diff options
Diffstat (limited to 'java.html.markdown')
| -rw-r--r-- | java.html.markdown | 193 |
1 files changed, 168 insertions, 25 deletions
diff --git a/java.html.markdown b/java.html.markdown index ba602d2e..50629ce1 100644 --- a/java.html.markdown +++ b/java.html.markdown @@ -6,6 +6,8 @@ contributors: - ["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 --- @@ -95,11 +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 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 @@ -114,7 +118,40 @@ public class LearnJava { // 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!"; @@ -138,7 +175,7 @@ public class LearnJava { // Another way to declare & initialize an array int[] y = {9000, 1000, 1337}; String names[] = {"Bob", "John", "Fred", "Juan Pedro"}; - boolean bools[] = new boolean[] {true, false, false}; + boolean bools[] = {true, false, false}; // Indexing an array - Accessing an element System.out.println("intArray @ 0: " + intArray[0]); @@ -153,8 +190,12 @@ 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 @@ -171,8 +212,8 @@ public class LearnJava { 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 (0.5 truncated down) - System.out.println("1/2 = " + (i1 / (i2*1.0))); // => 0.5 + System.out.println("1/2 = " + (i1 / i2)); // => 0 (int/int returns an int) + System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5 // Modulo System.out.println("11%3 = "+(11 % 3)); // => 2 @@ -219,7 +260,7 @@ 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"); @@ -254,7 +295,18 @@ 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++) { + 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. @@ -286,6 +338,23 @@ 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. @@ -348,19 +417,19 @@ public class LearnJava { validCodes.add("FINLAND"); } - // But there's a nifty way to achive the same thing in an + // 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>() {{ + private static final Set<String> COUNTRIES = new HashSet<String>() {{ add("DENMARK"); add("SWEDEN"); add("FINLAND"); }} - // The first brace is creating an new AnonymousInnerClass and the - // second one declares and instance initializer block. This block - // is called with the anonymous inner class is created. + // 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. @@ -368,8 +437,8 @@ public class LearnJava { } // End LearnJava class -// You can include other, non-public outer-level classes in a .java file, -// but it is good practice. Instead split classes into separate files. +// 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: @@ -386,10 +455,21 @@ class Bicycle { 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: + // You can also call another constructor: // this(1, 50, 5, "Bontrager"); gear = 1; cadence = 50; @@ -455,7 +535,7 @@ class PennyFarthing extends Bicycle { // (Penny Farthings are those bicycles with the big front wheel. // They have no gears.) - public PennyFarthing(int startCadence, int startSpeed){ + public PennyFarthing(int startCadence, int startSpeed) { // Call the parent constructor with super super(startCadence, startSpeed, 0, "PennyFarthing"); } @@ -489,6 +569,7 @@ public interface Digestible { // We can now create a class that implements both of these interfaces. public class Fruit implements Edible, Digestible { + @Override public void eat() { // ... @@ -504,6 +585,7 @@ public class Fruit implements Edible, Digestible { // interfaces. For example: public class ExampleClass extends ExampleClassParent implements InterfaceOne, InterfaceTwo { + @Override public void InterfaceOneMethod() { } @@ -511,6 +593,7 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne, @Override public void InterfaceTwoMethod() { } + } // Abstract Classes @@ -522,10 +605,10 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne, // } // 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 +// 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, +// 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. @@ -541,7 +624,7 @@ public abstract class Animal age = 30; } - // No need to initialize, however in an interface + // No need to initialize, however in an interface // a variable is implicitly final and hence has // to be initialized. protected int age; @@ -569,7 +652,7 @@ class Dog extends Animal // age = 30; ==> ERROR! age is private to Animal } - // NOTE: You will get an error if you used the + // 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. @@ -583,7 +666,7 @@ class Dog extends Animal } } -// Final Classes +// Final Classes // Final Class declaration syntax // <access-level> final <final-class-name> { @@ -592,8 +675,8 @@ class Dog extends Animal // } // 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 +// 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 { @@ -619,6 +702,66 @@ public abstract class Mammal() 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 se more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html + ``` ## Further Reading @@ -642,7 +785,7 @@ The links provided here below are just to get an understanding of the topic, fee * [Generics](http://docs.oracle.com/javase/tutorial/java/generics/index.html) -* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconv-138413.html) +* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconvtoc-136057.html) **Online Practice and Tutorials** |