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Diffstat (limited to 'java.html.markdown')
| -rw-r--r-- | java.html.markdown | 157 |
1 files changed, 103 insertions, 54 deletions
diff --git a/java.html.markdown b/java.html.markdown index 38c9e490..48e6ec75 100644 --- a/java.html.markdown +++ b/java.html.markdown @@ -17,9 +17,11 @@ programming language. ```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. @@ -30,11 +32,12 @@ 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. +// 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 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. @@ -60,7 +63,8 @@ public class LearnJava { */ // Declare a variable using <type> <name> int fooInt; - // Declare multiple variables of the same type <type> <name1>, <name2>, <name3> + // Declare multiple variables of the same + // type <type> <name1>, <name2>, <name3> int fooInt1, fooInt2, fooInt3; /* @@ -69,7 +73,8 @@ 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> + // Initialize multiple variables of same type with same + // value <type> <name1>, <name2>, <name3> = <val> int fooInt1, fooInt2, fooInt3; fooInt1 = fooInt2 = fooInt3 = 1; @@ -119,17 +124,14 @@ public class LearnJava { 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); - + // BigInteger can be initialized using an array of bytes or a string. + BigInteger fooBigInteger = new BigInteger(fooByteArray); // BigDecimal - Immutable, arbitrary-precision signed decimal number // @@ -142,10 +144,12 @@ 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. + BigDecimal tenCents = new BigDecimal("0.1"); // Strings String fooString = "My String Is Here!"; @@ -170,7 +174,7 @@ 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]); @@ -179,16 +183,16 @@ public class LearnJava { intArray[1] = 1; System.out.println("intArray @ 1: " + intArray[1]); // => 1 - // Others to check out + // 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 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 + // 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 @@ -207,8 +211,8 @@ 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 int) + System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5 // Modulo System.out.println("11%3 = "+(11 % 3)); // => 2 @@ -237,7 +241,7 @@ public class LearnJava { | Bitwise inclusive OR */ - // Incrementations + // Increment operators int i = 0; System.out.println("\n->Inc/Dec-rementation"); // The ++ and -- operators increment and decrement by 1 respectively. @@ -309,7 +313,6 @@ public class LearnJava { // 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 @@ -359,7 +362,6 @@ public class LearnJava { String bar = (foo < 10) ? "A" : "B"; System.out.println(bar); // Prints A, because the statement is true - //////////////////////////////////////// // Converting Data Types And Typecasting //////////////////////////////////////// @@ -382,7 +384,6 @@ public class LearnJava { // with some more intermediate concepts. Feel free to check it out here: // http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html - /////////////////////////////////////// // Classes And Functions /////////////////////////////////////// @@ -404,7 +405,6 @@ public class LearnJava { // 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"); @@ -415,8 +415,7 @@ public class LearnJava { // 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"); @@ -431,10 +430,8 @@ public class LearnJava { } // End main method } // 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. - +// but it is not good practice. Instead split classes into separate files. // Class Declaration Syntax: // <public/private/protected> class <class name> { @@ -449,6 +446,16 @@ class Bicycle { 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 @@ -460,7 +467,6 @@ class Bicycle { speed = 5; name = "Bontrager"; } - // This is a constructor that takes arguments public Bicycle(int startCadence, int startSpeed, int startGear, String name) { @@ -485,23 +491,18 @@ class Bicycle { 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; } @@ -519,7 +520,7 @@ 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"); } @@ -550,10 +551,8 @@ public interface Digestible { public void digest(); } - // We can now create a class that implements both of these interfaces. -public class Fruit implements Edible, Digestible { - +public class Fruit implements Edible, Digestible { @Override public void eat() { // ... @@ -569,7 +568,6 @@ public class Fruit implements Edible, Digestible { // interfaces. For example: public class ExampleClass extends ExampleClassParent implements InterfaceOne, InterfaceTwo { - @Override public void InterfaceOneMethod() { } @@ -588,14 +586,13 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne, // // 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 +// 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, -// unless the method is static, and variables are final by default, unlike an -// abstract class. Also abstract classes CAN have the "main" method. - +// 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. public abstract class Animal { public abstract void makeSound(); @@ -640,7 +637,7 @@ class Dog extends Animal // @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/ + // Check out this SO post: http://stackoverflow.com/questions/16313649/ public static void main(String[] args) { Dog pluto = new Dog(); @@ -679,13 +676,66 @@ 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. + // 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 se more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html + ``` ## Further Reading @@ -709,7 +759,7 @@ 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** @@ -717,7 +767,6 @@ The links provided here below are just to get an understanding of the topic, fee * [Codingbat.com](http://codingbat.com/java) - **Books**: * [Head First Java](http://www.headfirstlabs.com/books/hfjava/) @@ -726,4 +775,4 @@ The links provided here below are just to get an understanding of the topic, fee * [Objects First with Java](http://www.amazon.com/Objects-First-Java-Practical-Introduction/dp/0132492660) -* [Java The Complete Reference](http://www.amazon.com/gp/product/0071606300) +* [Java The Complete Reference](http://www.amazon.com/gp/product/0071606300)
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