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Variables and Data Types

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Variable Definition

a location in memory, referenced by a name (identifier), where data of a given type can be stored, changed, and retrieved

Using variables establish its data type and initial value

(declaration) set/change its value (assignment, input) use/display its value (expressions, output)

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Example:Using Variables

int dimecount; // declarationdouble dimevalue = 0.10; // declaration with initial

valuedouble totalvalue;

dimecount = Input.readInt(); // input (assignment)totalvalue = dimecount*dimevalue; // assignmentSystem.out.println(totalvalue); // output

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0.10

5.30

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dimevalue

dimecount

totalvalue

dimecount = Input.readInt();

totalvalue = dimecount*dimevalue;

INPUT

OUTPUT

System.out.println(totalvalue);

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Back toJava Program Structure A Java program (application or

applet) is a class that consists of methods main(), init(), paint(), action(), setup(),

onButtonPressed(), … Each method has a body

delimited by { } consists of a sequence of statements

(including declarations)

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Statements Declarations

int dimecount;double dimevalue = 0.10;

Assignment statementsdimecount = Input.readInt();totalvalue = dimecount*dimevalue;

Output statementsSystem.out.println(totalvalue);

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Identifier

A name in a Java program used for variables, classes, methods, ...

Rules in forming an identifier: consists of letters, digits, and underscores

(_) should start with letter or underscore

Examples: ateneo score5 total_credit BigBlue _one4three x if

Some identifiers are reserved words

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Data Type Describes a domain or pool of values Helps a compiler impose rules Programming language needs rules for

constructing literals for a given data type e.g., 234 is an integer literal, ‘A’ is a

character literal, 2.1e-3 is a double floating point literal

Some primitive data types in Java: int, char, float, double, boolean

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Understanding Data Types

Important components of a data type:

Range of values Literals Possible operations

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The int Data Type Range: -2,147,483,648 to

2,147,483,647 applies to all system platforms

Literals sequence of digits Examples: 22, 16, 1, 426, 0, 12900

Operations: usual arithmetic operations +, -, *, /, % negative numbers obtained using - as prefix

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The double Data Type Values: decimal numbers

Range: 4.94e-324 to 1.80e+308 precision: n.nnnnn... X 10(+/-)mmm

Literals (examples) 100.5, 0.33333, 200000.0 -8E10 (-80000000000), 2.1e-3 (0.0021)

Operations: arithmetic ops (division?) float: lower precision

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Constants Literal values in a program

appear often enough and may be associated with an appropriate name

declare as a “variable with a fixed value” Examples

public static final int MAX = 100; public static final double PI = 3.1415926; public static final double DIMEVALUE =

0.10;

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Input and Output

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I/O in Java Text output in Java

System.out.print & System.out.println Input in pure Java is not straightforward

need to handle exception cases uses notions of streams and files

Trend in current applications perform I/O through visual components GUIs

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Input.java A “home made” class designed to

make console input simpler For use in Java applications

make sure that Input.java is in your working directory

use Input.xxx() for text input of ints/doublesInput.readInt(), Input.readDouble()

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Input Statements areAssignment Statements

Examples:

double interestRate;...int count = Input.readInt();

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Applets To create applets

need to process GUI events need an init() method to set up visual

objects, an action() method to specify associated actions

use InputOutputApplet extend InputOutputApplet instead of

Applet define setup() and onButtonPressed()

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Using InputOutputApplet Make sure InputOutputApplet.class is

present in your directory In the setup() method

addInput(“name”) to add input objects addButton(“label”) to add a button addOutput() to add an output area

In the onButtonPressed method getInt(), getDouble() for retrieving data print(), println() for printing on output area

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Operators and Expressions

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Operators in Java Arithmetic operators

+, -, *, /, % Special operators

(, ) performs grouping = (assignment)

Other operators

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Understanding Operators Operands

count (binary/unary) type

Calculation performed value (and type) returned

Other effects

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Example: % Binary operation Both operands are ints Returns the (int) remainder when

left operand is divided by the right operand

No additional effects

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Another Example: = Binary operation Left operand must be a variable Returns the value of the right

operand Effect: value of right operand is

assigned to left operand

* Note that a = b = c = 0; is valid

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Other Operators Increment and decrement operators

++, -- post- or pre-

Assignment operators +=, -=, *=, /=, …

“Built-in” Functions not really operators (but similar) Math.abs(), Math.sqrt(), Math.pow(), ...

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Post-increment Operator: ++ Example: number++ Unary operator Operand must be a variable Returns the (original) value of the

operand Effect: variable is incremented

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Pre-increment Operator: ++ Example: ++number Unary operator Operand must be a variable Returns the incremented value of

the operand Effect: variable is incremented

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About ++ Notice that a++; and ++a; are

virtually equivalent return value is ignored in both cases could be used as shorthands for a = a+1;

Distinction apparent when the return value is used

a = 5; a = 5;b = a++; b = ++a;// values of a & b? // values of a & b?

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Decrement Operator: --

Analogous definitions for

Post-decrement number-- Pre-decrement --number

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Assignment Operators There is a shorthand for constructs

such as sum = sum + number;

sum += number;

+= is an operator: such an operator exists for virtually

every arithmetic operator +=, -=, *=. /=, %=, ...

effect: variable is updated returned value: the updated value

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Built-in Functions Provided in Java to provide for more

complex operations Example: Math.pow()

double result = Math.pow(5.5,3.0); can be viewed as a binary operation

that calculates some power of a number javap java.lang.Math

prints a list of available math functions

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Operand Types vsResult Type There are cases where the type of

the result differs from the types of the operands

Examples division between an int and a double

returns a double Math.round() has an operand

(argument) that is a float but returns an int

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Expressions Expression

sequence of variables, literals, operators, and function calls

Uses right operand of an assignment argument for System.out.println()

Expression-statement an expression terminated by a semicolon

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Strings

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Variables Revisited A variable holds a value

A variable may instead contain a reference

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“Hello”

int num = 5;

num

String s = “Hello”;

s

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String A special kind of data type

called a class allows for string objects

About Strings sequences of characters (letters, digits,

etc) literals: formed by delimiting the

sequence of characters with " " operations?

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Operations on Strings Concatenation

“Hello” + “ there” equals “Hello there” Obtain length of string

String s = “Hello”; int len = s.length(); // len = 5

Obtain a substring String s = “Hello”; String t = s.substring(0,3); // t = “Hel”