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Diffstat (limited to 'java.html.markdown')
| -rw-r--r-- | java.html.markdown | 386 |
1 files changed, 335 insertions, 51 deletions
diff --git a/java.html.markdown b/java.html.markdown index 928eb39f..48e6ec75 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 --- @@ -14,9 +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. @@ -27,11 +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 { - // 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 +51,36 @@ 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 +102,13 @@ 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 +118,38 @@ 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 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!"; @@ -112,7 +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]); @@ -121,14 +183,18 @@ 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 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 @@ -145,7 +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)); // => 0 (int/int returns int) + System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5 // Modulo System.out.println("11%3 = "+(11 % 3)); // => 2 @@ -158,18 +225,23 @@ 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 */ - // Incrementations + // Increment operators int i = 0; System.out.println("\n->Inc/Dec-rementation"); // The ++ and -- operators increment and decrement by 1 respectively. @@ -187,7 +259,7 @@ 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 +288,24 @@ 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. @@ -231,7 +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 @@ -255,6 +336,23 @@ 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. @@ -264,7 +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 //////////////////////////////////////// @@ -287,7 +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 /////////////////////////////////////// @@ -306,12 +402,36 @@ public class LearnJava { // toString returns this Object's string representation. System.out.println("trek info: " + trek.toString()); - } // End main method -} // End LearnJava class + // 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 = new 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. -// You can include other, non-public outer-level classes in a .java file + } // End main method +} // End LearnJava class +// 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: // <public/private/protected> class <class name> { @@ -326,16 +446,27 @@ 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 public Bicycle() { + // You can also call another constructor: + // this(1, 50, 5, "Bontrager"); gear = 1; cadence = 50; speed = 5; name = "Bontrager"; } - // This is a constructor that takes arguments public Bicycle(int startCadence, int startSpeed, int startGear, String name) { @@ -345,13 +476,13 @@ 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; } @@ -360,29 +491,24 @@ 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; } //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; @@ -394,7 +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"); } @@ -417,26 +543,25 @@ 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 { +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 @@ -444,13 +569,173 @@ public class Fruit implements Edible, Digestible { public class ExampleClass extends ExampleClassParent implements InterfaceOne, InterfaceTwo { @Override - public void InterfaceOneMethod() { - } + public void InterfaceOneMethod() { + } + + @Override + 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 +// 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. +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 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 InterfaceTwoMethod() { - } + 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; + } +} + +// 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 @@ -474,7 +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** @@ -482,7 +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/) @@ -491,4 +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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