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---
language: java
contributors:
- ["Jake Prather", "https://github.com/JakeHP"]
- ["Jakukyo Friel", "https://weakish.github.io"]
- ["Madison Dickson", "https://github.com/mix3d"]
- ["Simon Morgan", "https://sjm.io/"]
- ["Zachary Ferguson", "https://github.com/zfergus2"]
- ["Cameron Schermerhorn", "https://github.com/cschermerhorn"]
- ["Rachel Stiyer", "https://github.com/rstiyer"]
- ["Michael Dähnert", "https://github.com/JaXt0r"]
- ["Rob Rose", "https://github.com/RobRoseKnows"]
- ["Sean Nam", "https://github.com/seannam"]
- ["Shawn M. Hanes", "https://github.com/smhanes15"]
filename: LearnJava.java
---
Java is a general-purpose, concurrent, class-based, object-oriented computer
programming language.
[Read more here.](https://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.
* Main attributes:
*
* @author Name (and contact information such as email) of author(s).
* @version Current version of the program.
* @since When this part of the program was first added.
* @param For describing the different parameters for a method.
* @return For describing what the method returns.
* @deprecated For showing the code is outdated or shouldn't be used.
* @see Links to another part of documentation.
*/
// Import ArrayList class inside of the java.util package
import java.util.ArrayList;
// Import all classes inside of java.security package
import java.security.*;
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) {
///////////////////////////////////////
// Input/Output
///////////////////////////////////////
/*
* Output
*/
// 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
/*
* Input
*/
// use Scanner to read input
// must import java.util.Scanner;
Scanner scanner = new Scanner(System.in);
// read string input
String name = scanner.next();
// read byte input
byte numByte = scanner.nextByte();
// read int input
int numInt = scanner.nextInt();
// read long input
long numLong = scanner.nextLong();
// read float input
float numFloat = scanner.nextFloat();
// read double input
double numDouble = scanner.nextDouble();
// read boolean input
boolean bool = scanner.nextBoolean();
///////////////////////////////////////
// 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 barInt = 1;
// Initialize multiple variables of same type with same
// value <type> <name1>, <name2>, <name3>
// <name1> = <name2> = <name3> = <val>
int barInt1, barInt2, barInt3;
barInt1 = barInt2 = barInt3 = 1;
// Shorthand for multiple declarations
int barInt4 = 1, barInt5 = 2;
/*
* Variable types
*/
// Byte - 8-bit signed two's complement integer
// (-128 <= byte <= 127)
byte fooByte = 100;
// If you would like to interpret a byte as an unsigned integer
// then this simple operation can help
int unsignedIntLessThan256 = 0xff & fooByte;
// this contrasts a cast which can be negative.
int signedInt = (int) fooByte;
// 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 bazInt = 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: byte, short, int and long are signed. They can have positive and negative values.
// There are no unsigned variants.
// char, however, is 16-bit unsigned.
// 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,
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
// 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);
// String Building
// #1 - with plus operator
// That's the basic way to do it (optimized under the hood)
String plusConcatenated = "Strings can " + "be concatenated " + "via + operator.";
System.out.println(plusConcatenated);
// Output: Strings can be concatenated via + operator.
// #2 - with StringBuilder
// This way doesn't create any intermediate strings. It just stores the string pieces, and ties them together
// when toString() is called.
// Hint: This class is not thread safe. A thread-safe alternative (with some impact on performance) is StringBuffer.
StringBuilder builderConcatenated = new StringBuilder();
builderConcatenated.append("You ");
builderConcatenated.append("can use ");
builderConcatenated.append("the StringBuilder class.");
System.out.println(builderConcatenated.toString()); // only now is the string built
// Output: You can use the StringBuilder class.
// StringBuilder is efficient when the fully constructed String is not required until the end of some processing.
StringBuilder stringBuilder = new StringBuilder();
String inefficientString = "";
for (int i = 0 ; i < 10; i++) {
stringBuilder.append(i).append(" ");
inefficientString += i + " ";
}
System.out.println(inefficientString);
System.out.println(stringBuilder.toString());
// inefficientString requires a lot more work to produce, as it generates a String on every loop iteration.
// Simple concatenation with + is compiled to a StringBuilder and toString()
// Avoid string concatenation in loops.
// #3 - with String formatter
// Another alternative way to create strings. Fast and readable.
String.format("%s may prefer %s.", "Or you", "String.format()");
// Output: Or you may prefer String.format().
// 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 mapping of key Objects to value Objects. 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-amortized even for large sets.
// TreeMap - A Map that is sorted by its keys. Each modification
// maintains the sorting defined by either a Comparator
// supplied at instantiation, or comparisons of each Object
// if they implement the Comparable interface.
// Failure of keys to implement Comparable combined with failure to
// supply a Comparator will throw ClassCastExceptions.
// Insertion and removal operations take O(log(n)) time
// so avoid using this data structure unless you are taking
// advantage of the sorting.
///////////////////////////////////////
// Operators
///////////////////////////////////////
System.out.println("\n->Operators");
int i1 = 1, i2 = 2;
// 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("1/2.0 = " + (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 100 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.
// Starting in Java 7 and above, we can also use the String type.
// Note: Do remember that, not adding "break" at end any particular case ends up in
// executing the very next case(given it satisfies the condition provided) as well.
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);
// Try-with-resources (Java 7+)
// Try-catch-finally statements work as expected in Java but in Java 7+
// the try-with-resources statement is also available. Try-with-resources
// simplifies try-catch-finally statements by closing resources
// automatically.
// In order to use a try-with-resources, include an instance of a class
// in the try statement. The class must implement java.lang.AutoCloseable.
try (BufferedReader br = new BufferedReader(new FileReader("foo.txt"))) {
// You can attempt to do something that could throw an exception.
System.out.println(br.readLine());
// In Java 7, the resource will always be closed, even if it throws
// an Exception.
} catch (Exception ex) {
//The resource will be closed before the catch statement executes.
System.out.println("readLine() failed.");
}
// No need for a finally statement in this case, the BufferedReader is
// already closed. This can be used to avoid certain edge cases where
// a finally statement might not be called.
// To learn more:
// https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
// 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 : " + bar); // Prints "bar : A", because the
// statement is true.
// Or simply
System.out.println("bar : " + (foo < 10 ? "A" : "B"));
////////////////////////////////////////
// Converting Data Types
////////////////////////////////////////
// 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
///////////////////////////////////////
// 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());
} // End main method
private static class TestInitialization {
// Double Brace Initialization
// Before Java 11, the Java Language had no syntax for how to create
// static Collections in an easy way. Usually you end up like this:
private static final Set<String> COUNTRIES = new HashSet<String>();
static {
COUNTRIES.add("DENMARK");
COUNTRIES.add("SWEDEN");
COUNTRIES.add("FINLAND");
}
// There's a nifty way to achieve the same thing,
// by using something that is called Double Brace Initialization.
private static final Set<String> COUNTRIES_DOUBLE_BRACE =
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.
// Another option was to initialize the Collection from an array,
// using Arrays.asList() method:
private static final List<String> COUNTRIES_AS_LIST =
Arrays.asList("SWEDEN", "DENMARK", "NORWAY");
// This has one catch: the list we get is internally backed by the array,
// and since arrays can't change their size, the list backed by the array
// is not resizeable, which means we can't add new elements to it:
public static void main(String[] args) {
COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException!
// However, we can replace elements by index, just like in array:
COUNTRIES.set(1, "FINLAND");
System.out.println(COUNTRIES); // prints [SWEDEN, FINLAND, NORWAY]
}
// The resizing problem can be circumvented
// by creating another Collection from the List:
private static final Set<String> COUNTRIES_SET =
new HashSet<>(Arrays.asList("SWEDEN", "DENMARK", "NORWAY"));
// It's perfectly fine to add anything to the Set of COUNTRIES now.
} // End TestInitialization class
private static class TestJava11Initialization {
// Since Java 11, there is a convenient option to initialize Collections:
// Set.of() and List.of() methods.
private static final Set<String> COUNTRIES =
Set.of("SWEDEN", "DENMARK", "NORWAY");
// There is a massive catch, though: Lists and Sets initialized like this
// 1) are immutable
// 2) can't contain null elements (even check for null elements fails)!
public static void main(String[] args) {
COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException
COUNTRIES.remove("NORWAY"); // throws UnsupportedOperationException
COUNTRIES.contains(null); // throws NullPointerException
}
private static final Set<String> COUNTRIES_WITH_NULL =
Set.of("SWEDEN", null, "NORWAY"); // throws NullPointerException
} // End TestJava11Initialization class
} // 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) {
this.gear = 0;
}
}
// Object casting
// Since the PennyFarthing class is extending the Bicycle class, we can say
// a PennyFarthing is a Bicycle and write :
// Bicycle bicycle = new PennyFarthing();
// This is called object casting where an object is taken for another one. There
// are lots of details and deals with some more intermediate concepts here:
// https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
// 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();
// Since Java 8, interfaces can have default method.
public default void defaultMethod() {
System.out.println("Hi from default method ...");
}
}
// 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 class <abstract-class-name> extends
// <super-abstract-classes> {
// // Constants and variables
// // Method declarations
// }
// Abstract Classes cannot be instantiated.
// Abstract classes may define abstract methods.
// Abstract methods have no body and are marked abstract
// Non-abstract child classes must @Override all abstract methods
// from their super-classes.
// Abstract classes can be useful when combining repetitive logic
// with customised behavior, but as Abstract classes require
// inheritance, they violate "Composition over inheritance"
// so consider other approaches using composition.
// https://en.wikipedia.org/wiki/Composition_over_inheritance
public abstract class Animal
{
private int age;
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;
}
public void printAge()
{
System.out.println(age);
}
// Abstract classes can have main method.
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
// Getting Started with Lambda Expressions
//
// New to Java version 8 are lambda expressions. Lambdas are more commonly found
// in functional programming languages, which means they are methods which can
// be created without belonging to a class, passed around as if it were itself
// an object, and executed on demand.
//
// Final note, lambdas must implement a functional interface. A functional
// interface is one which has only a single abstract method declared. It can
// have any number of default methods. Lambda expressions can be used as an
// instance of that functional interface. Any interface meeting the requirements
// is treated as a functional interface. You can read more about interfaces
// above.
//
import java.util.Map;
import java.util.HashMap;
import java.util.function.*;
import java.security.SecureRandom;
public class Lambdas {
public static void main(String[] args) {
// Lambda declaration syntax:
// <zero or more parameters> -> <expression body or statement block>
// We will use this hashmap in our examples below.
Map<String, String> planets = new HashMap<>();
planets.put("Mercury", "87.969");
planets.put("Venus", "224.7");
planets.put("Earth", "365.2564");
planets.put("Mars", "687");
planets.put("Jupiter", "4,332.59");
planets.put("Saturn", "10,759");
planets.put("Uranus", "30,688.5");
planets.put("Neptune", "60,182");
// Lambda with zero parameters using the Supplier functional interface
// from java.util.function.Supplier. The actual lambda expression is
// what comes after numPlanets =.
Supplier<String> numPlanets = () -> Integer.toString(planets.size());
System.out.format("Number of Planets: %s\n\n", numPlanets.get());
// Lambda with one parameter and using the Consumer functional interface
// from java.util.function.Consumer. This is because planets is a Map,
// which implements both Collection and Iterable. The forEach used here,
// found in Iterable, applies the lambda expression to each member of
// the Collection. The default implementation of forEach behaves as if:
/*
for (T t : this)
action.accept(t);
*/
// The actual lambda expression is the parameter passed to forEach.
planets.keySet().forEach((p) -> System.out.format("%s\n", p));
// If you are only passing a single argument, then the above can also be
// written as (note absent parentheses around p):
planets.keySet().forEach(p -> System.out.format("%s\n", p));
// Tracing the above, we see that planets is a HashMap, keySet() returns
// a Set of its keys, forEach applies each element as the lambda
// expression of: (parameter p) -> System.out.format("%s\n", p). Each
// time, the element is said to be "consumed" and the statement(s)
// referred to in the lambda body is applied. Remember the lambda body
// is what comes after the ->.
// The above without use of lambdas would look more traditionally like:
for (String planet : planets.keySet()) {
System.out.format("%s\n", planet);
}
// This example differs from the above in that a different forEach
// implementation is used: the forEach found in the HashMap class
// implementing the Map interface. This forEach accepts a BiConsumer,
// which generically speaking is a fancy way of saying it handles
// the Set of each Key -> Value pairs. This default implementation
// behaves as if:
/*
for (Map.Entry<K, V> entry : map.entrySet())
action.accept(entry.getKey(), entry.getValue());
*/
// The actual lambda expression is the parameter passed to forEach.
String orbits = "%s orbits the Sun in %s Earth days.\n";
planets.forEach((K, V) -> System.out.format(orbits, K, V));
// The above without use of lambdas would look more traditionally like:
for (String planet : planets.keySet()) {
System.out.format(orbits, planet, planets.get(planet));
}
// Or, if following more closely the specification provided by the
// default implementation:
for (Map.Entry<String, String> planet : planets.entrySet()) {
System.out.format(orbits, planet.getKey(), planet.getValue());
}
// These examples cover only the very basic use of lambdas. It might not
// seem like much or even very useful, but remember that a lambda can be
// created as an object that can later be passed as parameters to other
// methods.
}
}
```
## 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](https://docs.oracle.com/javase/tutorial/index.html)
* [Java Access level modifiers](https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html)
* [Object-Oriented Programming Concepts](https://docs.oracle.com/javase/tutorial/java/concepts/index.html):
* [Inheritance](https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html)
* [Polymorphism](https://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html)
* [Abstraction](https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html)
* [Exceptions](https://docs.oracle.com/javase/tutorial/essential/exceptions/index.html)
* [Interfaces](https://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html)
* [Generics](https://docs.oracle.com/javase/tutorial/java/generics/index.html)
* [Java Code Conventions](https://www.oracle.com/technetwork/java/codeconvtoc-136057.html)
* New features in Java 8:
* [Lambda expressions (functional programming)](https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html)
* [Date and time API (java.time package)](http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html)
### Online Practice and Tutorials
* [Learneroo.com - Learn Java](http://www.learneroo.com)
* [Codingbat.com](http://codingbat.com/java)
* [Codewars - Java Katas](https://www.codewars.com/?language=java)
* [University of Helsinki - Object-Oriented programming with Java](http://moocfi.github.io/courses/2013/programming-part-1/)
### Books
* [Head First Java](http://www.headfirstlabs.com/books/hfjava/)
* [Thinking in Java](https://www.amazon.com/Thinking-Java-4th-Bruce-Eckel/dp/0131872486/)
* [Objects First with Java](https://www.amazon.com/Objects-First-Java-Practical-Introduction/dp/0132492660)
* [Java The Complete Reference](https://www.amazon.com/gp/product/0071606300)
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