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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/"]
+ - ["Zachary Ferguson", "http://github.com/zfergus2"]
+ - ["Cameron Schermerhorn", "http://github.com/cschermerhorn"]
+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 {
+
+ // In order to run a java program, it 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");
+
+ // Use System.out.printf() for easy formatted printing.
+ System.out.printf("pi = %.5f", Math.PI); // => pi = 3.14159
+
+ ///////////////////////////////////////
+ // 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
+ // 2^-149 <= float <= (2-2^-23) * 2^127
+ float fooFloat = 234.5f;
+ // f or 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
+ // 2^-1074 <= x <= (2-2^-52) * 2^1023
+ 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;
+ // but they can be initialized later.
+ final double E;
+ E = 2.71828;
+
+
+ // BigInteger - Immutable arbitrary-precision integers
+ //
+ // BigInteger is a data type that allows programmers to manipulate
+ // integers longer than 64-bits. Integers are stored as an array of
+ // of bytes and are manipulated using functions built into BigInteger
+ //
+ // BigInteger can be initialized using an array of bytes or a string.
+
+ BigInteger fooBigInteger = new BigDecimal(fooByteArray);
+
+
+ // BigDecimal - Immutable, arbitrary-precision signed decimal number
+ //
+ // A BigDecimal takes two parts: an arbitrary precision integer
+ // unscaled value and a 32-bit integer scale
+ //
+ // BigDecimal allows the programmer complete control over decimal
+ // rounding. It is recommended to use BigDecimal with currency values
+ // and where exact decimal percision is required.
+ //
+ // BigDecimal can be initialized with an int, long, double or String
+ // or by initializing the unscaled value (BigInteger) and scale (int).
+
+ BigDecimal fooBigDecimal = new BigDecimal(fooBigInteger, fooInt);
+
+
+
+ // 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)
+ System.out.println("1/2 = " + (i1 / (i2*1.0))); // => 0.5
+
+ // 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
+
+ // Boolean operators
+ System.out.println("3 > 2 && 2 > 3? " + ((3 > 2) && (2 > 3))); // => false
+ System.out.println("3 > 2 || 2 > 3? " + ((3 > 2) || (2 > 3))); // => true
+ System.out.println("!(3 == 2)? " + (!(3 == 2))); // => true
+
+ // Bitwise operators!
+ /*
+ ~ Unary bitwise complement
+ << Signed left shift
+ >> Signed/Arithmetic right shift
+ >>> Unsigned/Logical 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
+ // for loop structure => for(<start_statement>; <conditional>; <step>)
+ for (int 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);
+
+ // Starting in Java 7 and above, switching Strings works like this:
+ String myAnswer = "maybe";
+ switch(myAnswer){
+ case "yes":
+ System.out.println("You answered yes.");
+ break;
+ case "no":
+ System.out.println("You answered no.");
+ break;
+ case "maybe":
+ System.out.println("You answered maybe.");
+ break;
+ default:
+ System.out.println("You answered " + myAnswer);
+ break;
+ }
+
+ // 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 achieve 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 a new AnonymousInnerClass and the
+ // second one declares an instance initializer block. This block
+ // is called when 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,
+// but it is good practice. Instead split classes into separate files.
+
+
+// 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() {
+ // You can also call another constructor:
+ // this(1, 50, 5, "Bontrager");
+ 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;
+ }
+
+ // Method Syntax:
+ // <public/private/protected> <return type> <function name>(<args>)
+
+ // Java classes often implement getters and setters for their fields
+
+ // Method declaration syntax:
+ // <access modifier> <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 // Inherited from the Object class.
+ 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
+// }
+
+// Marking a class as abstract means that it contains abstract methods that must
+// be defined in a child class. Similar to interfaces, abstract classes cannot
+// be instantiated, but instead must be extended and the abstract methods
+// defined. Different from interfaces, abstract classes can contain a mixture of
+// concrete and abstract methods. Methods in an interface cannot have a body,
+// unless the method is static, and variables are final by default, unlike an
+// abstract class. Also abstract classes CAN have the "main" method.
+
+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.
+ protected 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();
+ }
+}
+
+// Final Classes
+
+// Final Class declaration syntax
+// <access-level> final <final-class-name> {
+// // Constants and variables
+// // Method declarations
+// }
+
+// Final classes are classes that cannot be inherited from and are therefore a
+// final child. In a way, final classes are the opposite of abstract classes
+// because abstract classes must be extended, but final classes cannot be
+// extended.
+public final class SaberToothedCat extends Animal
+{
+ // Note still have to override the abstract methods in the
+ // abstract class.
+ @Override
+ public void makeSound()
+ {
+ System.out.println("Roar");
+ }
+}
+
+// Final Methods
+public abstract class Mammal()
+{
+ // Final Method Syntax:
+ // <access modifier> final <return type> <function name>(<args>)
+
+ // Final methods, like, final classes cannot be overridden by a child class,
+ // and are therefore the final implementation of the method.
+ public final boolean isWarmBlooded()
+ {
+ return true;
+ }
+}
+```
+
+## 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)