diff options
Diffstat (limited to 'java.html.markdown')
| -rw-r--r-- | java.html.markdown | 127 |
1 files changed, 60 insertions, 67 deletions
diff --git a/java.html.markdown b/java.html.markdown index e8c15234..a27a68ca 100644 --- a/java.html.markdown +++ b/java.html.markdown @@ -48,9 +48,9 @@ import java.security.*; // as the file. public class LearnJava { - // In order to run a java program, it must have a main method as an entry + // In order to run a java program, it must have a main method as an entry // point. - public static void main (String[] args) { + public static void main(String[] args) { /////////////////////////////////////// // Input/Output @@ -109,7 +109,7 @@ public class LearnJava { */ // Declare a variable using <type> <name> int fooInt; - // Declare multiple variables of the same + // Declare multiple variables of the same // type <type> <name1>, <name2>, <name3> int fooInt1, fooInt2, fooInt3; @@ -119,8 +119,9 @@ public class LearnJava { // Initialize a variable using <type> <name> = <val> int barInt = 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> + // <name1> = <name2> = <name3> = <val> int barInt1, barInt2, barInt3; barInt1 = barInt2 = barInt3 = 1; @@ -130,7 +131,7 @@ public class LearnJava { // Byte - 8-bit signed two's complement integer // (-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; @@ -184,12 +185,12 @@ public class LearnJava { // 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 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 @@ -199,7 +200,7 @@ 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. @@ -231,13 +232,13 @@ public class LearnJava { builderConcatenated.append("You "); builderConcatenated.append("can use "); builderConcatenated.append("the StringBuilder class."); - System.out.println(builderConcatenated.toString()); // only now is the string built + 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++){ + for (int i = 0 ; i < 10; i++) { stringBuilder.append(i).append(" "); inefficientString += i + " "; } @@ -246,12 +247,12 @@ public class LearnJava { // 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 @@ -387,7 +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++) { @@ -398,7 +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. @@ -416,6 +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) { @@ -429,38 +431,21 @@ 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"))) { + 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. + // an Exception. } catch (Exception ex) { //The resource will be closed before the catch statement executes. System.out.println("readLine() failed."); @@ -470,18 +455,22 @@ public class LearnJava { // 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. // 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 + 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 @@ -497,11 +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: - // https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html - /////////////////////////////////////// // Classes And Functions /////////////////////////////////////// @@ -566,10 +550,10 @@ class Bicycle { 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"; @@ -652,6 +636,14 @@ class PennyFarthing extends Bicycle { } } +// 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> { @@ -667,10 +659,10 @@ public interface Edible { public interface Digestible { public void digest(); - // In Java 8, interfaces can have default method. - // public void digest() { - // System.out.println("digesting ..."); - // } + // 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. @@ -703,14 +695,15 @@ 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 +// 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, @@ -734,7 +727,7 @@ public abstract class Animal public void printAge() { - System.out.println(age); + System.out.println(age); } // Abstract classes can have main function. @@ -816,18 +809,18 @@ public abstract class Mammal() // 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: @@ -835,17 +828,17 @@ public class EnumTest { break; case FRIDAY: System.out.println("Fridays are better."); - break; - case SATURDAY: + break; + case SATURDAY: case SUNDAY: System.out.println("Weekends are best."); - break; + 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. @@ -854,7 +847,7 @@ 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 see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html |