What We Will Cover
Illuminations
Homework Questions?
Homework Discussion Questions
- Where do you turn in programming assignments?
- What was your impression of Blackboard?
- What is scholastic dishonesty?
- What is an example of acceptable help on programming assignments
- What is an example of unacceptable help on programming assignments
- What was your experience installing the JDK?
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2.1: Language Basics
Learner Outcomes
At the end of the lesson the student will be able to:
- Make comments in code
- Print data to a console
- Create program variables and assign them values
- Declare the data type of a variable
- Identify the data type of various literal values
- Create code to perform arithmetic
- Convert the data type of a primitive value to another type
- Distinguish between pre and post operators for both increment and decrement operations
- Generate code that uses literal strings
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2.1.1: Example Program
- Here is the example program like the one we looked at before
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/**
* Hello.java
* Purpose: Displays a greeting.
*
* @author Ed Parrish
* @version 1.1 8/17/08
*/
public class Hello {
public static void main(String[] args) {
System.out.println("Hello, world!");
} // end of main method
} // end of Hello class
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Brief Explanation by Line Number
- Lines 1-7: comments about the class -- notes to programmers
- Line 8: the class declaration
- Line 9: the
main() method where all programs start
- Line 10: a programming statement to display data to the console
- Line 11: the end of the
main() method followed by another comment
- Line 12: the end of the
Hello class followed by another comment
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2.1.2: Comments
Javadoc Comments
- Java also has a third type of comment known as a Javadoc comment
- Javadoc comments are used to generate documentation automatically
- Syntax:
/**
* Description part of a Javadoc comment
*
* @tag Comment for the tag
*/
- Note the extra star (
*) in the opening of the comment
- Javadoc comments have two parts:
- Description of the code
- Followed by zero or more tags
- For example, you should put a Javadoc comment like the following at the beginning of every class:
/**
* Hello.java
* Purpose: Prints a message to the screen.
*
* @author Ed Parrish
* @version 1.0 8/30/05
*/
- In this example, we have two tags:
@author followed by the name of the author@version followed by the version number or date
- In addition, you put Javadoc comments like the following before every method:
/**
* The main method for the Hello program.
*
* @param args Not used
*/
- Then you use a tool, known as Javadoc, to automatically create program documentation
- Also, a tool known as CheckStyle will check your comments (among other things) for correct usage
- I have some instructions for setting up TextPad to run these tools:
- Also, you can run CheckStyle from the command line following the instructions here: Running CheckStyle
More Information
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2.1.3: Primitive Data Types
Integer data types: byte, short, int, long
Floating-point types: float, double
Character data type: char
Boolean type: boolean
Table of Primitive Data Types
| Type Name |
Format |
Size |
Range |
byte |
integer |
1 byte |
-128 to 127 |
short |
integer |
2 bytes |
-32768 to 32767 |
int |
integer |
4 bytes |
-2,147,483,648 to 2,147,483,647 |
long |
integer |
8 bytes |
-9,223,372,036,854,775,808 to 9,223,374,036,854,775,807 |
float |
floating point |
4 bytes |
+/- 1.4023... x 10-45 to +/- 3.4028... x 1038 |
double |
floating point |
8 bytes |
+/- 4.940... x 10-324 to +/- 1.767... x 10308 |
char |
UTF-16 |
2 bytes |
All Unicode characters |
boolean |
true or false |
1 byte |
Not applicable |
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2.1.4: Variables and Assignment
- A variable is the name of a location in main memory
- All Java program variables must be declared before using them
- A declaration statement both names a variable and specifies the data type it can store
- General syntax:
dataType VariableName1, VariableName2, ...;
- Where:
- dataType: one of the Java data types
- VariableNameX: the name of the variable
- For example:
int num1, num2, total;
long dateNum;
float firstNum;
double secNum;
char letter;
- Variable names must follow the rules for valid identifiers
Naming a Variable
- A variable name is a sequence of letters, digits, underscores (
_ ) and currency symbols like: $, ¢, £, ¥
- However, Java has some limitations on what you chose for a variable name
- Specifically, the first character must be either a letter, underscore character "
_" or currency symbol
- Cannot be a number
- A currency symbol is allowed but its use is discouraged
- Also, the variable name cannot be one of the Java reserved words (keywords)
- For a list of reserved words, see: keywords
- Note that you cannot have spaces in a variable name
- A space is NOT a letter, digit, underscore character or currency symbol
- Also, variable names are cAsE sEnSiTiVe
id, ID, iD and Id are all valid but different names
Assigning Values to Variables
- When variables are declared, the computer allocates storage space for them
- Your program must then assign values to the variables
- To change the value of a variable, you use the assignment operator ("equals sign"):
=
variable = expression;
- The value of the
expression on the right side gets assigned to the variable on the left side
- For example:
int counter; // declare variable counter
counter = 10; // initialize counter to ten
- You can combine variable declaration and assignment into one statement:
int counter = 10; // declare and initialize counter
- Note that if you try to use an uninitialized local variable, the code fails to compile:
// These may not be initialized when declared
// and have unknown values
int sum, number1, number2;
sum = number1 + number2;
- Instead, you must initialize local variables
// These are initialized when declared
int sum = 0;
int number1 = 5, number2 = 10;
sum = number1 + number2;
Shortcut Assignment Operators
- You can use additional operators to calculate values and assign them to the variable on the left all in one statement
- Known as shortcut assignment operators
- The general syntax is:
variable op= expression;
- Where
op is one of the five arithmetic operators: +, -, *, /, %
- For example, the following two statements produce the same result:
x = x + 3;
x += 3;
Constant Variables
Example of a Blank Final Variable
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import java.util.Scanner;
public class BlankFinal {
public static void main(String[] args) {
final int CONST;
// Following causes a compiler error
//System.out.println("CONST = " + CONST);
System.out.print("Enter an integer number: ");
Scanner input = new Scanner(System.in);
int num = input.nextInt();
CONST = num;
System.out.println("CONST = " + CONST);
//CONST = 9; // Compiler error
}
}
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2.1.5: Arithmetic Operators and Variables
Arithmetic Operators (Binary)
| Operator |
Description |
Example |
+ |
Adds or combines two items |
men = 10;
women = 15;
total = men + women;
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- |
Subtracts one item from another |
income = 1000;
expenses = 750;
profit = income - expenses;
|
* |
Multiplies two items |
width = 20;
height = 30;
area = width * height;
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/ |
Divides one item by another |
persons = 20;
cost = 30;
costPerPerson = cost / persons;
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% |
Calculates the remainder after dividing one item by another |
numEggs = 65;
cartonSize = 12;
eggsLeft = numEggs % cartonSize;
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Truncation in Integer Division
Mixed-Mode Arithmetic
- No truncation occurs if at least one of the values is of type
float or double
- Instead, both values are promoted to the highest data type of the two using the following hierarchy:
byte => short => int => long => float => double
- For example:
int a = 4, b = 5, c;
double x = 1.5, y;
y = b / x; // value returned by b is promoted to double
// value of y is about 3.33333
c = b / a; // all values are ints so the division
// truncates: the value of c is 1
Arithmetic Operators (Unary)
- Java also has unary operators that work on only one item
- Shown below are three unary arithmetic operators with examples of how they are used
| Operator |
Description |
Example |
Equivalent To |
- |
Changes the sign of (negates) an item's value |
-x |
x = 0 - x |
++ |
Increases the item's value by one |
x++ |
x = x + 1 |
-- |
Decreases the item's value by one |
x-- |
x = x - 1 |
Check Yourself
- Lets check our understanding of the increment and decrement operators with the following code
- Write down what you think the program will display before you check the answer
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public class PrePost {
public static void main(String[] args) {
int a, b, c;
// Prefix order
a = 10;
b = ++a;
System.out.println("a=" + a + ", b=" + b);
c = b + --a;
System.out.println("a=" + a + ", c=" + c);
--a;
System.out.println("a=" + a + "\n");
// Postfix order
a = 10;
b = a++;
System.out.println("a=" + a + ", b=" + b);
c = b + a--;
System.out.println("a=" + a + ", c=" + c);
a--;
System.out.println("a=" + a);
}
}
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Parenthesis and Precedence
- Some arithmetic operators act before others (e.g., multiplication before addition)
- Java follows rules similar to real-number algebra
- Arithmetic operations are processed in algebraic order:
- Parentheses ( )
- Unary operations: +, -, ++, --
- Multiplication, division, modulus: *, /, %
- Addition, subtraction: +, -
- Binary operators of same precedence are evaluated from left to right
- To change the usual order, you can use parenthesis to group expressions
- Anything within parenthesis is evaluated first
- For example, to find the average of three variables a, b and c
- Do not use:
a + b + c / 3
- Instead use:
(a + b + c ) / 3
- You can have parenthesis within parenthesis
- The innermost parenthesis is evaluated first:
(2 * (10 + 5))
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2.1.6: Magic Numbers
- Imagine that you are a programmer hired to modify a payroll program
- You come across the following section of code:
double pay;
pay = hours * 7.5 + (hours / 40)
* (hours - 40) * 7.5 * 0.5;
- The numbers are important to the program, but what do they mean?
- Numbers like these are called "magic numbers"
- They are magic because the value or presence is unexplainable without more knowledge
- Often, no one knows what they mean after 3 months, including the author
- A programmer can often infer the meaning of numbers after reading the code carefully
- Much better code is to use named constants rather than literal numbers
- For example:
final double WAGE = 7.5;
final double OVERTIME_ADDER = 0.5;
final int HOURS_PER_WEEK = 40;
double pay;
pay = hours * WAGE + (hours / HOURS_PER_WEEK)
* (hours - HOURS_PER_WEEK) * WAGE * OVERTIME_ADDER;
- Now it is much easier to read and understand the code
- And see any problems or limitations
Programming Style: Constant Variables and Magic Numbers
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2.1.7: Type Conversion
- We often find ourselves converting one data type into another
- This process is called type conversion or typecasting
- Java is a strongly-typed language and checks for compatible data types both when compiling and while running
- To put a value of a different type in a variable, you must convert the type
- There are two types of conversion: implicit and explicit
- The term for implicit conversion is sometimes called coercion
- The most common form of explicit conversion is know as casting
Coercion
Coercion in an Arithmetic Expression
Explicit Casting
- Explicit casting changes the data type for a single use of the variable
- To cast, precede the variable name with the new data type in parentheses:
(typeName) variableName
- Where:
- typeName: one of the Java data types
- variableName: the name of the variable
- For example:
int n;
double x = 2.0;
n = (int) x;
- The value of
x is converted from type double to int before assigning the value to n
- Note that explicit casting is required to assign a broader type to a narrower type
- ILLEGAL: Implicit casting to a lower data type:
int n;
double x = 2.1;
n = x; // illegal in Java
Illegal since x is double, n is an int, and double is a higher data type than int
- LEGAL: Explicit casting to a lower data type:
int n;
double x = 2.1;
n = (int) x; // legal in java
- You can use an explicit cast even when an implicit one will be done automatically
Truncation When Casting double to Integer type
Converting Between char and int Values
- Casting a
char value to int produces the ASCII/Unicode value
- For example, what would the following display?
char answer = 'Y';
System.out.println(answer);
System.out.println((int) answer);
- Answer:
y
89
- You can convert characters to integer numbers as well:
char ch7 = '7';
char ch9 = (char) (ch7 + 2);
int num7 = ch7 - '0';
int num9 = num7 + 2;
System.out.println("ch9: " + ch9);
System.out.println("num9: " + num9);
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2.1.8: Strings
Declaring and Assigning string Variables
Joining Strings (Concatenation)
String Conversions
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2.1.9: Summary
- Every Java program starts at a
main() method
public static void main(String[] args) {
// statements to execute
}
- Java has three styles of comments
- Java has two classifications of data types: primitive and class
- Data types are important because they tell the computer:
- How much memory to allocate
- The format to use when storing the data in memory
- The operations permitted on the data
- Java has eight primitive data types in four categories:
- Integer:
byte, short, int and long
- Floating point:
float and double
- Character:
char
- Boolean:
boolean
- Variables store a value that can change as a program executes.
- Constants store a value that cannot change.
- Variables and constants must be declared before use.
- To declare a variable or constant, specify a data type and a name.
- You assign values to a variable using an assignment statement
- Simple assignment statements have a variable, equals sign and an expression:
variable = expression;
- The expression is computed before the assignment
- Java has assignment variations of the form:
variable <op>= expression;
- Java uses the following operators for arithmetic:
+ for addition- for subtraction* for multiplication/ for division% for modulus (remainder)
- Results of integer division is truncated
- Must use modulus operator (
%) to get the remainder value
- Operators have the same precedence as in algebra:
- Parenthesis:
( )
- Unary operators:
+, -
- Multiplication, division, modulus:
*, /, %
- Addition, subtraction:
+, -
- You can explicitly cast values of an expression to a different type.
- You must use an explicit cast when assigning a broader type to a narrower type:
int count = input.nextInt();
- Even though strings are objects in Java, you can assign them to a variable using the assignment operator
- Also, you can concatenate two strings using the "+" operator
Check Yourself
- Where does every Java program start executing?
- What styles of comments does Java allow?
- What is the purpose of Javadoc comments?
- What is the purpose of a data type?
- What are the eight primitive data types that Java supports?
- What are the rules for creating an identifier?
- What is the code to declare an
int variable named foo and assign it a value of 10?
- What is the code to declare a constant
double named BIG_NUM and assign it a value of 100,000?
- What are the five operators Java uses for arithmetic?
- How do you calculate the square root of a number like
27?
- What is meant by the term casting? coercion?
- What data type is returned from an expression when an
int and a double are added together?
- What is wrong with the following code?
double x = 1;
int y = x;
- What type of delimiters are used to encapsulate literal strings?
- What is the code to declare a string variable named
foo?
- What operator is used to join two strings?
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Exercise 2.1
Lets go through these one at a time. Take one minute to prepare an answer and then we will discuss it.
- Given the following code, which statement is true?
int a, b = 1;
- Variable
a is not declared
- Variable
b is not declared
- Variable
a is declared but not initialized
- Variable
b is declared but not initialized
- Neither variable is declared nor initialized
- What is the value returned by the following expressions?
- -1-3 * 10 / 5-1;
-8-67810
- Starting with the code:
int n = 3;
int m = 4;
int result;
What will be the value of m and result after each of these executes?
result = n * ++m; //preincrement mresult = n * m++; //postincrement mresult = n * --m; //predecrement mresult = n * m--; //postdecrement m
- What does the following code do?
char ch7 = '7';
int num7 = ch7 - '0';
System.out.println("answer: " + num7 + 2);
- Error
- prints
72
- prints
9
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2.2: Libraries, Classes and Console I/O
Learner Outcomes
At the end of the lesson the student will be able to:
- Import standard API classes
- Create objects and call methods
- Display information to a console
- Get user input from the console
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2.2.1: Importing Classes
- To make it easier to write programs, Java has many libraries
- These libraries contain prewritten code
- Collectively, these libraries are referred to as the Java Application Programming Interface (API)
- Recall that all Java code is stored in classes
- Groups of related classes are organized into packages
Some Commonly Used Packages
| Package Name |
Description |
java.lang |
Provides classes fundamental to Java. |
java.io |
Provides classes to read and write files. |
java.txt |
Provides classes to handle text, dates, and numbers |
java.util |
Provides classes to work with collections and miscellaneous utilities. |
javax.swing |
Provides classes to create graphical user interfaces and applets. |
Importing Classes and Packages
Navigating the API
- The Java API is located at: http://java.sun.com/javase/6/docs/api/index.html
- Related classes are organized into packages
- Shown in the upper left frame
- When you select a package, all the classes for that package are shown in the lower left frame
- Once you select a class, it is shown in the right frame
- Each class shows a variety of information including a summary of all its methods
- To view the information about a method, either click on its hyperlink or scroll down the page
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2.2.2: Creating Objects and Calling Methods
Calling Object Methods
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2.2.3: Printing to a Console
Escape Sequences
- Some characters are more difficult to code output for than others
- For example, what would the compiler do with the following statement?
System.out.println("Say, "Hey!"");
- Some characters cannot be output directly in a string
- Also, Not all of first 32 ASCII characters are visible on our monitors
- Known as control codes because they control the output device
- Even though these characters are not visible, we sometimes need to use them
- For example: a newline character
- We need some way to "print" invisible and hard-to-print characters
- Java can access some of the control codes and hard-to-print characters using escape sequences
- A backslash (
\) directly in front of a certain character tells the compiler to escape from the normal interpretation
- Some examples:
System.out.print('\u0007');
System.out.print('\n');
System.out.print("Left\tRight");
System.out.print("one\ntwo\nthree");
- The following table lists some nonprinting and hard-to-print characters:
Common Escape Sequences
| Sequence |
Meaning |
Unicode Value |
|
Alert |
\u0007 |
\b |
Backspace |
\u0008 |
\f |
Formfeed |
\u000C |
\n |
Newline |
\u000A |
\r |
Carriage return |
\u000D |
\t |
Horizontal tab |
\u0009 |
\\ |
Backslash |
\u005C |
\" |
Double quote |
\u0022 |
\' |
Single quote |
\u0027 |
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2.2.4: Reading Interactive Input
- Oftentimes you want a program to ask a user questions and read the input from the user
- You can read input interactively from the command line (console) using a
Scanner object
Scanner is located in the java.util package, so you need to import the class using either:
import java.util.Scanner;
or
import java.util.*; - You then create a scanner object like:
Scanner input = new Scanner(System.in);
- Note that
System.in is used to get data from the standard input stream
- To use one of the methods of a
Scanner object, call it like the following:
int count = input.nextInt();
- When you call a method of a
Scanner object, the program waits for the user to enter data with the keyboard
- Each group of characters that a user enters is called a token
- The program waits until the user presses the Enter key
- The user can enter multiple tokens by separating them with whitespace
- If the user enters the wrong type, an error occurs and the program aborts
- When you combine different scanner methods, you sometimes have to include an extra call to
nextLine() to get rid of a newline
Some Commonly Used Methods of a Scanner Object
| Method |
Description |
next() |
Returns the next token as a String object. |
nextLine() |
Returns the rest of the current line as a String object and positions the input cursor at the beginning of the next line. |
nextDouble() |
Returns the next token as an double value. |
nextInt() |
Returns the next token as an int value. |
Example Program Using Scanner for Processing User Input
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import java.util.Scanner;
public class InputApp {
public static void main(String[] args) {
String name;
int age;
double weight;
Scanner input = new Scanner(System.in);
System.out.print("Enter your name: ");
name = input.next();
System.out.print("Enter your age: ");
age = input.nextInt();
System.out.print("Enter your weight: ");
weight = input.nextDouble();
System.out.println("\nYou entered:");
System.out.println("Name:" + name);
System.out.println("Age:" + age);
System.out.println("Weight:" + weight);
}
}
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Scanner Pitfall: Working with Newlines
Other Input Methods
- The
Scanner class was introduced in Java version 5.0 (JDK 1.5)
- For console input without the
Scanner class, see: Using System.in
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2.2.5: Summary
- To make it easier to write programs, Java has many libraries of prewritten code
- You can make use of this code by importing packages:
import packagename.ClassName;
or
import packagename.*;
- For example, to import all the
java.util libraries:
import java.util.*;
- Alternatively, you can use the fully qualified package name:
import java.util.Scanner;
- To create an object for a class, you:
- Define a variable to store the object
- Create the object using the new operator and assign it to the variable
- You can combine both steps into one statement, like:
Scanner input = new Scanner(System.in);
- One you create an object, you can call its methods
- To call a method, you use the object name, a dot (period), the method name and a set of parenthesis
- For example:
input.nextInt();
- You can use the
System.out object to print to the console
System.out.print() leaves the cursor positioned after the last characterSystem.out.println() positions the cursor on a new line
- You can use a
Scanner object to read data from the console
- To use
Scanner, you need to need to import the java.util.Scanner class
- You then create a
Scanner object like:
Scanner input = new Scanner(System.in);
- Each group of characters that a user enters is called a token
- To read each token, you call a method of a
Scanner object like:
int count = input.nextInt();
Check Yourself
- How do you include libraries in your Java programs?
- What library do you include to create
Scanner objects?
- What does the acronym "API" stand for? What does it mean?
- How do you create an object?
- How do you call a method of an object?
- What is the code to create a
Scanner object?
- What is the code to get an
int value from the keyboard using a Scanner object?
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Exercise 2.2
Take one minute to prepare an answer to the following problem.
Write a complete program to read a single int from the keyboard and display it to the console.
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Wrap Up
Due Next:
A2-Metabolic Energy (2/24/10)
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Last Updated: February 15 2010 @16:48:34
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