JAVA Programming Guide - Quick Reference.pdf - Java - musli_com

Java Programming Guide - Quick Reference

Syntax for a standalone application in Java:

Java Comments:

class <classname>

{

Delimiters Use

// Used for commenting a single line

public static void main(String args[])

{

statements;

————————;

}

/* ————— */ Used for commenting a block of code

/** —————*/ Used for commenting a block of code.

Used by the Javadoc tool for

generating Java documentation.

Primitive datatypes in Java:

Steps to run the above application:

1. Type the program in the DOS editor or notepad. Save the

file with a .java extension.

2. The file name should be the same as the class, which has the

main method.

3. To compile the program, using javac compiler, type the

following on the command line:

Syntax: javac <filename.java>

Example: javac abc.java

4. After compilation, run the program using the Java

interpreter.

Syntax: java <filaname> (without the .java

extension)

Example: java abc

5. The program output will be displayed on the command line.

DataType Size Default Min Value

Max Value

byte

(Signed

-128

integer) 8 bits 0

+127

short

(Signed

-32,768

integer) 16 bits 0

+32,767

int

(Signed

-2,147,483,648

integer) 32 bits 0

+2,147,483,647

long

-9, 223, 372,036,854,

(Signed

775,808,

Integer)

+9,223,372,036,

64 bits 0

854, 775, 807

Java Programming Guide - Quick Reference

Java reserved words:

abstract default if package this

boolean do implements private throw

Break double import protected throws

Byte else instanceof public transient

case extends int return null

try Const for new switch

continue while goto

float 32 bits 0.0 1.4E-45

(IEEE 754

3.4028235E38

floating-point)

synchronized super

double 64 bits 0.0 4.9E-324

(IEEE 754 1.7976931348623157E308

floating-point)

Catch final

interface short

void

char finally long

static

volatile

class float

native

char 16 bits \u0000 \u0000

(Unicode

character)

Java naming conventions:

\uFFFF

boolean 1 bit false

Variable Names: Can start with a letter, ‘$’ (dollar symbol),

or ‘_’ (underscore); cannot start with a number; cannot be a

reserved word.

Method Names: Verbs or verb phrases with first letter in

lowercase, and the first letter of subsequent words

capitalized; cannot be reserved words.

Example: setColor()

Variable Declaration:

Example: int num1;

Variable Initialization:

<datatype> <variable name> = value

Example: double num2 = 3.1419;

Class And Interface Names: Descriptive names

that begin with a capital letter, by convention; cannot be a

reserved word.

Escape sequences:

Literal

Represents

Constant Names: They are in capitals.

Example: Font.BOLD, Font.ITALIC

New line

Horizontal tab

Backspace

Carriage return

Java Programming Guide - Quick Reference

Form feed

5. Switch statement

Syntax:

switch(variable)

{

case(value1):

statements;

break;

case(value2):

statements;

break;

default:

statements;

break;

}

Backslash

\”

Double quote

\ddd

Octal character

\xdd

Hexadecimal character

\udddd

Unicode character

Arrays: An array which can be of any datatype, is created in

two steps – array declaration and memory allocation.

Array declaration

<datatype> [] <arr ```````````ayname>;

Examples int[] myarray1;

double[] myarray2;

Memory Allocation

The new keyword allocates memory for an array.

Syntax

<arrayname> = new <array type> [<number of

elements>];

Examples

myarray1 = new int[10];

Myarray2 = new double[15];

Class Declaration: A class must be declared using the

keyword class followed by the class name.

Syntax

class <classname>

{

———— Body of the class

A typical class declaration is as follows:

<modifier> class <classname> extends

<superclass name> implements <interface name>

{

—————Member variable declarations;

—————Method declarations and definitions

}

Multi-dimensional arrays:

Syntax:

[number of rows][number of columns];

Example:

int mdarray[][] = new int[4][5];

Java Programming Guide - Quick Reference

Flow Control:

Member variable declarations:

1. If……..else statements

Syntax:

if(condition)

{

statements;

}

else

{

statements;

}

<access specifier> <static/final/transient/

volatile> <datatype> <variable name>

Example public final int num1;

Method declarations:

{

Method body;

}

Example public static void main(String args[])

{

}

2. For loop

Syntax:

for(initialization; condition; increment)

{

statements;

}

Interface declaration: Create an interface. Save the file

with a.java extension, and with the same name as the

interface. Interface methods do not have any implementation

and are abstract by default.

3. While loop

Syntax:

while(condition)

{

statements;

}

Syntax

interface <interface name>

{

void abc();

void xyz();

}

4. Do….While loop

Syntax:

{

statements;

}

while(condition);

Using an interface: A class implements an interface with the

implements keyword.

Java Programming Guide - Quick Reference

Syntax

class <classname> extends <superclass name>

implements <interface name>

{

class body;

—————————;

}

final

Class

Cannot be subclassed.

Method

Cannot be overridden.

Variable

Value cannot be changed

(Constant)

Creating A Package:

native

Method

Implemented in a language

other than Java like C,C++,

assembly etc. Methods do not

have bodies.

1. Identify the hierarchy in which the .class files have to

be organized.

2. Create a directory corresponding to every package, with

names similar to the packages.

3. Include the package statement as the first statement in

the program.

4. Declare the various classes.

5. Save the file with a .java extension.

6. Compile the program which will create a .class file in

the same directory.

7. Execute the .class file.

static

Method

Class method. It cannot refer to

nonstatic variables and methods

of the class. Static methods are

implicitly final and invoked

through the class name.

Variable Class variable. It has only one

copy regardless of how many

instances are created. Accessed

only through the class name.

Packages and Access Protection:

Accessed

Public Protected Package Private

From the

same class ?

synchronized

Method

A class which has a synchronized

method automatically acts as a

lock. Only one synchronized

method can run for each class.

Yes

From a non

subclass in

the same

package ?

Yes

Java Programming Guide - Quick Reference

List of exceptions in Java(part of java.lang package):

From a non

subclass

outside the

package?

Essential exception classes include -

Yes

Exception Description

From a

subclass

in the same

package?

ArithmeticException Caused by exceptional

conditions like divide by

zero

Yes

ArrayIndexOfBounds Thrown when an array is

Exception

From a

subclass

outside the

package ?

accessed beyond its bounds

Yes

Yes No No

ArrayStoreException Thrown when an incompatible

type is stored in an array

ClassCastException Thrown when there is an invalid

cast

Attribute modifiers in Java:

IllegalArgument Thrown when an inappropriate

Exception argument is passed to a method

Modifier Acts on Description

abstract Class Contains abstract

methods.Cannot

be instantiated.

IllegalMonitorState Illegal monitor operations such as

Exception waiting on an unlocked thread

Interface All interfaces are implicitly

abstract. The modifier is

optional.

IllegalThreadState Thrown when a requested

Exception operation is incompatible with

the current thread state.

Method

Method without a body.

Signature is followed by a

semicolon. The class must also

be abstract.

IndexOutOfBounds Thrown to indicate that an index

Exception is out of range.

NegativeArraySize Thrown when an array is created

Exception with negative size.

Java Programming Guide - Quick Reference

NullPointerException Invalid use of a null reference.

setPriority()

Changes the priority of the thread

NumberFormatException Invalid conversion of a string to a

number.

currentThread() Returns a reference to the

currently executing thread

SecurityException

Thrown when security is violated.

activeCount()

Returns the number of active

threads in a thread group

ClassNotFound Thrown when a class is not found.

Exception

Exception Handling Syntax:

CloneNotSupported Attempt to clone an object that

Exception does not implement the Cloneable

interface.

try

{

//code to be tried for errors

}

catch(ExceptionType1 obj1)

{

//Exception handler for ExceptionType1

}

catch(ExceptionType2 obj2)

{

//Exception handler for ExceptionType2

}

finally{

//code to be executed before try block ends.

This executes whether or not an //

exception occurs in the try block.

}

IllegalAccess Thrown when a method does not

Exception have access to a class.

Instantiation Thrown when an attempt is made

Exception to instantiate an abstract class or

an interface.

InterruptedException Thrown when a second thread

interrupts a waiting, sleeping, or

paused thread.

The java.lang.Thread class

I/O classes in Java (part of the java.io package):

The Thread class creates individual threads. To create a thread

either (i) extend the Thread class or (ii) implement the Runnable

interface. In both cases, the run() method defines operations

I/O class name

Description

BufferedInputStream Provides the ability to buffer the

Java Programming Guide - Quick Reference

performed by the thread.

Methods of the Thread class:

input. Supports mark() and

reset() methods.

BufferedOutputStream Provides the ability to write bytes

to the underlying output stream

without making a call to the

underlying system.

Methods

Description

run()

Must be overridden by

Runnable object; contains code

that the thread should perform

BufferedReader

Reads text from a character

input stream

start()

Causes the run method to

execute and start the thread

BufferedWriter

Writes text to character

output stream

sleep()

Causes the currently executing

thread to wait for a specified time

before allowing other threads to

execute

DataInputStream

Allows an application to read

primitive datatypes from an

underlying input stream

DataOutputStream

Allows an application to write

primitive datatypes to an output

stream

interrupt()

Interrupts the current thread

File

Represents disk files and

directories

Yield()

Yields the CPU to other runnable

threads

FileInputStream

Reads bytes from a file in a file

system

getName()

Returns the current thread’s name

FileOutputStream

Writes bytes to a file

ObjectInputStream

Reads bytes i.e. deserializes

objects using the

readObject() method

getPriority()

Returns the thread’s priority as an

integer

ObjectOutputStream

Writes bytes i.e. serializes

objects using the

writeObject() method

isAlive()

Tests if the thread is alive; returns

a Boolean value

PrintStream

Provides the ability to print

different data values in an

efficient manner

join()

Waits for specified number of

milliseconds for a thread to die

RandomAccessFile

Supports reading and writing to

a random access file

setName()

Changes the name of the thread

Java Programming Guide - Quick Reference

StringReader

Character stream that reads

from a string

getName() Returns the name of the file and directory

denoted by the path name

isDirectory() Tests whether the file represented by the

pathname is a directory

lastModified() Returns the time when the file was last

modified

l length() Returns the length of the file represented by

the pathname

listFiles() Returns an array of files in the directory

represented by the pathname

setReadOnly() Marks the file or directory so that only

read operations can be performed

renameTo() Renames the file represented by the

pathname

delete() Deletes the file or directory represented by

the pathname

canRead() Checks whether the application can read

from the specified file

canWrite() Checks whether an application can write to

a specified file

StringWriter

Character stream that writes to

a StringBuffer that is later

converted to a String

The java.io.InputStream class: The InputStream class is

at the top of the input stream hierarchy. This is an abstract class

which cannot be instantiated. Hence, subclasses like the

DataInputStream class are used for input purposes.

Methods of the InputStream class:

Method Description

available() Returns the number of bytes that can be

read

close()

Closes the input stream and releases

associated system resources

mark()

Marks the current position in the input

stream

Creating applets:

mark

Supported() Returns true if mark() and reset() methods

are supported by the input stream

1. Write the source code and save it with a .java

extension

2. Compile the program

3. Create an HTML file and embed the .class file with the

<applet> tag into it.

4. To execute the applet, open the HTML file in the browser

or use the appletviewer utility, whch is part of the Java

Development Kit.

read()

Abstract method which reads the next byte

of data from the input stream

read(byte b[]) Reads bytes from the input stream and

stores them in the buffer array

Java Programming Guide - Quick Reference

skip()

Skips a specified number of bytes from the

input stream

The <applet> tag: Code, width , and height are

mandatory attributes of the <applet> tag. Optional attributes

include codebase, alt,name, align, vspace, and

hspace . The code attribute takes the name of the class file as

its value.

Syntax:

<applet code = “abc.class” height=300

width=300>

</applet>

The java.io.OutputStream class: The OutputStream class

which is at the top of the output stream hierarchy, is also an

abstract class, which cannot be instantiated. Hence, subclasses

like DataOutputStream and PrintStream are used for

output purposes.

Methods of the OutputStream class:

Method

Description

Using the Appletviewer: Appletviewer.exe is an

application found in the BIN folder as part of the JDK. Once an

HTML file containing the class file is created (eg. abc.html) ,

type in the command line:

Appletviewer abc.html

close()

Closes the output stream, and releases

associated system resources

write(int b) Writes a byte to the output stream

java.applet.Applet class:

write(byte b[]) Writes bytes from the byte array to the

output stream

Methods of the java.applet.Applet class:

flush()

Flushes the ouput stream, and writes

buffered output bytes

Method

Description

init()

Invoked by the browser or the

applet viewer to inform that the

applet has been loaded

java.io.File class: The File class abstracts information

about files and directories.

start()

Invoked by the browser or the

applet viewer to inform that

applet execution has started

Methods of the File class:

stop()

Invoked by the browser or the

applet viewer to inform that

applet execution has stopped

Method

Description

exists()

Checks whether a specified file exists

JAVA Programming Guide - Quick Reference.pdf

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