The Java Language

Duke, Javas official maskot.

What is Java?

Java is a programming language originally developed by James Gosling at Sun Microsystems (which has since merged into Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them. Java applications are typically compiled to bytecode (class file) that can run on any Java virtual machine (JVM) regardless of computer architecture. Java is a general-purpose, concurrent, class-based, object-oriented language that is specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" (WORA), meaning that code that runs on one platform does not need to be recompiled to run on another. Java is as of 2012 one of the most popular programming languages in use, particularly for client-server web applications, with a reported 10 million users.
The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1991 and first released in 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of its Java technologies under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java and GNU Classpath.

Why use Java?

Because you can write applications that can run on all kinds of systems.

You can write a desktop application that can run on windows, mac, linux and solaris.

Java is used on more than 850 million machines, and you can develop applications for mobile phones, android and much more!

Classes

To declare a class, you do this. The class name is RedHat.

Code:

public class RedHat {

}

Now you have made a class in Java.

inside the class, you can add methods and fields.

Code:

public class RedHat { 
  private String name = "redpois0n";
  private int age = 18;
  private boolean male = true;
}

Now we got 3 fields in our RedHat class, called name, age, and male.

Since these fields are private, we cant access them from the outside.

Code:

RedHat hat = new RedHat();
int age = hat.age; //wont work

It wont work since the fields are private, so we need to generate getters.

Getters is a kind of method that will return the field.

So instead of getting the field directly, you call a method that will return it for you.

Code:

public class RedHat { 
  private String name = "redpois0n";
  private int age = 18;
  private boolean male = true;

  public String getName() {
  return this.name;
  }

  public int getAge() {
  return this.age;
  }

  public boolean getMale() {
  return this.male;
  }

}

Now, we can access the fields.

Code:

RedHat hat = new RedHat();
int age = hat.getAge(); //will return hat.age

But what if you want to change the fields?

Code:

public class RedHat { 
  private String name = "redpois0n";
  private int age = 18;
  private boolean male = true;

  public void setName(String name) {
  this.name = name;
  }

  public void setAge(int age) {
  this.age = age;
  }

  public void setMale(boolean male) {
  this.male = male;
  }

}

Keywords
Taken from wikipedia.

abstract

The abstract keyword is used to declare a class or method to be abstract. An abstract method has no implementation; all classes containing abstract methods must themselves be abstract, although not all abstract classes have abstract methods. Objects of a class which is abstract cannot be instantiated, but can be extended by other classes. All subclasses of an abstract class must either provide implementations for all abstract methods, or must also be abstract.

assert

The assert keyword, which was added in J2SE 1.4, is used to make an assertion—a statement which the programmer believes is always true at that point in the program. If assertions are enabled when the program is run and it turns out that an assertion is false, an AssertionError is thrown and the program terminates. This keyword is intended to aid in debugging.

boolean

The boolean keyword is used to declare a field that can store a boolean value; that is, either true or false. This keyword is also used to declare that a method returns a value of type boolean.

break

Used to resume program execution at the statement immediately following the current enclosing block or statement. If followed by a label, the program resumes execution at the statement immediately following the enclosing labeled statement or block.

byte

The byte keyword is used to declare a field that can store an 8-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of type byte.

case

The case keyword is used to create individual cases in a switch statement; see switch.

catch

Defines an exception handler—a group of statements that are executed if an exception is thrown in the block defined by a preceding try keyword. The code is executed only if the class of the thrown exception is assignment compatible with the exception class declared by the catch clause.

char

The char keyword is used to declare a field that can store a 16-bit Unicode character. This keyword is also used to declare that a method returns a value of type char.

class

A type that defines the implementation of a particular kind of object. A class definition defines instance and class fields, methods, and inner classes as well as specifying the interfaces the class implements and the immediate superclass of the class. If the superclass is not explicitly specified, the superclass is implicitly Object.

const

Unused, cant be used for anything

Although reserved as a keyword in Java, const is not used and has no function. For defining constants in java, see the 'final' reserved word.

continue

Used to resume program execution at the end of the current loop body. If followed by a label, continue resumes execution at the end of the enclosing labeled loop body.

default

The default can optionally be used in a switch statement to label a block of statements to be executed if no case matches the specified value; see switch.

do

The do keyword is used in conjunction with while to create a do-while loop, which executes a block of statements associated with the loop and then tests a boolean expression associated with the while. If the expression evaluates to true, the block is executed again; this continues until the expression evaluates to false.

double

The double keyword is used to declare a field that can hold a 64-bit double precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of type double.

else

The else keyword is used in conjunction with if to create an if-else statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if are evaluated; if it evaluates to false, the block of statements associated with the else are evaluated.

enum

A Java keyword used to declare an enumerated type. Enumerations extend the base class Enum.

extends

Used in a class declaration to specify the superclass; used in an interface declaration to specify one or more superinterfaces. Class X extends class Y to add functionality, either by adding fields or methods to class Y, or by overriding methods of class Y. An interface Z extends one or more interfaces by adding methods. Class X is said to be a subclass of class Y; Interface Z is said to be a subinterface of the interfaces it extends.
Also used to specify an upper bound on a type parameter in Generics.

final

Define an entity once that cannot be changed nor derived from later. More specifically: a final class cannot be subclassed, a final method cannot be overridden, and a final variable can occur at most once as a left-hand expression. All methods in a final class are implicitly final.

finally

Used to define a block of statements for a block defined previously by the try keyword. The finally block is executed after execution exits the try block and any associated catch clauses regardless of whether an exception was thrown or caught, or execution left method in the middle of the try or catch blocks using the return keyword.

float

The float keyword is used to declare a field that can hold a 32-bit single precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of type float.

for

The for keyword is used to create a for loop, which specifies a variable initialization, a boolean expression, and an incrementation. The variable initialization is performed first, and then the boolean expression is evaluated. If the expression evaluates to true, the block of statements associated with the loop are executed, and then the incrementation is performed. The boolean expression is then evaluated again; this continues until the expression evaluates to false.
As of J2SE 5.0, the for keyword can also be used to create a so-called "enhanced for loop"[17], which specifies an array or Iterable object; each iteration of the loop executes the associated block of statements using a different element in the array or Iterable.

goto

Unused, cant be used for anything

Although reserved as a keyword in Java, goto is not used and has no function

if

The if keyword is used to create an if statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if statement is executed. This keyword can also be used to create an if-else statement; see else

implements

Included in a class declaration to specify one or more interfaces that are implemented by the current class. A class inherits the types and abstract methods declared by the interfaces.

import

Used at the beginning of a source file to specify classes or entire Java packages to be referred to later without including their package names in the reference. Since J2SE 5.0, import statements can import static members of a class.

instanceof

A binary operator that takes an object reference as its first operand and a class or interface as its second operand and produces a boolean result. The instanceof operator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.

int

The int keyword is used to declare a field that can hold a 32-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of type int.

interface

Used to declare a special type of class that only contains abstract methods, constant (static final) fields and static interfaces. It can later be implemented by classes that declare the interface with the implements keyword.

long

The long keyword is used to declare a field that can hold a 64-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of type long.

native

Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.

new

Used to create an instance of a class or array/an object.

package

A group of types. Packages are declared with the package keyword.

private

The private keyword is used in the declaration of a method, field, or inner class; private members can only be accessed by other members of their own class.

protected

The protected keyword is used in the declaration of a method, field, or inner class; protected members can only be accessed by members of their own class, that class's subclasses or classes from the same package.

public

The public keyword is used in the declaration of a class, method, or field; public classes, methods, and fields can be accessed by the members of any class.

return

Used to finish the execution of a method. It can be followed by a value required by the method definition that is returned to the caller.

short

The short keyword is used to declare a field that can hold a 16-bit signed two's complement integer[7][8]. This keyword is also used to declare that a method returns a value of type short.

static

Used to declare a field, method, or inner class as a class field. Classes maintain one copy of class fields regardless of how many instances exist of that class. static also is used to define a method as a class method. Class methods are bound to the class instead of to a specific instance, and can only operate on class fields. (Classes and interfaces declared as static members of another class or interface are actually top-level classes and are not inner classes.)

strictfp

A Java keyword used to restrict the precision and rounding of floating point calculations to ensure portability.

super

Used to access members of a class inherited by the class in which it appears. Allows a subclass to access overridden methods and hidden members of its superclass. The super keyword is also used to forward a call from a constructor to a constructor in the superclass.
Also used to specify a lower bound on a type parameter in Generics.

switch

The switch keyword is used in conjunction with case and default to create a switch statement, which evaluates a variable, matches its value to a specific case, and executes the block of statements associated with that case. If no case matches the value, the optional block labelled by default is executed, if included.

synchronized

Used in the declaration of a method or code block to acquire the mutex lock for an object while the current thread executes the code. For static methods, the object locked is the class' Class. Guarantees that at most one thread at a time operating on the same object executes that code. The mutex lock is automatically released when execution exits the synchronized code. Fields, classes and interfaces cannot be declared as synchronized.

this

Used to represent an instance of the class in which it appears. this can be used to access class members and as a reference to the current instance. The this keyword is also used to forward a call from one constructor in a class to another constructor in the same class.

throw

Causes the declared exception instance to be thrown. This causes execution to continue with the first enclosing exception handler declared by the catch keyword to handle an assignment compatible exception type. If no such exception handler is found in the current method, then the method returns and the process is repeated in the calling method. If no exception handler is found in any method call on the stack, then the exception is passed to the thread's uncaught exception handler.

throws

Used in method declarations to specify which exceptions are not handled within the method but rather passed to the next higher level of the program. All uncaught exceptions in a method that are not instances of RuntimeException must be declared using the throws keyword.

transient

Declares that an instance field is not part of the default serialized form of an object. When an object is serialized, only the values of its non-transient instance fields are included in the default serial representation. When an object is deserialized, transient fields are initialized only to their default value. If the default form is not used, e.g. when a serialPersistentFields table is declared in the class hierarchy, all transient keywords are ignored.

try

Defines a block of statements that have exception handling. If an exception is thrown inside the try block, an optional catch block can handle declared exception types. Also, an optional finally block can be declared that will be executed when execution exits the try block and catch clauses, regardless of whether an exception is thrown or not. A try block must have at least one catch clause or a finally block.

void

The void keyword is used to declare that a method does not return any value.

volatile

Used in field declarations to specify that the variable is modified asynchronously by concurrently running threads. Methods, classes and interfaces thus cannot be declared volatile.

while

The while keyword is used to create a while loop, which tests a boolean expression and executes the block of statements associated with the loop if the expression evaluates to true; this continues until the expression evaluates to false. This keyword can also be used to create a do-while loop; see do.

Reserved words for literal values

false

A boolean literal value, false.

true

A boolean literal value, true.

null

A reference literal value, null is nothing.

Getting started

When you start Java, I would suggest having previous knowledge in a similar language, by that way you will have easier learning.

I had easy time learning Java since I knew basic C#.

You need

• Java installed
• JDK (Java Development Kit) installed
• An IDE (Eclipse)

To install this, a simple google search can guide you there.

Java: - http://www.java.com/getjava/
JDK (Java Development Kit) - http://www.oracle.com/technetwork/java/j...index.html
Eclipse: - http://www.eclipse.org/

Now, you install all of these, of course.

Writing first application

Here you are, if you just started.



You create a new Java Project.



You open the project in the package explorer, open it, then right click the "src" folder and click "new" then "class".

You create a class like this.



public static void main(String[]) is the main entry point in a java program.

Lets write a hello world program.



You do like that.

Now we press this button: 

The program should run, as we told it to, and print out "Hello hackforums!" in the console.



You just made your first hello world program!

Operators

Commonly used: (by me)

Simple Assignment Operator

= - Simple assignment operator - Used to assign things

Arithmetic Operators

+ - Additive operator (also used for String concatenation)
- - Subtraction operator
* - Multiplication operator
/ - Division operator
% - Remainder operator

Unary Operators

+ - Unary plus operator; indicates positive value (numbers are positive without this, however)
- - Unary minus operator; negates an expression
++ - Increment operator; increments a value by 1
-- - Decrement operator; decrements a value by 1
! - Logical complement operator; inverts the value of a boolean

Equality and Relational Operators

== - Equal to
!= - Not equal to
> - Greater than
>= - Greater than or equal
< - Less than
<= - Less than or equal

Conditional operators

&& - and
|| - or
?: - Ternary (shorthand for if-then-else statement)

Type Comparison Operator

instanceof - Compares an object to a specified type 

Rarely used: (by me)

Bitwise and Bit Shift Operators

~ - Unary bitwise complement
<< - Signed left shift
>> - Signed right shift
>>> - Unsigned right shift
& - Bitwise and
^ - Bitwise exclusive or
| - Bitwise inclusive or

Android

Android is a mobile operating system made for mobile devices, most commonly smartphones and table computers.
It is currently owned and developed by google.
The android system is open source and is linux based.

You can develop and run java on android systems.