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1 files changed, 346 insertions, 121 deletions
diff --git a/java.html.markdown b/java.html.markdown index 10dd498c..38c9e490 100644 --- a/java.html.markdown +++ b/java.html.markdown @@ -1,16 +1,19 @@ --- - language: java contributors: - ["Jake Prather", "http://github.com/JakeHP"] - - ["Madison Dickson", "http://github.com/mix3d"] - ["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 - --- -Java is a general-purpose, concurrent, class-based, object-oriented computer programming language. -[Read more here.](http://docs.oracle.com/javase/tutorial/java/index.html) +Java is a general-purpose, concurrent, class-based, object-oriented computer +programming language. +[Read more here.](http://docs.oracle.com/javase/tutorial/java/) ```java // Single-line comments start with // @@ -31,26 +34,48 @@ 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 + // Use System.out.println() to print lines. System.out.println("Hello World!"); System.out.println( "Integer: " + 10 + " Double: " + 3.14 + " Boolean: " + true); - // To print without a newline, use System.out.print + // To print without a newline, use System.out.print(). 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; @@ -69,14 +94,16 @@ public class LearnJava { // L is used to denote that this variable value is of type Long; // anything without is treated as integer by default. - // Note: Java has no unsigned types + // 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!"; @@ -101,10 +159,10 @@ public class LearnJava { System.out.println(bazString); // Arrays - //The array size must be decided upon instantiation - //The following formats work for declaring an array - //<datatype>[] <var name> = new <datatype>[<array size>]; - //<datatype> <var name>[] = new <datatype>[<array size>]; + // The array size must be decided upon instantiation + // The following formats work for declaring an array + // <datatype>[] <var name> = new <datatype>[<array size>]; + // <datatype> <var name>[] = new <datatype>[<array size>]; int[] intArray = new int[10]; String[] stringArray = new String[1]; boolean boolArray[] = new boolean[100]; @@ -122,17 +180,21 @@ public class LearnJava { System.out.println("intArray @ 1: " + intArray[1]); // => 1 // Others to check out - // ArrayLists - Like arrays except more functionality is offered, - // and the size is mutable + // 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. - // 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 constant even - // for large sets. + // 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 + // 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 + // constant even for large sets. /////////////////////////////////////// // Operators @@ -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 @@ -158,15 +221,20 @@ 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 - & Bitwise AND - ^ Bitwise exclusive OR - | Bitwise inclusive OR + ~ Unary bitwise complement + << Signed left shift + >> Signed/Arithmetic right shift + >>> Unsigned/Logical right shift + & Bitwise AND + ^ Bitwise exclusive OR + | Bitwise inclusive OR */ // Incrementations @@ -175,10 +243,10 @@ public class LearnJava { // The ++ and -- operators increment and decrement by 1 respectively. // If they are placed before the variable, they increment then return; // after the variable they return then increment. - System.out.println(i++); //i = 1, prints 0 (post-increment) - System.out.println(++i); //i = 2, prints 2 (pre-increment) - System.out.println(i--); //i = 1, prints 2 (post-decrement) - System.out.println(--i); //i = 0, prints 0 (pre-decrement) + System.out.println(i++); // i = 1, prints 0 (post-increment) + System.out.println(++i); // i = 2, prints 2 (pre-increment) + System.out.println(i--); // i = 1, prints 2 (post-decrement) + System.out.println(--i); // i = 0, prints 0 (pre-decrement) /////////////////////////////////////// // Control Structures @@ -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"); @@ -197,73 +265,96 @@ public class LearnJava { // While loop int fooWhile = 0; - while(fooWhile < 100) - { - //System.out.println(fooWhile); - //Increment the counter - //Iterated 100 times, fooWhile 0,1,2...99 + while(fooWhile < 100) { + System.out.println(fooWhile); + // Increment the counter + // Iterated 100 times, fooWhile 0,1,2...99 fooWhile++; } System.out.println("fooWhile Value: " + fooWhile); // Do While Loop int fooDoWhile = 0; - do - { - //System.out.println(fooDoWhile); - //Increment the counter - //Iterated 99 times, fooDoWhile 0->99 + do { + System.out.println(fooDoWhile); + // Increment the counter + // Iterated 99 times, fooDoWhile 0->99 fooDoWhile++; - }while(fooDoWhile < 100); + } while(fooDoWhile < 100); System.out.println("fooDoWhile Value: " + fooDoWhile); // For Loop - int fooFor; - //for loop structure => for(<start_statement>; <conditional>; <step>) - for(fooFor=0; fooFor<10; fooFor++){ - //System.out.println(fooFor); - //Iterated 10 times, fooFor 0->9 + // for loop structure => for(<start_statement>; <conditional>; <step>) + 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 - // An automatic iteration through an array or list of objects. - int[] fooList = {1,2,3,4,5,6,7,8,9}; - //for each loop structure => for(<object> : <array_object>) - //reads as: for each object in the array - //note: the object type must match the array. - - for( int bar : fooList ){ - //System.out.println(bar); + // The for loop is also able to iterate over arrays as well as objects + // that implement the Iterable interface. + int[] fooList = {1, 2, 3, 4, 5, 6, 7, 8, 9}; + // 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 } // Switch Case // A switch works with the byte, short, char, and int data types. - // It also works with enumerated types (discussed in Enum Types), - // the String class, and a few special classes that wrap - // primitive types: Character, Byte, Short, and Integer. + // It also works with enumerated types (discussed in Enum Types), the + // String class, and a few special classes that wrap primitive types: + // Character, Byte, Short, and Integer. int month = 3; String monthString; - switch (month){ - case 1: - monthString = "January"; - break; - case 2: - monthString = "February"; + switch (month) { + case 1: monthString = "January"; break; - case 3: - monthString = "March"; + case 2: monthString = "February"; break; - default: - monthString = "Some other month"; + case 3: monthString = "March"; break; + default: monthString = "Some other month"; + 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. - // Reads as "If (statement) is true, use <first value>, otherwise, use <second value>" + // Reads as "If (statement) is true, use <first value>, otherwise, use + // <second value>" int foo = 5; String bar = (foo < 10) ? "A" : "B"; System.out.println(bar); // Prints A, because the statement is true @@ -287,9 +378,8 @@ public class LearnJava { // String // Typecasting - // You can also cast java objects, there's a lot of details and - // deals with some more intermediate concepts. - // Feel free to check it out here: + // You can also cast Java objects, there's a lot of details and deals + // with some more intermediate concepts. Feel free to check it out here: // http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html @@ -311,17 +401,45 @@ 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: -// <public/private/protected> class <class name>{ -// //data fields, constructors, functions all inside. -// //functions are called as methods in Java. +// <public/private/protected> class <class name> { +// // data fields, constructors, functions all inside. +// // functions are called as methods in Java. // } class Bicycle { @@ -335,6 +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; @@ -342,20 +462,21 @@ class Bicycle { } // This is a constructor that takes arguments - public Bicycle(int startCadence, int startSpeed, int startGear, String name) { + public Bicycle(int startCadence, int startSpeed, int startGear, + String name) { this.gear = startGear; this.cadence = startCadence; this.speed = startSpeed; 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; } @@ -386,12 +507,10 @@ 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; + return "gear: " + gear + " cadence: " + cadence + " speed: " + speed + + " name: " + name; } } // end class Bicycle @@ -405,60 +524,168 @@ class PennyFarthing extends Bicycle { super(startCadence, startSpeed, 0, "PennyFarthing"); } - // You should mark a method you're overriding with an @annotation - // To learn more about what annotations are and their purpose - // check this out: http://docs.oracle.com/javase/tutorial/java/annotations/ + // You should mark a method you're overriding with an @annotation. + // To learn more about what annotations are and their purpose check this + // out: http://docs.oracle.com/javase/tutorial/java/annotations/ @Override public void setGear(int gear) { gear = 0; } - } -//Interfaces -//Interface declaration syntax -//<access-level> interface <interface-name> extends <super-interfaces> { -// //Constants -// //Method declarations -//} +// Interfaces +// Interface declaration syntax +// <access-level> interface <interface-name> extends <super-interfaces> { +// // Constants +// // Method declarations +// } -//Example - Food: +// Example - Food: public interface Edible { - public void eat(); //Any class that implements this interface, must implement this method + 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 +// 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 { +// 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() { + } + +} + +// 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 InterfaceTwoMethod() { + 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; + } +} ``` ## Further Reading @@ -500,5 +727,3 @@ 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) - - |