Merge branch 'master' of https://github.com/torgeek/learnxinyminutes-docs

1 files changed, 241 insertions, 99 deletions

diff --git a/java.html.markdown b/java.html.markdown
index 12de0c73..79769352 100644
--- a/java.html.markdown
+++ b/java.html.markdown
@@ -10,6 +10,8 @@ contributors:
     - ["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
 ---
 
@@ -29,13 +31,13 @@ Multi-line comments look like this.
  * 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.
+ * @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  * @deprecated  For showing the code is outdated or shouldn't be used.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.
+ * @see         Links to another part of documentation.
 */
 
 // Import ArrayList class inside of the java.util package
@@ -43,13 +45,19 @@ 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 
+    // In order to run a java program, it must have a main method as an entry
     // point.
-    public static void main (String[] args) {
+    public static void main(String[] args) {
+
+    ///////////////////////////////////////
+    // Input/Output
+    ///////////////////////////////////////
+
+        /*
+        * Output
+        */
 
         // Use System.out.println() to print lines.
         System.out.println("Hello World!");
@@ -65,6 +73,32 @@ public class LearnJava {
         // 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
+        float numFloat = scanner.nextFloat();
+
+        // read double input
+        double numDouble = scanner.nextDouble();
+
+        // read boolean input
+        boolean bool = scanner.nextBoolean();
+
         ///////////////////////////////////////
         // Variables
         ///////////////////////////////////////
@@ -74,7 +108,7 @@ public class LearnJava {
         */
         // Declare a variable using <type> <name>
         int fooInt;
-        // Declare multiple variables of the same 
+        // Declare multiple variables of the same
         // type <type> <name1>, <name2>, <name3>
         int fooInt1, fooInt2, fooInt3;
 
@@ -83,11 +117,12 @@ public class LearnJava {
         */
 
         // 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;
+        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;
 
         /*
         *  Variable types
@@ -95,7 +130,7 @@ public class LearnJava {
         // 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;
@@ -137,7 +172,7 @@ public class LearnJava {
         // Char - A single 16-bit Unicode character
         char fooChar = 'A';
 
-        // final variables can't be reassigned to another object,
+        // final variables can't be reassigned,
         final int HOURS_I_WORK_PER_WEEK = 9001;
         // but they can be initialized later.
         final double E;
@@ -149,12 +184,12 @@ public class LearnJava {
         // 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 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 
+        // 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
@@ -164,7 +199,7 @@ public class LearnJava {
         // 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.
@@ -196,13 +231,13 @@ public class LearnJava {
         builderConcatenated.append("You ");
         builderConcatenated.append("can use ");
         builderConcatenated.append("the StringBuilder class.");
-        System.out.println(builderConcatenated.toString()); // only now is the string built 
+        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++){
+        for (int i = 0 ; i < 10; i++) {
             stringBuilder.append(i).append(" ");
             inefficientString += i + " ";
         }
@@ -211,12 +246,12 @@ public class LearnJava {
         // 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
@@ -244,7 +279,7 @@ public class LearnJava {
         // 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
+        // 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
@@ -253,10 +288,16 @@ public class LearnJava {
         // 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.
-        // TreeMap - This class is a sorted tree structure. It implements a red
-        //           black tree and sorts the entries based on the key value or
-        //           the comparator provided while creating the object
+        //            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
@@ -270,7 +311,7 @@ public class LearnJava {
         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
+        System.out.println("1/2.0 = " + (i1 / (double)i2)); // => 0.5
 
         // Modulo
         System.out.println("11%3 = "+(11 % 3)); // => 2
@@ -340,7 +381,7 @@ public class LearnJava {
         do {
             System.out.println(fooDoWhile);
             // Increment the counter
-            // Iterated 99 times, fooDoWhile 0->99
+            // Iterated 100 times, fooDoWhile 0->99
             fooDoWhile++;
         } while(fooDoWhile < 100);
         System.out.println("fooDoWhile Value: " + fooDoWhile);
@@ -352,7 +393,7 @@ public class LearnJava {
             // 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++) {
@@ -363,7 +404,7 @@ public class LearnJava {
             }
           }
         }
-        
+
         // For Each Loop
         // The for loop is also able to iterate over arrays as well as objects
         // that implement the Iterable interface.
@@ -381,6 +422,9 @@ public class LearnJava {
         // 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) {
@@ -394,38 +438,21 @@ public class LearnJava {
                      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;
-        }
-        
-        
+
+
         // 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 be closing resources
+        // simplifies try-catch-finally statements by closing resources
         // automatically.
-        
-        // In order to use a try-with-resources, include a an instance of a class
+
+        // 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"))) {
+        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. 
+            // an Exception.
         } catch (Exception ex) {
             //The resource will be closed before the catch statement executes.
             System.out.println("readLine() failed.");
@@ -435,18 +462,22 @@ public class LearnJava {
         // 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); // Prints A, because the statement is true
+        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 And Typecasting
+        // Converting Data Types
         ////////////////////////////////////////
 
         // Converting data
@@ -462,11 +493,6 @@ public class LearnJava {
         // 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:
-        // https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
-
         ///////////////////////////////////////
         // Classes And Functions
         ///////////////////////////////////////
@@ -531,10 +557,10 @@ class Bicycle {
     String name; // default: Only accessible from within this package
     static String className; // Static class variable
 
-    // Static block 
+    // Static block
     // Java has no implementation of static constructors, but
-    // has a static block that can be used to initialize class variables 
-    // (static variables). 
+    // 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";
@@ -617,6 +643,14 @@ class PennyFarthing extends Bicycle {
     }
 }
 
+// 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> {
@@ -632,10 +666,10 @@ public interface Edible {
 
 public interface Digestible {
     public void digest();
-    // In Java 8, interfaces can have default method.
-    // public void digest() {
-    //     System.out.println("digesting ...");
-    // }
+    // 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.
@@ -668,20 +702,27 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne,
 // Abstract Classes
 
 // Abstract Class declaration syntax
-// <access-level> abstract <abstract-class-name> extends <super-abstract-classes> {
+// <access-level> abstract class <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.
+// 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
@@ -692,17 +733,12 @@ public abstract class Animal
         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);  
+        System.out.println(age);
     }
 
-    // Abstract classes can have main function.
+    // Abstract classes can have main method.
     public static void main(String[] args)
     {
         System.out.println("I am abstract");
@@ -717,7 +753,7 @@ class Dog extends Animal
     public void makeSound()
     {
         System.out.println("Bark");
-        // age = 30;	==> ERROR!	age is private to Animal
+        // age = 30;    ==> ERROR!    age is private to Animal
     }
 
     // NOTE: You will get an error if you used the
@@ -781,18 +817,18 @@ public abstract class Mammal()
 // you would specify a days-of-the-week enum type as:
 public enum Day {
     SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
-    THURSDAY, FRIDAY, SATURDAY 
+    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:
@@ -800,17 +836,17 @@ public class EnumTest {
                 break;
             case FRIDAY:
                 System.out.println("Fridays are better.");
-                break;   
-            case SATURDAY: 
+                break;
+            case SATURDAY:
             case SUNDAY:
                 System.out.println("Weekends are best.");
-                break;     
+                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.
@@ -819,10 +855,112 @@ public class EnumTest {
     }
 }
 
-// Enum types are much more powerful than we show above. 
+// 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
@@ -858,6 +996,10 @@ The links provided here below are just to get an understanding of the topic, fee
 
 * [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/)