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Java - Quick Reference Guide

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What is Java?

Java is:

Java Environment Setup:

Java SE is freely available from the link Download Java. So you download a version based on your operating system.

You can refere to installation guide for a complete detail.

Java Basic Syntax:

First Java Program:

Let us look at a simple code that would print the words Hello World.

public class MyFirstJavaProgram{

   /* This is my first java program.  
    * This will print 'Hello World' as the output
    */

    public static void main(String []args){
       System.out.println("Hello World"); // prints Hello World
    }
} 

About Java programs, it is very important to keep in mind the following points.

Java Identifiers:

All java components require names. Names used for classes, variables and methods are called identifiers.

In java there are several points to remember about identifiers. They are as follows:

Java Modifiers:

Like other languages it is possible to modify classes, methods etc by using modifiers. There are two categories of modifiers.

We will be looking into more details about modifiers in the next section.

Java Variables:

We would see following type of variables in Java:

Java Arrays:

Arrays are objects that store multiple variables of the same type. However an Array itself is an object on the heap. We will look into how to declare, construct and initialize in the upcoming chapters.

Java Enums:

Enums were introduced in java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.

With the use of enums it is possible to reduce the number of bugs in your code.

For example if we consider an application for a fresh juice shop it would be possible to restrict the glass size to small, medium and Large. This would make sure that it would not allow anyone to order any size other than the small, medium or large.

Example:

class  FreshJuice{
   enum FreshJuiceSize{ SIZE, MEDUIM, LARGE }
   FreshJuiceSize size;
}

public class FreshJuiceTest{
   public static void main(String args[]){
      FreshJuice juice = new FreshJuice();
      juice.size = FreshJuice. FreshJuiceSize.MEDUIM ;
   }
}

Note: enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.

Java Keywords:

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.

abstractassertbooleanbreak
bytecasecatchchar
classconstcontinuedefault
dodoubleelseenum
extendsfinalfinallyfloat
forgotoifimplements
importinstanceofintinterface
longnativenewpackage
privateprotectedpublicreturn
shortstaticstrictfpsuper
switchsynchronizedthisthrow
throwstransienttryvoid
volatilewhile

Comments in Java

Java supports single line and multi-line comments very similar to c and c++. All characters available inside any comment are ignored by Java compiler.

public class MyFirstJavaProgram{

   /* This is my first java program.
    * This will print 'Hello World' as the output
    * This is an example of multi-line comments.
    */

    public static void main(String []args){
       // This is an example of single line comment
       /* This is also an example of single line comment. */
       System.out.println("Hello World"); 
    }
} 

Data Types in Java

There are two data types available in Java:

  1. Primitive Data Types

  2. Reference/Object Data Types

Primitive Data Types:

There are eight primitive data types supported by Java. Primitive data types are predefined by the language and named by a key word. Let us now look into detail about the eight primitive data types.

Reference Data Types:

Java Literals:

A literal is a source code representation of a fixed value. They are represented directly in the code without any computation.

Literals can be assigned to any primitive type variable. For example:

byte a = 68;
char a = 'A'

String literals in Java are specified like they are in most other languages by enclosing a sequence of characters between a pair of double quotes. Examples of string literals are:

"Hello World"
"two\nlines"
"\"This is in quotes\""

Java language supports few special escape sequences for String and char literals as well. They are:

NotationCharacter represented
\nNewline (0x0a)
\rCarriage return (0x0d)
\fFormfeed (0x0c)
\bBackspace (0x08)
\sSpace (0x20)
\ttab
\" Double quote
\'Single quote
\\backslash
\dddOctal character (ddd)
\uxxxxHexadecimal UNICODE character (xxxx)

Java Access Modifiers:

Java provides a number of access modifiers to set access levels for classes, variables, methods and constructors. The four access levels are:

  1. Visible to the package. the default. No modifiers are needed.

  2. Visible to the class only (private).

  3. Visible to the world (public).

  4. Visible to the package and all subclasses (protected).

Java Basic Operators:

Java provides a rich set of operators to manipulate variables. We can divide all the Java operators into the following groups:

The Arithmetic Operators:

OperatorDescriptionExample
+Addition - Adds values on either side of the operator A + B will give 30
-Subtraction - Subtracts right hand operand from left hand operand A - B will give -10
*Multiplication - Multiplies values on either side of the operator A * B will give 200
/Division - Divides left hand operand by right hand operand B / A will give 2
%Modulus - Divides left hand operand by right hand operand and returns remainder B % A will give 0
++Increment - Increase the value of operand by 1 B++ gives 21
--Decrement - Decrease the value of operand by 1 B-- gives 19

The Relational Operators:

OperatorDescriptionExample
== Checks if the value of two operands are equal or not, if yes then condition becomes true. (A == B) is not true.
!= Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. (A != B) is true.
> Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. (A > B) is not true.
< Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (A < B) is true.
>= Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. (A >= B) is not true.
<= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. (A <= B) is true.

The Bitwise Operators:

OperatorDescriptionExample
& Binary AND Operator copies a bit to the result if it exists in both operands. (A & B) will give 12 which is 0000 1100
| Binary OR Operator copies a bit if it exists in eather operand. (A | B) will give 61 which is 0011 1101
^ Binary XOR Operator copies the bit if it is set in one operand but not both. (A ^ B) will give 49 which is 0011 0001
~ Binary Ones Complement Operator is unary and has the efect of 'flipping' bits. (~A ) will give -60 which is 1100 0011
<< Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand. A << 2 will give 240 which is 1111 0000
>> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. A >> 2 will give 15 which is 1111
>>> Shift right zero fill operator. The left operands value is moved right by the number of bits specified by the right operand and shifted values are filled up with zeros. A >>>2 will give 15 which is 0000 1111

The Logical Operators:

OperatorDescriptionExample
&& Called Logical AND operator. If both the operands are non zero then then condition becomes true. (A && B) is false.
||Called Logical OR Operator. If any of the two operands are non zero then then condition becomes true. (A || B) is true.
!Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false. !(A && B) is true.

The Assignment Operators:

OperatorDescriptionExample
=Simple assignment operator, Assigns values from right side operands to left side operand C = A + B will assigne value of A + B into C
+=Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand C += A is equivalent to C = C + A
-=Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand C -= A is equivalent to C = C - A
*=Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand C *= A is equivalent to C = C * A
/=Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand C /= A is equivalent to C = C / A
%=Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand C %= A is equivalent to C = C % A
<<=Left shift AND assignment operator C <<= 2 is same as C = C << 2
>>=Right shift AND assignment operator C >>= 2 is same as C = C >> 2
&=Bitwise AND assignment operator C &= 2 is same as C = C & 2
^=bitwise exclusive OR and assignment operator C ^= 2 is same as C = C ^ 2
|=bitwise inclusive OR and assignment operator C |= 2 is same as C = C | 2

Misc Operators

There are few other operators supported by Java Language.

Conditional Operator ( ? : ):

Conditional operator is also known as the ternary operator. This operator consists of three operands and is used to evaluate boolean expressions. The goal of the operator is to decide which value should be assigned to the variable. The operator is written as :

variable x = (expression) ? value if true : value if false

instanceOf Operator:

This operator is used only for object reference variables. The operator checks whether the object is of a particular type(class type or interface type). instanceOf operator is wriiten as:

( Object reference variable ) instanceOf  (class/interface type)

Precedence of Java Operators:

Category  Operator  Associativity 
Postfix  () [] . (dot operator) Left to right 
Unary  ++ - - ! ~ Right to left 
Multiplicative   * / %  Left to right 
Additive   + -  Left to right 
Shift   >> >>> <<   Left to right 
Relational   > >= < <=   Left to right 
Equality   == !=  Left to right 
Bitwise AND  Left to right 
Bitwise XOR  Left to right 
Bitwise OR  Left to right 
Logical AND  &&  Left to right 
Logical OR  ||  Left to right 
Conditional  ?:  Right to left 
Assignment  = += -= *= /= %= >>= <<= &= ^= |=  Right to left 
Comma  Left to right 

The while Loop:

A while loop is a control structure that allows you to repeat a task a certain number of times.

Syntax:

The syntax of a while loop is:

while(Boolean_expression)
{
   //Statements
}

The do...while Loop:

A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.

Syntax:

The syntax of a do...while loop is:

do
{
   //Statements
}while(Boolean_expression);

The for Loop:

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

A for loop is useful when you know how many times a task is to be repeated.

Syntax:

The syntax of a for loop is:

for(initialization; Boolean_expression; update)
{
   //Statements
}

Enhanced for loop in Java:

As of java 5 the enhanced for loop was introduced. This is mainly used for Arrays.

Syntax:

The syntax of enhanced for loop is:

for(declaration : expression)
{
   //Statements
}

The break Keyword:

The break keyword is used to stop the entire loop. The break keyword must be used inside any loop or a switch statement.

The break keyword will stop the execution of the innermost loop and start executing the next line of code after the block.

The continue Keyword:

The continue keyword can be used in any of the loop control structures. It causes the loop to immediately jump to the next iteration of the loop.

Syntax:

The syntax of a continue is a single statement inside any loop:

continue;

The if Statement:

An if statement consists of a Boolean expression followed by one or more statements.

Syntax:

The syntax of an if statement is:

if(Boolean_expression)
{
   //Statements will execute if the Boolean expression is true
}

The if...else Statement:

An if statement can be followed by an optional else statement, which executes when the Boolean expression is false.

Syntax:

The syntax of a if...else is:

if(Boolean_expression){
   //Executes when the Boolean expression is true
}else{
   //Executes when the Boolean expression is false
}

The if...else if...else Statement:

An if statement can be followed by an optional else if...else statement, which is very usefull to test various conditions using single if...else if statement.

Syntax:

The syntax of a if...else is:

if(Boolean_expression 1){
   //Executes when the Boolean expression 1 is true
}else if(Boolean_expression 2){
   //Executes when the Boolean expression 2 is true
}else if(Boolean_expression 3){
   //Executes when the Boolean expression 3 is true
}else {
   //Executes when the one of the above condition is true.
}

Nested if...else Statement:

It is always legal to nest if-else statements. When using if , else if , else statements there are few points to keep in mind.

Syntax:

The syntax for a nested if...else is as follows:

if(Boolean_expression 1){
   //Executes when the Boolean expression 1 is true
   if(Boolean_expression 2){
      //Executes when the Boolean expression 2 is true
   }
}

The switch Statement:

A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case.

Syntax:

The syntax of enhanced for loop is:

switch(expression){
    case value :
       //Statements
       break; //optional
    case value :
       //Statements
       break; //optional
    //You can have any number of case statements.
    default : //Optional
       //Statements
}

Java Methods:

A Java method is a collection of statements that are grouped together to perform an operation. When you call the System.out.println method, for example, the system actually executes several statements in order to display a message on the console.

In general, a method has the following syntax:

modifier returnValueType methodName(list of parameters) {
  // Method body;
}

A method definition consists of a method header and a method body. Here are all the parts of a method:

Java Classes & Objects:

A sample of a class is given below:

public class Dog{
   String breed;
   int age;
   String color;

   void barking(){
   }
   
   void hungry(){
   }
   
   void sleeping(){
   }
}

A class can contain any of the following variable types.

Exceptions Handling:

A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:

try
{
   //Protected code
}catch(ExceptionName e1)
{
   //Catch block
}

Multiple catch Blocks:

A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following:

try
{
   //Protected code
}catch(ExceptionType1 e1)
{
   //Catch block
}catch(ExceptionType2 e2)
{
   //Catch block
}catch(ExceptionType3 e3)
{
   //Catch block
}

The throws/throw Keywords:

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.

You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the different in throws and throw keywords.

The finally Keyword

The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.

Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code.

A finally block appears at the end of the catch blocks and has the following syntax:

try
{
   //Protected code
}catch(ExceptionType1 e1)
{
   //Catch block
}catch(ExceptionType2 e2)
{
   //Catch block
}catch(ExceptionType3 e3)
{
   //Catch block
}finally
{
   //The finally block always executes.
}

For a complete detail of the Java Programming language, it is recommended to go through our simple Java Tutorial.

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