Variables and Constants Objectives F To understand Identifiers, Variables, and Constants.
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Transcript of Variables and Constants Objectives F To understand Identifiers, Variables, and Constants.
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Variables and Constants Objectives
To understand Identifiers, Variables, and Constants
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IdentifiersYou need to give a name to something that you will refer to
in your program – variables, constants, methods, classes, packages
An identifier is a sequence of characters that consist of letters, digits, underscores (_), and dollar signs ($).
An identifier must start with a letter, an underscore (_), or a dollar sign ($). It cannot start with a digit.
An identifier cannot be a reserved word. (See Appendix A, “Java Keywords,” for a list of reserved words).
An identifier cannot be true, false, ornull.
An identifier can be of any length. Examples: $2, computeArea, area, radius, showMessageDialog
Java is case sensitive. An x and X are different variables
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VariablesVariables are used to store data input,data output, or intermediate data // Compute the first arearadius = 1.0;area = radius * radius * 3.14159;System.out.println("The area is “ + area + "
for radius “ + radius);
// Compute the second arearadius = 2.0;area = radius * radius * 3.14159;System.out.println("The area is “ + area + "
for radius “ + radius);
Note the + symbol is a concatenation operator
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Declaring VariablesVariables are used for representing data of a
certain type. To use a variable, you must first declare it.
int x; // Declare x to be an // integer variable;
double radius; // Declare radius to // be a double variable;
char a; // Declare a to be a // character variable;
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Assignment StatementsAfter a variable is declared, you can assign a
value to it using the assignment statement
x = 1; // Assign 1 to x;
radius = 1.0; // Assign 1.0 to radius;
a = 'A'; // Assign 'A' to a;
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Declaring and Initializinga Variable in One Step
int x = 1;
double d = 1.4;
float f = 1.4;
Is this statement correct?
If incorrect, what should it be?
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Note: In an assignment statement, the data type of the variable on the left must be compatible with the data type on the right
int x = 1.4; //double assigned
// to an int
Is this statement correct?
If incorrect, what should the statement be?
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Tip:
A variable must be declared before it can be assigned a value.
A variable declared in a method (local variable) must be assigned a value before it is used
Whenever possible declare a variable and assign its initial value in one step.
int x = 1;
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Constants
The value of a variable may change
during the execution of the program.
But a constant represents permanent
data that never change.
final datatype CONSTANT_NAME = VALUE;
final double PI = 3.14159;
final int SIZE = 3;
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Constants
Caution
A constant must be declared and initialized
before it can be used. You cannot change a
constant’s value once it is declared.
By convention, constants are named in upper
case: PI, not pi or Pi
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Benefits of Constants
There are three benefits of using constants: You don’t have to repeatedly type the same
value The value can be changed in a single
location, if necessary The program is easier to read