diff options
Diffstat (limited to 'java.html.markdown')
-rw-r--r-- | java.html.markdown | 352 |
1 files changed, 222 insertions, 130 deletions
diff --git a/java.html.markdown b/java.html.markdown index 74140120..a27a68ca 100644 --- a/java.html.markdown +++ b/java.html.markdown @@ -1,28 +1,42 @@ --- language: java contributors: - - ["Jake Prather", "http://github.com/JakeHP"] - - ["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"] + - ["Jake Prather", "https://github.com/JakeHP"] + - ["Jakukyo Friel", "https://weakish.github.io"] + - ["Madison Dickson", "https://github.com/mix3d"] + - ["Simon Morgan", "https://sjm.io/"] + - ["Zachary Ferguson", "https://github.com/zfergus2"] + - ["Cameron Schermerhorn", "https://github.com/cschermerhorn"] - ["Rachel Stiyer", "https://github.com/rstiyer"] + - ["Michael Dähnert", "https://github.com/JaXt0r"] + - ["Rob Rose", "https://github.com/RobRoseKnows"] + - ["Sean Nam", "https://github.com/seannam"] 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/) +[Read more here.](https://docs.oracle.com/javase/tutorial/java/) ```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. + * JavaDoc comments look like this. Used to describe the Class or various + * attributes of a Class. + * Main attributes: + * + * @author Name (and contact information such as email) of author(s). + * @version Current version of the program. + * @since When this part of the program was first added. + * @param For describing the different parameters for a method. + * @return For describing what the method returns. + * @deprecated For showing the code is outdated or shouldn't be used. + * @see Links to another part of documentation. */ // Import ArrayList class inside of the java.util package @@ -30,12 +44,21 @@ 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. - public static void main (String[] args) { + // In order to run a java program, it must have a main method as an entry + // point. + public static void main(String[] args) { + + /////////////////////////////////////// + // Input/Output + /////////////////////////////////////// + + /* + * Ouput + */ // Use System.out.println() to print lines. System.out.println("Hello World!"); @@ -51,6 +74,32 @@ public class LearnJava { // Use System.out.printf() for easy formatted printing. System.out.printf("pi = %.5f", Math.PI); // => pi = 3.14159 + /* + * Input + */ + + // use Scanner to read input + // must import java.util.Scanner; + Scanner scanner = new Scanner(System.in); + + // read string input + String name = scanner.next(); + + // read byte input + byte numByte = scanner.nextByte(); + + // read int input + int numInt = scanner.nextInt(); + + // read long input + float numFloat = scanner.nextFloat(); + + // read double input + double numDouble = scanner.nextDouble(); + + // read boolean input + boolean bool = scanner.nextBoolean(); + /////////////////////////////////////// // Variables /////////////////////////////////////// @@ -60,7 +109,8 @@ 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; /* @@ -68,10 +118,12 @@ 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> - int fooInt1, fooInt2, fooInt3; - fooInt1 = fooInt2 = fooInt3 = 1; + int barInt = 1; + // Initialize multiple variables of same type with same + // value <type> <name1>, <name2>, <name3> + // <name1> = <name2> = <name3> = <val> + int barInt1, barInt2, barInt3; + barInt1 = barInt2 = barInt3 = 1; /* * Variable types @@ -80,13 +132,19 @@ public class LearnJava { // (-128 <= byte <= 127) byte fooByte = 100; + // If you would like to interpret a byte as an unsigned integer + // then this simple operation can help + int unsignedIntLessThan256 = 0xff & fooByte; + // this contrasts a cast which can be negative. + int signedInt = (int) fooByte; + // Short - 16-bit signed two's complement integer // (-32,768 <= short <= 32,767) short fooShort = 10000; // Integer - 32-bit signed two's complement integer // (-2,147,483,648 <= int <= 2,147,483,647) - int fooInt = 1; + int bazInt = 1; // Long - 64-bit signed two's complement integer // (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807) @@ -94,7 +152,9 @@ 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: byte, short, int and long are signed. They can have positive and negative values. + // There are no unsigned variants. + // char, however, is 16-bit unsigned. // Float - Single-precision 32-bit IEEE 754 Floating Point // 2^-149 <= float <= (2-2^-23) * 2^127 @@ -119,7 +179,6 @@ public class LearnJava { final double E; E = 2.71828; - // BigInteger - Immutable arbitrary-precision integers // // BigInteger is a data type that allows programmers to manipulate @@ -127,13 +186,11 @@ public class LearnJava { // 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 + // 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 @@ -142,16 +199,13 @@ 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!"; @@ -163,6 +217,42 @@ public class LearnJava { System.out.println(barString); System.out.println(bazString); + // String Building + // #1 - with plus operator + // That's the basic way to do it (optimized under the hood) + String plusConcatenated = "Strings can " + "be concatenated " + "via + operator."; + System.out.println(plusConcatenated); + // Output: Strings can be concatenated via + operator. + + // #2 - with StringBuilder + // This way doesn't create any intermediate strings. It just stores the string pieces, and ties them together + // when toString() is called. + // Hint: This class is not thread safe. A thread-safe alternative (with some impact on performance) is StringBuffer. + StringBuilder builderConcatenated = new StringBuilder(); + builderConcatenated.append("You "); + builderConcatenated.append("can use "); + builderConcatenated.append("the StringBuilder class."); + System.out.println(builderConcatenated.toString()); // only now is the string built + // Output: You can use the StringBuilder class. + + // StringBuilder is efficient when the fully constructed String is not required until the end of some processing. + StringBuilder stringBuilder = new StringBuilder(); + String inefficientString = ""; + for (int i = 0 ; i < 10; i++) { + stringBuilder.append(i).append(" "); + inefficientString += i + " "; + } + System.out.println(inefficientString); + System.out.println(stringBuilder.toString()); + // inefficientString requires a lot more work to produce, as it generates a String on every loop iteration. + // Simple concatenation with + is compiled to a StringBuilder and toString() + // Avoid string concatenation in loops. + + // #3 - with String formatter + // Another alternative way to create strings. Fast and readable. + String.format("%s may prefer %s.", "Or you", "String.format()"); + // Output: Or you may prefer String.format(). + // Arrays // The array size must be decided upon instantiation // The following formats work for declaring an array @@ -175,7 +265,7 @@ 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]); @@ -184,7 +274,7 @@ 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 @@ -200,6 +290,9 @@ public class LearnJava { // interface. This allows the execution time of basic // operations, such as get and insert element, to remain // constant even for large sets. + // TreeMap - This class is a sorted tree structure. It implements a red + // black tree and sorts the entries based on the key value or + // the comparator provided while creating the object /////////////////////////////////////// // Operators @@ -212,7 +305,7 @@ 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 (int/int returns an int) + System.out.println("1/2 = " + (i1 / i2)); // => 0 (int/int returns int) System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5 // Modulo @@ -242,7 +335,7 @@ 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. @@ -295,7 +388,7 @@ 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++) { @@ -306,7 +399,7 @@ public class LearnJava { } } } - + // For Each Loop // The for loop is also able to iterate over arrays as well as objects // that implement the Iterable interface. @@ -314,7 +407,6 @@ 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 @@ -325,6 +417,7 @@ public class LearnJava { // 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. + // Starting in Java 7 and above, we can also use the String type. int month = 3; String monthString; switch (month) { @@ -338,23 +431,31 @@ 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; + + + // Try-with-resources (Java 7+) + // Try-catch-finally statements work as expected in Java but in Java 7+ + // the try-with-resources statement is also available. Try-with-resources + // simplifies try-catch-finally statements by closing resources + // automatically. + + // In order to use a try-with-resources, include an instance of a class + // in the try statement. The class must implement java.lang.AutoCloseable. + try (BufferedReader br = new BufferedReader(new FileReader("foo.txt"))) { + // You can attempt to do something that could throw an exception. + System.out.println(br.readLine()); + // In Java 7, the resource will always be closed, even if it throws + // an Exception. + } catch (Exception ex) { + //The resource will be closed before the catch statement executes. + System.out.println("readLine() failed."); } + // No need for a finally statement in this case, the BufferedReader is + // already closed. This can be used to avoid certain edge cases where + // a finally statement might not be called. + // To learn more: + // https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html + // Conditional Shorthand // You can use the '?' operator for quick assignments or logic forks. @@ -362,11 +463,14 @@ public class LearnJava { // <second value>" int foo = 5; String bar = (foo < 10) ? "A" : "B"; - System.out.println(bar); // Prints A, because the statement is true - + System.out.println("bar : " + bar); // Prints "bar : A", because the + // statement is true. + // Or simply + System.out.println("bar : " + (foo < 10 ? "A" : "B")); + //////////////////////////////////////// - // Converting Data Types And Typecasting + // Converting Data Types //////////////////////////////////////// // Converting data @@ -382,12 +486,6 @@ public class LearnJava { // Long // 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: - // http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html - - /////////////////////////////////////// // Classes And Functions /////////////////////////////////////// @@ -409,18 +507,16 @@ 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"); - validCodes.add("SWEDEN"); - validCodes.add("FINLAND"); + COUNTRIES.add("DENMARK"); + COUNTRIES.add("SWEDEN"); + COUNTRIES.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"); @@ -436,10 +532,8 @@ 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> { @@ -454,13 +548,12 @@ 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 + // Static block // Java has no implementation of static constructors, but - // has a static block that can be used to initialize class variables - // (static variables). + // 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"; @@ -476,7 +569,6 @@ class Bicycle { speed = 5; name = "Bontrager"; } - // This is a constructor that takes arguments public Bicycle(int startCadence, int startSpeed, int startGear, String name) { @@ -501,23 +593,18 @@ 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; } @@ -545,10 +632,18 @@ class PennyFarthing extends Bicycle { // out: http://docs.oracle.com/javase/tutorial/java/annotations/ @Override public void setGear(int gear) { - gear = 0; + this.gear = 0; } } +// Object casting +// Since the PennyFarthing class is extending the Bicycle class, we can say +// a PennyFarthing is a Bicycle and write : +// Bicycle bicycle = new PennyFarthing(); +// This is called object casting where an object is taken for another one. There +// are lots of details and deals with some more intermediate concepts here: +// https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html + // Interfaces // Interface declaration syntax // <access-level> interface <interface-name> extends <super-interfaces> { @@ -564,12 +659,14 @@ public interface Edible { public interface Digestible { public void digest(); + // Since Java 8, interfaces can have default method. + public void defaultMethod() { + System.out.println("Hi from default method ..."); + } } - // We can now create a class that implements both of these interfaces. public class Fruit implements Edible, Digestible { - @Override public void eat() { // ... @@ -585,7 +682,6 @@ public class Fruit implements Edible, Digestible { // interfaces. For example: public class ExampleClass extends ExampleClassParent implements InterfaceOne, InterfaceTwo { - @Override public void InterfaceOneMethod() { } @@ -599,19 +695,19 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne, // Abstract Classes // Abstract Class declaration syntax -// <access-level> abstract <abstract-class-name> extends <super-abstract-classes> { +// <access-level> abstract class <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. - +// Marking a class as abstract means that it contains at least one abstract +// method 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(); @@ -619,7 +715,7 @@ public abstract class Animal // Method can have a body public void eat() { - System.out.println("I am an animal and I am Eating."); + System.out.println("I am an animal and I am Eating."); // Note: We can access private variable here. age = 30; } @@ -627,11 +723,11 @@ public abstract class Animal // No need to initialize, however in an interface // a variable is implicitly final and hence has // to be initialized. - protected int age; + private int age; public void printAge() { - System.out.println(age); + System.out.println(age); } // Abstract classes can have main function. @@ -649,14 +745,14 @@ class Dog extends Animal public void makeSound() { System.out.println("Bark"); - // age = 30; ==> ERROR! age is private to Animal + // 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/ + // Check out this SO post: http://stackoverflow.com/questions/16313649/ public static void main(String[] args) { Dog pluto = new Dog(); @@ -695,61 +791,54 @@ 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: - +// 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 + 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 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. @@ -758,9 +847,9 @@ public class EnumTest { } } -// Enum types are much more powerful than we show above. +// 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 +// You can see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html ``` @@ -770,22 +859,26 @@ The links provided here below are just to get an understanding of the topic, fee **Official Oracle Guides**: -* [Java Tutorial Trail from Sun / Oracle](http://docs.oracle.com/javase/tutorial/index.html) +* [Java Tutorial Trail from Sun / Oracle](https://docs.oracle.com/javase/tutorial/index.html) -* [Java Access level modifiers](http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html) +* [Java Access level modifiers](https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html) -* [Object-Oriented Programming Concepts](http://docs.oracle.com/javase/tutorial/java/concepts/index.html): - * [Inheritance](http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html) - * [Polymorphism](http://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html) - * [Abstraction](http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html) +* [Object-Oriented Programming Concepts](https://docs.oracle.com/javase/tutorial/java/concepts/index.html): + * [Inheritance](https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html) + * [Polymorphism](https://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html) + * [Abstraction](https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html) -* [Exceptions](http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html) +* [Exceptions](https://docs.oracle.com/javase/tutorial/essential/exceptions/index.html) -* [Interfaces](http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html) +* [Interfaces](https://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html) -* [Generics](http://docs.oracle.com/javase/tutorial/java/generics/index.html) +* [Generics](https://docs.oracle.com/javase/tutorial/java/generics/index.html) -* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconvtoc-136057.html) +* [Java Code Conventions](https://www.oracle.com/technetwork/java/codeconvtoc-136057.html) + +* New features in Java 8: + * [Lambda expressions (functional programming)](https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html) + * [Date and time API (java.time package)](http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html) **Online Practice and Tutorials** @@ -793,13 +886,12 @@ 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/) * [Thinking in Java](http://www.mindview.net/Books/TIJ/) -* [Objects First with Java](http://www.amazon.com/Objects-First-Java-Practical-Introduction/dp/0132492660) +* [Objects First with Java](https://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](https://www.amazon.com/gp/product/0071606300) |