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LEC. 02: DATA TYPES AND OPERATORS (1/2)

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CONTENT

Data types Literals Variable declaration and initialization Scope rules Operators Expressions Type conversion and casting Wrapper classes for primitive types

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DATA TYPES

A data type is a collection of two sets: Value set – all the possible and legal values of the data type Operation set – all the possible and legal operations

applicable on the values in the value set Example: Integer data type

Value set: { …, -2, -1, 0, 1, 2, …} Operation set: {+, –, *, /, mod}

In computer, the data type determines how to represent a value, such as the encoding, and how many bytes needed.

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JAVA DATA TYPES

Two categories of Java data types Object-oriented Non-object-oriented

Object-oriented Each class is a data type. Object-oriented types are designed by programmers to represent

problem-oriented types, such as the type for student records, also called user-defined data type.

All the possible states of a class object form the value set. All the methods defined in a class form the operation set.

Non-object-oriented Java provides 8 primitive types. Each primitive type is associated with a specified value range and

set of operations.

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SIZE AND RANGE OF PRIMITIVE TYPES

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Primitive data type

Length in bits

Ranges

boolean N/A true and false

char 16 16-bit Unicode characters

byte 8 -27 ~ 27-1

short 16 -215 ~ 215-1

int 32 -231 ~ 231-1

long 64 -263 ~ 263-1

float 32 Based on IEEE 754

double 64 Based on IEEE 754

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INTEGRAL TYPES

Java provides four integral types for integers, including both positive and negative.

Four integral types byte short int long

The difference among these four types is the number of bytes used to store an integer and the ranges.

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FLOATING TYPES

Java provides two types for real numbers , including both positive and negative.

Two types of floating types float double

The difference between these two types is the number of bytes used to store a real number and the ranges.

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CHARACTERS

Java uses Unicode to represent characters. Unicode defines a encoding for character set that can

represent all of the characters found in all human languages. In Java, char is an unsigned 16-bit type having a range of

0 to 65,535. The standard 8-bit ASCII character set is a subset of Unicode

and ranges from 0 to 127. Since char is an unsigned 16-bit type, it is possible to

perform various arithmetic manipulations on a char variable.

Example The following program segment prints Y

char ch; ch = ‘X’; System.out.print(++ch);

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BOOLEAN

The boolean type represents true/false values. Java defines the values true and false using the reserved

words true and false.

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DEMO PROGRAM FOR USING boolean

class BoolDemo {

public static void main(String args[]) {

boolean b;

b = false;

System.out.println("b is " + b);

b = true;

System.out.println("b is " + b);

if(b) System.out.println("This is executed.");

b = false;

if(b) System.out.println("This is not executed.");

System.out.println("10 > 9 is " + (10 > 9));

}

}

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What is the result if you remove the parentheses of

(10 > 9) ?

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LITERALS

In Java, literals refer to fixed values that are represented in their human-readable form.

A literal is a string representing a value by itself. The literal forms of different types are distinct, except

for byte, short and int.

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LITERAL TABLE

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Primitive data type

Example comment

boolean true, false Only two literals

char ‘a’‘\n’‘\\’‘\u03A6’

The letter aThe escape sequence for “new line”Representing the back-slash16-bit Unicode, using hexadecimals

byte 10, -30

short 227, -102

int 25-0310xA1F2

Integer in decimalInteger in Octal formInteger in Hexadecimal form (not case sensitive)

long 25L077l-0xA3CBL

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LITERAL TABLE

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Primitive data type

Example comment

float 52.56f, 32.9F

double 100.25, 100.345d,100.0112D12.34e2

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MORE ON LITERALS By default, integer literals are of type int.

If you want to specify a long literal, append an l or an L, e.g. 12 is an int, but 12L is a long.

Although integer literals create an int value by default, they can still be assigned to variables of type char, byte, or short as long as the value being assigned can be represented by the target type. byte b; b= 12; are legal. byte b; b= 128; are illegal.

Beginning with JDK 7, you can embed one or more underscores into an integer or floating point literal. Example: 123_45_1234 is equal to 123451234

By default, floating-point literals are of type double. If you want to specify a float literal, append an f or an F, e.g. 12.5 is a

double, but 12.5F is a float.

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HEXADECIMAL, OCTAL AND BINARY FORMS

In Java, an integer beginning with 0 is a octal number(八進位 ). 011 represents a octal number equal to 910. 081 is illegal.

In Java, an integer beginning with 0x or 0X is a hexadecimal number(十六進位 ). 0x11 represents a hexadecimal number equal to 1710. 0xG1 is illegal.

Beginning with JDK 7, an integer beginning with 0b or 0B is a binary number(二進位 ). 0b11 represents a hexadecimal number equal to 310. 0b21 is illegal.

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CHARACTER ESCAPE SEQUENCES (跳脫字元 )

Java provides escape sequences, also referred to as backslash character constants, to represent some characters which are not printable or have special meanings, such as the carriage return, the single and double quotes.

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TABLE OF CHARACTER ESCAPE SEQUENCES

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STRING LITERALS

A string is a sequence of zero or more characters enclosed by double quotes.

In Java, strings are represented by a standard built-in class String.

In Java, a string literal is a sequence of characters enclosed by a pair of double quotes.

Example Legal: "", "a", "abc123", "a\"hello", "a\t\uA123hello“ Illegal: ‘X’, "a""

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VARIABLES

A variable is a named memory space used to save value. All variables in Java must be declared prior to their use

by using the following form.

<type> <variable_name>; The capabilities of a variable are determined by its type. The type of a variable cannot be changed once it is

declared. There are 4 types of variables

Class fields Instance fields Local variables Parameters

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VARIABLES

Initializing variables

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byte count = 10; // give count an initial value of 10char ch = 'X'; // initialize ch with the letter Xfloat f = 1.2F; // f is initialized with 1.2int a, b = 8, c = 19, d; // b and c have initializationsdouble radius = 4, height = 5;double volume = 3.1416 * radius * radius * height; //dynamic

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SCOPE RULES OF VARIABLES

Scope rules defines the association between the use of a variable and the declaration of a variable. It determines what variables are visible to other parts of your

program and the lifetime of variables. Java allows variables to be declared within any code block.

Remember that the definitions of classes and methods are all code block.

A block defines a scope. A variables declared inside a scope are NOT visible (that is,

accessible) to codes which are defined outside that scope. For nested scopes, a variable declared in a outer scope will be

visible to codes within inner scopes. Nested scopes mean a scope is enclosed by another scope, such as a

method’s scope is enclosed by a class’ scope.

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EXAMPLE FOR SCOPE RULES

class ScopeDemo {

public static void main(String args[]) {

int x; // known to all code within main

x = 10;

if(x == 10) { // start new scope

int y = 20; // known only to this block

// x and y both known here.

// x cannot be re-defined here

System.out.println("x and y: " + x + " " + y);

x = y * 2;

}

// y = 100; // Error! y not known here

// x is still known here.

System.out.println("x is " + x);

}

}

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LIFETIME OF VARIABLES

The lifetime of a variable describes if there is a memory space associated with the variable.

The variables are created, associated with memory space, when their scope is entered/activated, and destroyed, disassociated with memory space, when their scope is left/inactivated. Variables declared within a method will not hold their values

between calls to that method.

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