte, short, int, and long are all integer data types. They can hold whole numbers such as 5, 10, 23, 89, etc. Integer data types cannot hold numbers that have a decimal point in them. Integers embedded into Java source code are called integer literals.

12 Integer Data Type Integer Data Type: Integer Data Type is used to store integer value. Integer Data Type is Primitive Data Type in Java Programming Language. Integer Data Type have respective Wrapper Class Integer Integer Data Type is able to store both unsigned and signed integer values just like in C/C++ Integer Data Type Can have 4 types of values as listed below: byte short int long

13 Constants The final Keyword WRITTEN IN STONE

14 Creating Named Constants with final Many programs have data that does not need to be changed. Littering programs with literal values can make the program hard do read and maintain. Replacing literal values with constants remedies this problem. Constants allow the programmer to use a name rather than a value throughout the program. Constants also give a singular point for changing those values when needed.

15 Creating Named Constants with final Constants keep the program organized and easier to maintain. Constants are identifiers that can hold only a single value. Constants are declared using the keyword final Constants need not be initialized when declared; however, they must be initialized before they are used or a compiler error will be generated.

16 Creating Named Constants with final Once initialized with a value, constants cannot be changed programmatically. By convention, constants are all upper case and words are separated by the underscore _ character. EXAMPLES: final int MONTHS_IN_YEAR = 12; final float CAL_SALES_TAX = 0.79; Both the Java and Becker API libraries have several constants built in programmatically by default. For example: Java has math.pi (where PI = ) Becker has direction.north (including EAST, SOUTH, WEST) where the direction represents specific degrees on a compass like 0, 90, 180, 270

17 New Topic: Counters and Counting Loops Review: if and while Statements Boolean conditions (true or false) Example code of both statements in action Counters What Good are They? How are They Used? Example code of counter in action

18 REVIEW: if and while Statements Both the if and while statements evaluate Boolean conditions They test to determine whether a condition is true or false True, do something False, don t do something Do it! Don t do it! 18

19 CONCEPT: The if Statement IF EXAMPLE: Taking a free right-turn at a red light If it is clear (no vehicle or pedestrian coming), then take a free right turn. Do This The Next Thing The Next Thing 19

20 The if Statement If true, do it once True False 20

21 if Example Code WALK-THROUGH: RobotWall.java 21

22 CONCEPT: The while Statement (Loop) WHILE EXAMPLE: Buying Gum Balls While I have enough quarters, buy a gum ball; keep buying gum balls as long as I have enough quarters. Do This The Next Thing The Next Thing 22

23 The while Statement While true do it again True False 23

24 while Example Code WALK-THROUGH: RobotWall.java 24

25 If and While When the simplest form of an if statement asks a question and the answer is true, it executes a group of statements once and then continues with the rest of the program. If the answer to the question is false, that group of statements is not executed. When a while statement asks a question and the answer is true, it executes a group of statements (just like the if statement). However, instead of continuing down to the rest of the program, the while statement asks the question again. If the answer is still true, that same group of statements is executed again. This continues until the answer to the question is false. The if statement s question is Should I execute these statements at least once? if (test condition) { // Statements of things to do } The while statement s question is Should I execute these statements again? while (test condition) { // Statements of things to do }

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28 QUESTION With the while loop, what would happen if the condition was always true?

29 ANSWER An infinite loop!

30 while Example Code WALK-THROUGH: RobotWall.java 30

31 QUESTION Is there a way to configure a set number of times to go through the loop, and then stop or jump out to prevent an infinite loop?

32 ANSWER Yes! You can do this by setting up and using a counter! I m a counter. I m a pretty big deal!

33 Counters

34 What is a Counter? A counter is a variable, usually of an integer int data type, typically initialized to start with a 0. A variable is a small section of memory that has been set aside based on the size and type of data it will contain. It is given a unique name and then an initial starter value. As the program is run this value can be changed, it can vary, hence the name variable. 34

35 Setting Up a Counter In the code, setting up a counter for use with a loop is a 3-Step Process: 1. Create and initialize the counter 2. Create the count condition 3. Increment the counter by 1 Let s take a look at what this means and how it works with our while loop program 35

36 Counter Example Code int counter = 0; // (1) Create the counter while(counter < 5) // (2) Create the count condition { lisa.putthing(); lisa.move(); counter = counter + 1; // (3) Increment the counter by 1 } // Read from Right to Left! WALK-THROUGH: RobotWall.java 36

37 Counter Increment/Decrement int counter = 0; while(counter < 5) { lisa.move(); counter = counter + 1; } int counter = 5; while(counter > 0) { lisa.move(); counter = counter - 1; } 37

38 Counters int X = 5; // Imagine we have this variable x Increment Add 1 to the counter X X = X + 1; // Left Right: ADD 1 to X NOW X is 6 or Decrement Subtract 1 from the counter X X = X - 1; // Left Right: SUBTRACT 1 from X NOW X is 4

39 Another Way to Show Increment & Decrement nummoves = nummoves + 1; numthings = numthings 1; SAME AS nummoves++; numthings ; So: Using a Counter X = X + 1 is the same as X++ X = X - 1 is the same as X-- X = X++ is INCORRECT! X = X-- is INCORRECT!

40 Counters CONTINUED But why use a counter? Because counters are great with loops!

41 Counters CONTINUED You can use initialize counters outside of loops and inside of loops, which affects their scope (which we ll talk about in a moment), all depending on the logic of the code. int counter = 0; while(counter < 5) // As long as this is true, loop { Rex.move(); counter = counter + 1; // Same as counter++; } See: counterexample.java

42 Counters CONTINUED Here s another couple of examples! Suppose you wanted to spin a robot all the way around four times (that is, do a complete 360 circle four times. You will have to adjust your logic accordingly. If one complete circle is 4 left turns, then four complete circles would be 16 left turns. There are two ways you could do this, either by incrementing a value to the counter, or decrementing a value from the counter. First you need to set your value of the counter accordingly: int counter = 0 (used for increment, count up ) while(counter < 16) //count up to 16 counter = counter+1; //Same as counter++ or See: increment16.java int counter = 16 (used for decrement. count down ) while(counter > 0) //count down to 0 counter = counter-1; //Same as counter-- See: decrement16.java

43 Counters Declare the datatype, and give it a name: int counter; Then, initialize it with a value: counter = 0; So, putting it together it might look like this: int counter; counter = 0; You can also do this all on the same line by combining the declaration and the initialization, which saves keystrokes: int counter = 0;

44 A Quick Word About Scope

45 A Quick Word About Scope

46 Hey! It s time for another ICE Lecture 6 Counting Loops Follow the In-Class Exercises Directions ICE_06_CL_Trace.java ( Example Solution )DEMO ICE_06_Count.java ICE_06_CountingLoops.java