Download Variables

Declaring Variables
Definition: A variable is a place in memory that can hold a value
• You must first declare a variable before you can use it!
• Declaring involves:
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Establishing the variable’s spot in memory
Specifying what kind of data it will contain (its type)
Give it a name (its identifier)
Optionally give it an initial value
• Note: A program may declare as many variables as it needs
• Example of Java declarations:
– Statement to: Declare an integer variable: int grade;
– Statement to: Declare an integer variable with a value: int count = 5;
– Statement to: Declare two integer variables: int grade, count = 5;
• Questions: What are the identifiers, initial value, and type in the above examples?
Important Note: Equal does NOT mean equal, it means assign
Variable types
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Definition: A type specifies a kind of data
There are many data types in Java
There is a reserved word for each
Examples: Integers, Fractions, String, boolean, and character
Integer variables
– byte i = 100; (a byte variable holds values from -128 to 127)
– int x = 32768; (an int variable holds values to billions)
– long total = 111111; (a long variable holds up to quintillians)
• Fraction variables
– float percentage; (float variables are accurate to six digits)
– double distance; (double variables are accurate to fifteen digits)
• Boolean variables: boolean isBig = true; or boolean isBig = false;
• String variables are a sequences of letters
– String name = “bill”; (Enclose your strings with double quotes)
• Character variable: char c = ‘3’; // A single letter
Question: Why all the types?
Answer: Different variable types are stored differently in memory
Literals
• Definition: A literal is a constant that we do
not have to declare.
• Examples:
– String literal: “abcdef”
– Integer literal: -10
– Fraction literal: 32.75
– Character literal: ‘t’
– Boolean literal: true
Expressions and Casting
Definition: To cast means to access a variable as if it were a different type
• Expression:
– A sequence of operations and operators
– Example: x + y / 3 + (4 + z)/a
• Java is fussy about expressions with different
types of data
• Casting Example:
– z becomes 0: int x = 5, y = 8; float z = x/y;
– z becomes 0.625: int x=5, y=8; float z = (float)x/y;
Identifier names
Definition: An identifier is the name we give to a variable
• Rules
– All identifiers start with an alphabetic letter or underscore
– Subsequent letters of an identifier can be alphabetic
letters, underscores, or numeric digits (No Spaces)
– Identifiers cannot be a Java reserved word
• Conventions followed by most programmers
– Keep the identifiers relatively short
– Identifiers should be easy to remember
– Naming conventions
• Variables: start lower case, and first letter of subequent words
should be upper case (ex: salesTotal)
• Classes: First letter of every word upper clase (ex: MilesPerGal)
• Symbolic constants: public final int MAX = 100;
Note: the final modifier means that the variable cannot be altered .
Programming conventions
• It is important to keep your programs readable
• Most programmers will
– Indent blocks of instructions by several spaces
– Add comments to the top with name, date, programmer name, source
file name, purpose, modification purpose and date
– Add comments to the top of each method to include its purpose and
for automatic documentation generation
• Example:
/** main method calculate miles per gallon
* @param args command line arguments (unused)
*/
public static void main(String[] args)
{ String data = “I’m indented”;
System.out.println(data);
}
Concatenation
• Definition: concatenation is gluing strings together.
– String s = “abc” + “def” puts “abcdef” in s
– Numbers and Strings are different
• 33 + 44 yields 77
• “33” + “44” yields “3344”
• “33” / “44” generates an error
– String s = “abc ” + 33 puts “abc 33” in s because Java
makes the 33 into a string and then does concatenation
– What does the following print?
x = 32.95;
System.out.println(“you earned $” + x + “dollars”);
Note: Assign means do what is on the right of the equal sign
and store in the variable on the left
Escape Sequences ( or Characters)
• Definition: An escape sequence is two letters
starting with a backslash (\) that has special
meaning to Java
• Purpose: To represent characters that are not
able to easily be displayed in a program
• Examples:
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System.out.println(“don’t worry\tbe happy”);
System.out.println(“first line\nsecond line”);
System.out.println(“He said, \”hi\””);
System.out.println(“\fclear screen or new page”);
Numbering systems
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Binary: each digit is 2 times the previous
Octal: each digit is 8 times the previous
Decimal: each digit is 10 times the previous
Hexadecimal: each digit is 16 times the previous
Problems:
– Convert 110 to decimal if it is binary, octal, or
hexadecimal.
– Convert: If FCA is a hexadecimal number, convert it to
binary
Scope and Life
Example
{ // Outer block
Block: the instructions within braces { } // Next instruction fails
System.out.println(outer);
• Scope:
int outer = 1;
– Where in a program can a variable
can be accessed
{
// inner block
– A variable’s scope is from its
int inner = 2;
declaration to the end of the block
// The next is OK
• Life:
System.out.println
– When memory for a variable is
(outer+inner);
assigned for that variable
} // End of outer block
– Variables live within the block
where they are declared.
// Next instruction fails
• Programming conventions:
System.out.println(inner);
– limit scope as much as possible
// Next instruction is OK.
– This convention will result in
System.out.println(outer);
less errors and make programs
} // End of outer block
easier to maintain
Inputting Data
• On the class web-site there is a java class called IO
– Download by right clicking on the hyperlink
– Store it in your lab folder so it can be used
– You need the .class file, or compile the .java file to make
it
• We can use methods in this class to do input
• Examples:
– int data = IO.readInt(“Enter a grade: ”);
– String str = IO.readString(“Enter your name: ”);
– double value = IO.readDouble(“Enter sales total:” );
Putting it all together
import javax.swing.*;
public class Average
{ public static final int NUM = 3;
public static void main(String[] args)
{ int x = IO.readInt(“enter first: ”);
int y = IO.readInt(“enter second: ”);
int z = IO.readInt(“enter third: ”);
double average = ((double)x + y + z)/NUM;
JOptionPane.showMessageDialog
(null, “The average is “ + average);
} // End of main() method
}
// End of Average class
What prints?
int x = 3;
int y = x + 4;
x = x + 2;
x = x/2;
int z = x + 2;
String s = "abc";
System.out.println(s+x + y + z);
Remember: instructions execute one by one, in order
Review
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What is a data type? What is its purpose? Give some examples?
What does it mean to cast?
What is precision? Give some examples?
What is the difference between a variable and an identifier?
What is a symbolic constant? Why are they desirable?
Can you declare a double and int in the same statement? Why or why not?
How would you describe hexadecimal to someone without a clue?
What is the difference between the scope and life of a variable?
How are {} used in Java? How is ; used in Java? How is [] used?
What does static mean? How about public and private?
What is an escape sequence?
What is concatenation?
What are the programming conventions for naming variables, for
indenting, and for commenting?