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-rw-r--r--java.html.markdown196
1 files changed, 137 insertions, 59 deletions
diff --git a/java.html.markdown b/java.html.markdown
index aae64ccf..56bffd88 100644
--- a/java.html.markdown
+++ b/java.html.markdown
@@ -8,6 +8,7 @@ contributors:
- ["Zachary Ferguson", "http://github.com/zfergus2"]
- ["Cameron Schermerhorn", "http://github.com/cschermerhorn"]
- ["Rachel Stiyer", "https://github.com/rstiyer"]
+ - ["Michael Dähnert", "http://github.com/JaXt0r"]
filename: LearnJava.java
---
@@ -17,12 +18,23 @@ 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.
+ * 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+ * @deprecated For showing the code is outdated or shouldn't be used.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
@@ -30,11 +42,12 @@ 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 {
- // In order to run a java program, it 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.
@@ -60,7 +73,8 @@ 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;
/*
@@ -69,7 +83,8 @@ 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>
+ // Initialize multiple variables of same type with same
+ // value <type> <name1>, <name2>, <name3> = <val>
int fooInt1, fooInt2, fooInt3;
fooInt1 = fooInt2 = fooInt3 = 1;
@@ -119,17 +134,14 @@ public class LearnJava {
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);
-
+ // BigInteger can be initialized using an array of bytes or a string.
+ BigInteger fooBigInteger = new BigInteger(fooByteArray);
// BigDecimal - Immutable, arbitrary-precision signed decimal number
//
@@ -142,10 +154,12 @@ 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!";
@@ -158,6 +172,29 @@ 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.
+
+ // #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
@@ -170,7 +207,7 @@ 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]);
@@ -179,22 +216,25 @@ 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 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
// 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
@@ -207,8 +247,8 @@ 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 int)
+ System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5
// Modulo
System.out.println("11%3 = "+(11 % 3)); // => 2
@@ -237,7 +277,7 @@ 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.
@@ -309,7 +349,6 @@ 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
@@ -359,7 +398,6 @@ 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
////////////////////////////////////////
@@ -382,7 +420,6 @@ 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
///////////////////////////////////////
@@ -404,19 +441,17 @@ 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 = HashSet<String>() {{
+ private static final Set<String> COUNTRIES = new HashSet<String>() {{
add("DENMARK");
add("SWEDEN");
add("FINLAND");
@@ -431,10 +466,8 @@ 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> {
@@ -449,7 +482,6 @@ 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
@@ -471,7 +503,6 @@ class Bicycle {
speed = 5;
name = "Bontrager";
}
-
// This is a constructor that takes arguments
public Bicycle(int startCadence, int startSpeed, int startGear,
String name) {
@@ -496,23 +527,18 @@ 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;
}
@@ -530,7 +556,7 @@ 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");
}
@@ -559,12 +585,14 @@ public interface Edible {
public interface Digestible {
public void digest();
+ // In Java 8, interfaces can have default method.
+ // public void digest() {
+ // System.out.println("digesting ...");
+ // }
}
-
// We can now create a class that implements both of these interfaces.
public class Fruit implements Edible, Digestible {
-
@Override
public void eat() {
// ...
@@ -580,7 +608,6 @@ public class Fruit implements Edible, Digestible {
// interfaces. For example:
public class ExampleClass extends ExampleClassParent implements InterfaceOne,
InterfaceTwo {
-
@Override
public void InterfaceOneMethod() {
}
@@ -599,14 +626,13 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne,
// // 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
+// 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,
-// unless the method is static, and variables are final by default, unlike an
-// abstract class. Also abstract classes CAN have the "main" method.
-
+// 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();
@@ -614,7 +640,7 @@ 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;
}
@@ -651,7 +677,7 @@ class Dog extends Animal
// @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();
@@ -690,13 +716,66 @@ 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:
+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 see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
+
```
## Further Reading
@@ -720,7 +799,7 @@ 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**
@@ -728,7 +807,6 @@ 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/)