diff options
Diffstat (limited to 'java.html.markdown')
| -rw-r--r-- | java.html.markdown | 100 |
1 files changed, 88 insertions, 12 deletions
diff --git a/java.html.markdown b/java.html.markdown index 38c9e490..50629ce1 100644 --- a/java.html.markdown +++ b/java.html.markdown @@ -128,7 +128,7 @@ public class LearnJava { // // BigInteger can be initialized using an array of bytes or a string. - BigInteger fooBigInteger = new BigDecimal(fooByteArray); + BigInteger fooBigInteger = new BigInteger(fooByteArray); // BigDecimal - Immutable, arbitrary-precision signed decimal number @@ -144,7 +144,12 @@ public class LearnJava { // 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 @@ -170,7 +175,7 @@ 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]); @@ -186,9 +191,9 @@ public class LearnJava { // 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 +212,8 @@ 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 an int) + System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5 // Modulo System.out.println("11%3 = "+(11 % 3)); // => 2 @@ -416,7 +421,7 @@ public class LearnJava { // 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"); @@ -433,7 +438,7 @@ public class LearnJava { // 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: @@ -450,6 +455,17 @@ class Bicycle { 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() { @@ -519,7 +535,7 @@ 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"); } @@ -686,6 +702,66 @@ public abstract class Mammal() 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 +785,7 @@ 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** |