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-rw-r--r--java.html.markdown291
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;
+ }
}
```