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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)