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Review 1) Two advantages of using a named constant: 2) A String is not a primitive type in Java, it is a… 3) A Class Type Variable (reference variable) does not hold the actual value, but... 4) A Named Constant is… 5) A String object is not created for a String variable until… 6) Define the String Methods: a. length() b. charAt(index) c. toLowerCase() d. toUpperCase() Scope Every variable has scope. o Scope refers to where in a program an entity can be accessed by name. o A variable is only visible to statements that are inside it’s scope. Scoping rules are complex, we will only discuss a few right now. We’ll add to these later. o So far we’ve only seen variables declared inside of the main method. Variables declared inside of a method are called local variables, and have local scope. o The scope of a local variable begins where they were declared and ends at the end of the method in which they were declared. o Also, you cannot have two local variables with the same name in the same scope Scope Example (What problems do you see?) //Scope - shows an example of a local variable being used out of its scope // and with another variable with the same name being in the same scope public class Scope { public static void main(String[] args) { x = 11; int x; System.out.println(x); int x = 15; System.out.println(x); } } Rules for Comments Internal Documentation takes the form of Comments in Java. o A Comment is… line(s) in a program that is ignored by the compiler entirely. o Why do we have comments? Include documentation inside of the code to allow other programmers (or ourselves) to understand what the code does, who wrote it, when, etc. To temporarily make lines of code not executed instead of removing them outright o Three kinds of comments in Java Single-Line – begin with // and the entire line is ignored // Here is a comment Multi-Line – begin with /* and end with */, and everything between is ignored /* Here is a comment and it is still going on here */ Documentation Comments – special comments that can be used to make attractively formatted HTML files that document the source code. (Not demonstrated here) Your comments should do three things at this point o Give a block comment at the top of the file to give information about the file. In this class I want them to include: Author’s name The course number (CS0007) The date created A short description about what program does o Give information about what segments of code do in the program, especially if you are worried the reader will not know what the code does, but even if you think it is obvious. Either goes above the line, or next to it. You can NEVER have too many comments. o Cite the source if a small snippet of code is taken from somewhere. Later we will talk more about how to document different constructs as we get to them. Block Comments There are many styles to making block comments: //////////////////////////////////////// // One Style // Looks like this /////////////////////////////////////// /* * Another * Looks like this */ //-------------------------------------// Yet another is // Looks like this //-------------------------------------- Programming Example of Comments. /************************************************************* * Author: Mr. O’Hara * Course: CS0007 * Date Created: 5/25/2011 *Description: This file holds the triangle stats program, which computes the * perimeter and area of a triangle with sides of length 2.3, 5.9, and 7.2, then * displays the lengths of the sides, the perimeter and the area to the screen ***********************************************************/ public class Commenting { public static void main(String[] args) { //Declarations for the sides, the perimeter, the area, and the //semiperimeter s double side1 = 2.3, side2 = 5.9, side3 = 7.2, perimeter, s, area; perimeter = side1 + side2 + side3;//Computes the perimeter s = perimeter/2; //Computes the semiperimeter //Computes the area area = Math.sqrt(s * (s - side1) * (s - side2) * (s - side3)); //Displays the triangle's sides, the perimeter, and the area to the screen System.out.print("The lengths of the triangle's sides are " + side1); System.out.println(", " + side2 + ", " + side3 + "."); System.out.println("The perimeter of the triangle is " + perimeter ); System.out.println("The area of the triangle is " + area ); } } Programming Style Programming Style refers to the way a programmer uses spaces, indentation, blank lines, and punctuation characters to visually arrange a program’s source code. o This has NOTHING to do with syntax. o General Rule: Make your code easy to read. We could write a program like this: public class Compact {public static void main(String [] args){int number = 5; System.out.println(number);}} o It even compiles and runs, but its really difficult to read. Rule 1: All statements inside of a block should be indented one tab more than the statements directly outside of the block. public class Neat { public static void main(String [] args) { int number; … } } o Notice that all lines inside of the public class Neat headed block are tabbed once, and all lines inside of the public static void main… block are tabbed again. o Also note that the closing braces are in the same column as their headers. Rule 2: Lines that are wrap onto the next line should be vertically aligned: System.out.println("Here I am going to " + "display a variable " + number); Notice that instead of having the string wrap to the next line, the concatenation operator is used and the string begins where the last one did. Also, if you are commenting variables, do something similar: int fahrenheit, //holds the Fahrenheit temperature celsius, //holds the Celsius temperature kelvin; //holds the Kelvin temperature We’ve also went over some naming conventions: o Self-documenting code o Classes start with a capital letter o Variables start with a lowercase letter o Named Constants are all caps with underscores *We will add more good programming practices throughout the semester. Reading Keyboard Input Just like System.out object refers to the standard output device, the Java API provides an object, System.in, that refers to the standard input device. The standard input device is normally the keyboard. Unfortunately, using System.in is not as straight-forward as using System.out System.in reads all input as bytes…which is often not very useful. Fortunately, the Java API provides the Scanner class that allows us to retrieve input as primitive types or strings. The Scanner Class To create an object from the Scanner class that takes in keyboard input we use the line: Scanner keyboard = new Scanner(System.in); o Scanner keyboard declares a variable named keyboard that is type Scanner. Because Scanner is a class, keyboard is a reference variable. o = is the assignment operator, so we are initializing the keyboard variable. o new is a Java keyword that creates an object in memory, what follows it tells the compiler what object is to be created. o Scanner(System.in) tells the compiler that the object to be created is a Scanner object, and it should be associated with standard input. This is called a constructor, and it creates the object in memory. o We will talk more about constructors much later in the course. o The result is that we have a reference variable, keyboard, that references a scanner object that is associated with standard input. Some classes provided by the Java API are not automatically available for use with all programs, and the Java compiler needs to be told where to find them. o Scanner is one of these classes. For this reason we must put the following line near the beginning of the file, outside of the class definition: import java.util.Scanner; o This tells the compiler where to find the definition for the Scanner class The Scanner class provides methods for reading input as different types: o nextByte() – Returns input as a byte o nextDouble() – Returns input a double o nextFloat() – Returns input as a float o nextInt() – Returns input as an int o nextLine() – Returns input as a String o nextLong() – Returns input as a long o nextShort() – Returns input as a short o When associated with standard input, the user will be able to type characters on their keyboard and finish by pressing enter. The result is then returned as the type specified by the method. o Notice there is no nextChar() method. If you want to take in a single character, you must use the nextLine() method and use charAt(0) to retrieve the first character. import java.util.Scanner; //Needed for the Scanner class /** * This program demonstrates the use of the Scanner class */ public class Scanner1 { public static void main(String[] args) { String name; int hours; double payRate; double grossPay; //creates a scanner object to read standard input Scanner input = new Scanner(System.in); //Get the user's name System.out.print("Enter your name: "); name = input.nextLine(); //Get the number of hours worked this week //ASSUMES INTEGER INPUT System.out.print("Enter the number of hours you've worked this” + week: "); hours = input.nextInt(); //Get the user's hourly pay rate System.out.print("Enter your hourly pay rate: "); payRate = input.nextDouble(); //Calculate Gross Pay grossPay = hours * payRate; //Display the resulting information System.out.println("Hello, " + name); System.out.println("Your gross pay is $" + grossPay); } } import java.util.Scanner; /** * This programs shows how to take a single character as input */ public class ReadCharacter { public static void main(String[] args) { String input; //Holds the input char answer; //Holds a single character //Creates a scanner object with standard input Scanner keyboard = new Scanner(System.in); //Ask the user a question System.out.print("Is your name Eric (Y=yes, N=no)? "); input = keyboard.nextLine(); //Get the user's input answer = input.charAt(0); //Gets the 1st char from the input System.out.println("The user entered " + answer); } } The nextLine() method problem . Let’s look at the program InputProblem.java. It didn’t take in the user’s name at all! The problem is that nextLine() works differently than the other Scanner class methods. When the user types keystrokes at the keyboard, those keystrokes are stored in an area of memory sometimes called the keyboard buffer. When the user pressed enter, the new line character is stored in the keyboard buffer. When a user inputs a number for nextInt(), everything the user entered is stored in the keyboard buffer, including the newline character. Then, the value entered by the user is read from the buffer, leaving the newline character still in the buffer. The nextDouble() method is designed so that it skips any leading newline characters it encounters, so when it tries to read the keyboard buffer, it sees the newline from nextInt(), it ignores it. However, when nextLine() encounters the same situation, it is NOT designed to skip leading newline characters and assumes the user has pressed enter, stopping keyboard input. The newLine() method problem. Solution: If you use another Scanner method before the nextLine() method, simply put another call to nextine() before the one that you want to take the user’s input. INPUT PROBLEM import java.util.Scanner; /** * This program demonstrates the problem with using nextLine after * another Scanner method */ public class InputProblem { public static void main(String[] args) { String name; int hours; double payRate; double grossPay; //creates a scanner object to read standard input Scanner input = new Scanner(System.in); //Get the number of hours worked this week //ASSUMES INTEGER INPUT System.out.print("Enter the number of hours you've worked” + “ this week: "); hours = input.nextInt(); //Get the user's hourly pay rate System.out.print("Enter your hourly pay rate: "); payRate = input.nextDouble(); //Get the user's name System.out.print("Enter your name: "); name = input.nextLine(); //Calculate Gross Pay grossPay = hours * payRate; //Display the resulting information System.out.println("Hello, " + name); System.out.println("Your gross pay is $" + grossPay); } } Corrected Input Problem import java.util.Scanner; /** * This program demonstrates a solution to the problem with using nextLine * after another Scanner method */ public class CorrectedInputProblem { public static void main(String[] args) { String name; int hours; double payRate; double grossPay; //creates a scanner object to read standard input Scanner input = new Scanner(System.in); //Get the number of hours worked this week //ASSUMES INTEGER INPUT System.out.print("Enter the number of hours you've” + “worked this week: "); hours = input.nextInt(); //Get the user's hourly pay rate System.out.print("Enter your hourly pay rate: "); payRate = input.nextDouble(); //THIS FIXES THE PROBLEM input.nextLine(); //Get the user's name System.out.print("Enter your name: "); name = input.nextLine(); //Calculate Gross Pay grossPay = hours * payRate; //Display the resulting information System.out.println("Hello, " + name); System.out.println("Your gross pay is $" + grossPay); } }