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---
language: java
contributors:
- ["Jake Prather", "http://github.com/JakeHP"]
- ["Jakukyo Friel", "http://weakish.github.io"]
- ["Madison Dickson", "http://github.com/mix3d"]
- ["Simon Morgan", "http://sjm.io/"]
filename: LearnJava.java
---
Java is a general-purpose, concurrent, class-based, object-oriented computer
programming language.
[Read more here.](http://docs.oracle.com/javase/tutorial/java/)
```java
// Single-line comments start with //
/*
Multi-line comments look like this.
*/
/**
JavaDoc comments look like this. Used to describe the Class or various
attributes of a Class.
*/
// Import ArrayList class inside of the java.util package
import java.util.ArrayList;
// Import all classes inside of java.security package
import java.security.*;
// Each .java file contains one outer-level public class, with the same name as
// the file.
public class LearnJava {
// A program must have a main method as an entry point.
public static void main (String[] args) {
// Use System.out.println() to print lines.
System.out.println("Hello World!");
System.out.println(
"Integer: " + 10 +
" Double: " + 3.14 +
" Boolean: " + true);
// To print without a newline, use System.out.print().
System.out.print("Hello ");
System.out.print("World");
///////////////////////////////////////
// Variables
///////////////////////////////////////
/*
* Variable Declaration
*/
// Declare a variable using <type> <name>
int fooInt;
// Declare multiple variables of the same type <type> <name1>, <name2>, <name3>
int fooInt1, fooInt2, fooInt3;
/*
* Variable Initialization
*/
// Initialize a variable using <type> <name> = <val>
int fooInt = 1;
// Initialize multiple variables of same type with same value <type> <name1>, <name2>, <name3> = <val>
int fooInt1, fooInt2, fooInt3;
fooInt1 = fooInt2 = fooInt3 = 1;
/*
* Variable types
*/
// Byte - 8-bit signed two's complement integer
// (-128 <= byte <= 127)
byte fooByte = 100;
// Short - 16-bit signed two's complement integer
// (-32,768 <= short <= 32,767)
short fooShort = 10000;
// Integer - 32-bit signed two's complement integer
// (-2,147,483,648 <= int <= 2,147,483,647)
int fooInt = 1;
// Long - 64-bit signed two's complement integer
// (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
long fooLong = 100000L;
// L is used to denote that this variable value is of type Long;
// anything without is treated as integer by default.
// Note: Java has no unsigned types.
// Float - Single-precision 32-bit IEEE 754 Floating Point
float fooFloat = 234.5f;
// f is used to denote that this variable value is of type float;
// otherwise it is treated as double.
// Double - Double-precision 64-bit IEEE 754 Floating Point
double fooDouble = 123.4;
// Boolean - true & false
boolean fooBoolean = true;
boolean barBoolean = false;
// Char - A single 16-bit Unicode character
char fooChar = 'A';
// final variables can't be reassigned to another object.
final int HOURS_I_WORK_PER_WEEK = 9001;
// Strings
String fooString = "My String Is Here!";
// \n is an escaped character that starts a new line
String barString = "Printing on a new line?\nNo Problem!";
// \t is an escaped character that adds a tab character
String bazString = "Do you want to add a tab?\tNo Problem!";
System.out.println(fooString);
System.out.println(barString);
System.out.println(bazString);
// Arrays
// The array size must be decided upon instantiation
// The following formats work for declaring an array
// <datatype>[] <var name> = new <datatype>[<array size>];
// <datatype> <var name>[] = new <datatype>[<array size>];
int[] intArray = new int[10];
String[] stringArray = new String[1];
boolean boolArray[] = new boolean[100];
// Another way to declare & initialize an array
int[] y = {9000, 1000, 1337};
String names[] = {"Bob", "John", "Fred", "Juan Pedro"};
boolean bools[] = new boolean[] {true, false, false};
// Indexing an array - Accessing an element
System.out.println("intArray @ 0: " + intArray[0]);
// Arrays are zero-indexed and mutable.
intArray[1] = 1;
System.out.println("intArray @ 1: " + intArray[1]); // => 1
// Others to check out
// ArrayLists - Like arrays except more functionality is offered, and
// the size is mutable.
// LinkedLists - Implementation of doubly-linked list. All of the
// operations perform as could be expected for a
// doubly-linked list.
// Maps - A set of objects that maps keys to values. A map cannot
// contain duplicate keys; each key can map to at most one value.
// HashMaps - This class uses a hashtable to implement the Map
// interface. This allows the execution time of basic
// operations, such as get and insert element, to remain
// constant even for large sets.
///////////////////////////////////////
// Operators
///////////////////////////////////////
System.out.println("\n->Operators");
int i1 = 1, i2 = 2; // Shorthand for multiple declarations
// Arithmetic is straightforward
System.out.println("1+2 = " + (i1 + i2)); // => 3
System.out.println("2-1 = " + (i2 - i1)); // => 1
System.out.println("2*1 = " + (i2 * i1)); // => 2
System.out.println("1/2 = " + (i1 / i2)); // => 0 (0.5 truncated down)
// Modulo
System.out.println("11%3 = "+(11 % 3)); // => 2
// Comparison operators
System.out.println("3 == 2? " + (3 == 2)); // => false
System.out.println("3 != 2? " + (3 != 2)); // => true
System.out.println("3 > 2? " + (3 > 2)); // => true
System.out.println("3 < 2? " + (3 < 2)); // => false
System.out.println("2 <= 2? " + (2 <= 2)); // => true
System.out.println("2 >= 2? " + (2 >= 2)); // => true
// Bitwise operators!
/*
~ Unary bitwise complement
<< Signed left shift
>> Signed right shift
>>> Unsigned right shift
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise inclusive OR
*/
// Incrementations
int i = 0;
System.out.println("\n->Inc/Dec-rementation");
// The ++ and -- operators increment and decrement by 1 respectively.
// If they are placed before the variable, they increment then return;
// after the variable they return then increment.
System.out.println(i++); // i = 1, prints 0 (post-increment)
System.out.println(++i); // i = 2, prints 2 (pre-increment)
System.out.println(i--); // i = 1, prints 2 (post-decrement)
System.out.println(--i); // i = 0, prints 0 (pre-decrement)
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
System.out.println("\n->Control Structures");
// If statements are c-like
int j = 10;
if (j == 10){
System.out.println("I get printed");
} else if (j > 10) {
System.out.println("I don't");
} else {
System.out.println("I also don't");
}
// While loop
int fooWhile = 0;
while(fooWhile < 100) {
System.out.println(fooWhile);
// Increment the counter
// Iterated 100 times, fooWhile 0,1,2...99
fooWhile++;
}
System.out.println("fooWhile Value: " + fooWhile);
// Do While Loop
int fooDoWhile = 0;
do {
System.out.println(fooDoWhile);
// Increment the counter
// Iterated 99 times, fooDoWhile 0->99
fooDoWhile++;
} while(fooDoWhile < 100);
System.out.println("fooDoWhile Value: " + fooDoWhile);
// For Loop
int fooFor;
// for loop structure => for(<start_statement>; <conditional>; <step>)
for (fooFor = 0; fooFor < 10; fooFor++) {
System.out.println(fooFor);
// Iterated 10 times, fooFor 0->9
}
System.out.println("fooFor Value: " + fooFor);
// For Each Loop
// The for loop is also able to iterate over arrays as well as objects
// that implement the Iterable interface.
int[] fooList = {1, 2, 3, 4, 5, 6, 7, 8, 9};
// for each loop structure => for (<object> : <iterable>)
// reads as: for each element in the iterable
// note: the object type must match the element type of the iterable.
for (int bar : fooList) {
System.out.println(bar);
//Iterates 9 times and prints 1-9 on new lines
}
// Switch Case
// A switch works with the byte, short, char, and int data types.
// It also works with enumerated types (discussed in Enum Types), the
// String class, and a few special classes that wrap primitive types:
// Character, Byte, Short, and Integer.
int month = 3;
String monthString;
switch (month) {
case 1: monthString = "January";
break;
case 2: monthString = "February";
break;
case 3: monthString = "March";
break;
default: monthString = "Some other month";
break;
}
System.out.println("Switch Case Result: " + monthString);
// Conditional Shorthand
// You can use the '?' operator for quick assignments or logic forks.
// Reads as "If (statement) is true, use <first value>, otherwise, use
// <second value>"
int foo = 5;
String bar = (foo < 10) ? "A" : "B";
System.out.println(bar); // Prints A, because the statement is true
////////////////////////////////////////
// Converting Data Types And Typecasting
////////////////////////////////////////
// Converting data
// Convert String To Integer
Integer.parseInt("123");//returns an integer version of "123"
// Convert Integer To String
Integer.toString(123);//returns a string version of 123
// For other conversions check out the following classes:
// Double
// Long
// String
// Typecasting
// You can also cast Java objects, there's a lot of details and deals
// with some more intermediate concepts. Feel free to check it out here:
// http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
///////////////////////////////////////
// Classes And Functions
///////////////////////////////////////
System.out.println("\n->Classes & Functions");
// (definition of the Bicycle class follows)
// Use new to instantiate a class
Bicycle trek = new Bicycle();
// Call object methods
trek.speedUp(3); // You should always use setter and getter methods
trek.setCadence(100);
// toString returns this Object's string representation.
System.out.println("trek info: " + trek.toString());
// Double Brace Initialization
// The Java Language has no syntax for how to create static Collections
// in an easy way. Usually you end up in the following way:
private static final Set<String> COUNTRIES = new HashSet<String>();
static {
validCodes.add("DENMARK");
validCodes.add("SWEDEN");
validCodes.add("FINLAND");
}
// But there's a nifty way to achive the same thing in an
// easier way, by using something that is called Double Brace
// Initialization.
private static final Set<String> COUNTRIES = HashSet<String>() {{
add("DENMARK");
add("SWEDEN");
add("FINLAND");
}}
// The first brace is creating an new AnonymousInnerClass and the
// second one declares and instance initializer block. This block
// is called with the anonymous inner class is created.
// This does not only work for Collections, it works for all
// non-final classes.
} // End main method
} // End LearnJava class
// You can include other, non-public outer-level classes in a .java file
// Class Declaration Syntax:
// <public/private/protected> class <class name> {
// // data fields, constructors, functions all inside.
// // functions are called as methods in Java.
// }
class Bicycle {
// Bicycle's Fields/Variables
public int cadence; // Public: Can be accessed from anywhere
private int speed; // Private: Only accessible from within the class
protected int gear; // Protected: Accessible from the class and subclasses
String name; // default: Only accessible from within this package
// Constructors are a way of creating classes
// This is a constructor
public Bicycle() {
gear = 1;
cadence = 50;
speed = 5;
name = "Bontrager";
}
// This is a constructor that takes arguments
public Bicycle(int startCadence, int startSpeed, int startGear,
String name) {
this.gear = startGear;
this.cadence = startCadence;
this.speed = startSpeed;
this.name = name;
}
// Function Syntax:
// <public/private/protected> <return type> <function name>(<args>)
// Java classes often implement getters and setters for their fields
// Method declaration syntax:
// <scope> <return type> <method name>(<args>)
public int getCadence() {
return cadence;
}
// void methods require no return statement
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void speedUp(int increment) {
speed += increment;
}
public void slowDown(int decrement) {
speed -= decrement;
}
public void setName(String newName) {
name = newName;
}
public String getName() {
return name;
}
//Method to display the attribute values of this Object.
@Override
public String toString() {
return "gear: " + gear + " cadence: " + cadence + " speed: " + speed +
" name: " + name;
}
} // end class Bicycle
// PennyFarthing is a subclass of Bicycle
class PennyFarthing extends Bicycle {
// (Penny Farthings are those bicycles with the big front wheel.
// They have no gears.)
public PennyFarthing(int startCadence, int startSpeed){
// Call the parent constructor with super
super(startCadence, startSpeed, 0, "PennyFarthing");
}
// You should mark a method you're overriding with an @annotation.
// To learn more about what annotations are and their purpose check this
// out: http://docs.oracle.com/javase/tutorial/java/annotations/
@Override
public void setGear(int gear) {
gear = 0;
}
}
// Interfaces
// Interface declaration syntax
// <access-level> interface <interface-name> extends <super-interfaces> {
// // Constants
// // Method declarations
// }
// Example - Food:
public interface Edible {
public void eat(); // Any class that implements this interface, must
// implement this method.
}
public interface Digestible {
public void digest();
}
// We can now create a class that implements both of these interfaces.
public class Fruit implements Edible, Digestible {
@Override
public void eat() {
// ...
}
@Override
public void digest() {
// ...
}
}
// In Java, you can extend only one class, but you can implement many
// interfaces. For example:
public class ExampleClass extends ExampleClassParent implements InterfaceOne,
InterfaceTwo {
@Override
public void InterfaceOneMethod() {
}
@Override
public void InterfaceTwoMethod() {
}
}
// Abstract Classes
// Abstract Class declaration syntax
// <access-level> abstract <abstract-class-name> extends <super-abstract-classes> {
// // Constants and variables
// // Method declarations
// }
// Methods can't have bodies in an interface, unless the method is
// static. Also variables are NOT final by default, unlike an interface.
// Also abstract classes CAN have the "main" method.
// Abstract classes solve these problems.
public abstract class Animal
{
public abstract void makeSound();
// Method can have a body
public void eat()
{
System.out.println("I am an animal and I am Eating.");
// Note: We can access private variable here.
age = 30;
}
// No need to initialize, however in an interface
// a variable is implicitly final and hence has
// to be initialized.
private int age;
public void printAge()
{
System.out.println(age);
}
// Abstract classes can have main function.
public static void main(String[] args)
{
System.out.println("I am abstract");
}
}
class Dog extends Animal
{
// Note still have to override the abstract methods in the
// abstract class.
@Override
public void makeSound()
{
System.out.println("Bark");
// age = 30; ==> ERROR! age is private to Animal
}
// NOTE: You will get an error if you used the
// @Override annotation here, since java doesn't allow
// overriding of static methods.
// What is happening here is called METHOD HIDING.
// Check out this awesome SO post: http://stackoverflow.com/questions/16313649/
public static void main(String[] args)
{
Dog pluto = new Dog();
pluto.makeSound();
pluto.eat();
pluto.printAge();
}
}
```
## Further Reading
The links provided here below are just to get an understanding of the topic, feel free to Google and find specific examples.
**Official Oracle Guides**:
* [Java Tutorial Trail from Sun / Oracle](http://docs.oracle.com/javase/tutorial/index.html)
* [Java Access level modifiers](http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html)
* [Object-Oriented Programming Concepts](http://docs.oracle.com/javase/tutorial/java/concepts/index.html):
* [Inheritance](http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html)
* [Polymorphism](http://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html)
* [Abstraction](http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html)
* [Exceptions](http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html)
* [Interfaces](http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html)
* [Generics](http://docs.oracle.com/javase/tutorial/java/generics/index.html)
* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconv-138413.html)
**Online Practice and Tutorials**
* [Learneroo.com - Learn Java](http://www.learneroo.com)
* [Codingbat.com](http://codingbat.com/java)
**Books**:
* [Head First Java](http://www.headfirstlabs.com/books/hfjava/)
* [Thinking in Java](http://www.mindview.net/Books/TIJ/)
* [Objects First with Java](http://www.amazon.com/Objects-First-Java-Practical-Introduction/dp/0132492660)
* [Java The Complete Reference](http://www.amazon.com/gp/product/0071606300)
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