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

1 files changed, 263 insertions, 28 deletions

diff --git a/java.html.markdown b/java.html.markdown
index 928eb39f..38c9e490 100644
--- a/java.html.markdown
+++ b/java.html.markdown
@@ -5,6 +5,9 @@ contributors:
     - ["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"]
+    - ["Rachel Stiyer", "https://github.com/rstiyer"]
 filename: LearnJava.java
 ---
 
@@ -31,7 +34,7 @@ import java.security.*;
 // the file.
 public class LearnJava {
 
-    // A program must have a main method as an entry point.
+    // 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.
@@ -45,12 +48,34 @@ public class LearnJava {
         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
 
         ///////////////////////////////////////
-        // Types & Variables
+        // 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;
@@ -72,11 +97,13 @@ public class LearnJava {
         // 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 is used to denote that this variable value is of type float;
+        // 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
@@ -86,8 +113,39 @@ 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 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 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);
+
+
 
         // Strings
         String fooString = "My String Is Here!";
@@ -127,8 +185,12 @@ 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 maps keys to values. A map cannot
-        //        contain duplicate keys; each key can map to at most one value.
+        // 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
@@ -146,6 +208,7 @@ 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 (0.5 truncated down)
+        System.out.println("1/2 = " + (i1 / (i2*1.0))); // => 0.5
 
         // Modulo
         System.out.println("11%3 = "+(11 % 3)); // => 2
@@ -158,12 +221,17 @@ public class LearnJava {
         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 right shift
-        >>>    Unsigned right shift
+        >>     Signed/Arithmetic right shift
+        >>>    Unsigned/Logical right shift
         &      Bitwise AND
         ^      Bitwise exclusive OR
         |      Bitwise inclusive OR
@@ -187,7 +255,7 @@ public class LearnJava {
 
         // If statements are c-like
         int j = 10;
-        if (j == 10){
+        if (j == 10) {
             System.out.println("I get printed");
         } else if (j > 10) {
             System.out.println("I don't");
@@ -216,14 +284,24 @@ public class LearnJava {
         System.out.println("fooDoWhile Value: " + fooDoWhile);
 
         // For Loop
-        int fooFor;
         // for loop structure => for(<start_statement>; <conditional>; <step>)
-        for (fooFor = 0; fooFor < 10; fooFor++) {
+        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.
@@ -255,6 +333,23 @@ 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;
+        }
 
         // Conditional Shorthand
         // You can use the '?' operator for quick assignments or logic forks.
@@ -306,11 +401,39 @@ public class LearnJava {
         // 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
+// 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:
@@ -330,6 +453,8 @@ class 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;
@@ -345,13 +470,13 @@ class Bicycle {
         this.name = name;
     }
 
-    // Function Syntax:
+    // Method Syntax:
     // <public/private/protected> <return type> <function name>(<args>)
 
     // Java classes often implement getters and setters for their fields
 
     // Method declaration syntax:
-    // <scope> <return type> <method name>(<args>)
+    // <access modifier> <return type> <method name>(<args>)
     public int getCadence() {
         return cadence;
     }
@@ -382,7 +507,7 @@ class Bicycle {
     }
 
     //Method to display the attribute values of this Object.
-    @Override
+    @Override // Inherited from the Object class.
     public String toString() {
         return "gear: " + gear + " cadence: " + cadence + " speed: " + speed +
             " name: " + name;
@@ -417,39 +542,149 @@ class PennyFarthing extends Bicycle {
 
 // Example - Food:
 public interface Edible {
-	public void eat(); // Any class that implements this interface, must
+    public void eat(); // Any class that implements this interface, must
                        // implement this method.
 }
 
 public interface Digestible {
-	public void digest();
+    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() {
-		// ...
-	}
+    public void eat() {
+        // ...
+    }
 
     @Override
-	public void digest() {
-		// ...
-	}
+    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() {
-	}
+    public void InterfaceOneMethod() {
+    }
 
     @Override
-	public void InterfaceTwoMethod() {
-	}
+    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;
+    }
 }
 ```