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Learn X in Y minutes

Where X=java

Java is a general-purpose, concurrent, class-based, object-oriented computer programming language. Read more here.

// 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 BigInteger(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 precision 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);

        // Be wary of the constructor that takes a float or double as
        // the inaccuracy of the float/double will be copied in BigDecimal.
        // Prefer the String constructor when you need an exact value.
        BigDecimal tenCents = new BigDecimal("0.1");

        // 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[] = {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

        // Other data types worth checking 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 map keys to values. Map is
        //        an interface and therefore cannot be instantiated.
        //        The type of keys and values contained in a Map must
        //        be specified upon instantiation of the implementing
        //        class. Each key may map to only one corresponding value,
        //        and each key may appear only once (no duplicates).
        // 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 (int/int returns int)
        System.out.println("1/2 = " + (i1 / (double)i2)); // => 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
        */

        // Increment operators
        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);

        // Nested For Loop Exit with Label
        outer:
        for (int i = 0; i < 10; i++) {
          for (int j = 0; j < 10; j++) {
            if (i == 5 && j ==5) {
              break outer;
              // breaks out of outer loop instead of only the inner one
            }
          }
        }

        // 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 {
           COUNTRIES.add("DENMARK");
           COUNTRIES.add("SWEDEN");
           COUNTRIES.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 = new 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 not 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
    static String className; // Static class variable

    // Static block 
    // Java has no implementation of static constructors, but
    // has a static block that can be used to initialize class variables 
    // (static variables). 
    // This block will be called when the class is loaded.
    static {
        className = "Bicycle";
    }

    // 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
// concrete and abstract methods. Methods in an interface cannot have a body,
// mixture of 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 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;
    }
}

// Enum Type
//
// An enum type is a special data type that enables for a variable to be a set
// of predefined constants. The variable must be equal to one of the values
// that have been predefined for it. Because they are constants, the names of
// an enum type's fields are in uppercase letters. In the Java programming
// language, you define an enum type by using the enum keyword. For example,
// you would specify a days-of-the-week enum type as:
public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
}

// We can use our enum Day like that:
public class EnumTest {
    // Variable Enum
    Day day;

    public EnumTest(Day day) {
        this.day = day;
    }

    public void tellItLikeItIs() {
        switch (day) {
            case MONDAY:
                System.out.println("Mondays are bad.");
                break;
            case FRIDAY:
                System.out.println("Fridays are better.");
                break;   
            case SATURDAY: 
            case SUNDAY:
                System.out.println("Weekends are best.");
                break;     
            default:
                System.out.println("Midweek days are so-so.");
                break;
        }
    }

    public static void main(String[] args) {
        EnumTest firstDay = new EnumTest(Day.MONDAY);
        firstDay.tellItLikeItIs(); // => Mondays are bad.
        EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
        thirdDay.tellItLikeItIs(); // => Midweek days are so-so.
    }
}

// Enum types are much more powerful than we show above. 
// The enum body can include methods and other fields.
// You can see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html

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:

Online Practice and Tutorials

Books:


Got a suggestion? A correction, perhaps? Open an Issue on the Github Repo, or make a pull request yourself!

Originally contributed by Jake Prather, and updated by 37 contributor(s).