learnxinyminutes-docs/java.html.markdown

425 lines
14 KiB
Markdown
Raw Normal View History

2013-06-29 17:12:23 +00:00
---
2013-06-30 03:12:03 +00:00
2013-06-29 17:12:23 +00:00
language: java
2013-07-04 05:59:13 +00:00
contributors:
- ["Jake Prather", "http://github.com/JakeHP"]
filename: LearnJava.java
2013-06-29 17:12:23 +00:00
---
Java is a general-purpose, concurrent, class-based, object-oriented computer programming language.
2013-06-30 12:50:04 +00:00
[Read more here.](http://docs.oracle.com/javase/tutorial/java/index.html)
2013-06-29 17:12:23 +00:00
```java
2013-06-30 06:13:53 +00:00
// 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.
*/
2013-06-30 06:13:53 +00:00
// Import ArrayList class inside of the java.util package
2013-06-30 06:13:53 +00:00
import java.util.ArrayList;
2013-07-01 23:08:48 +00:00
// Import all classes inside of java.security package
2013-07-01 16:24:57 +00:00
import java.security.*;
2013-06-30 06:13:53 +00:00
2013-07-01 22:50:18 +00:00
// Each .java file contains one public class, with the same name as the file.
public class LearnJava {
2013-06-30 06:13:53 +00:00
2013-07-01 22:50:18 +00:00
// A program must have a main method as an entry point
public static void main (String[] args) {
2013-06-30 06:13:53 +00:00
2013-07-01 22:50:18 +00:00
// Use System.out.println to print lines
System.out.println("Hello World!");
2013-07-01 22:51:55 +00:00
System.out.println(
"Integer: " + 10 +
" Double: " + 3.14 +
" Boolean: " + true);
2013-06-30 06:13:53 +00:00
2013-07-01 22:50:18 +00:00
// To print without a newline, use System.out.print
System.out.print("Hello ");
System.out.print("World");
2013-06-30 06:13:53 +00:00
2013-07-01 22:50:18 +00:00
///////////////////////////////////////
// Types & Variables
///////////////////////////////////////
// Declare a variable using <type> <name> [
// Byte - 8-bit signed two's complement integer
// (-128 <= byte <= 127)
byte fooByte = 100;
// 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;
// Long - 64-bit signed two's complement integer
// (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
long fooLong = 100000L; // L is used to denote that this variable value is of type Long; else it is treated as integer by default.
2013-07-01 22:50:18 +00:00
// Note: Java has no unsigned types
2013-07-01 22:50:18 +00:00
// Float - Single-precision 32-bit IEEE 754 Floating Point
float fooFloat = 234.5f; // f is used to denote that this variable value is of type float; else it is treated as double.
2013-07-01 22:50:18 +00:00
// Double - Double-precision 64-bit IEEE 754 Floating Point
double fooDouble = 123.4;
// Boolean - true & false
boolean fooBoolean = true;
boolean barBoolean = false;
// Char - A single 16-bit Unicode character
char fooChar = 'A';
// Use final to make a variable immutable
final int HOURS_I_WORK_PER_WEEK = 9001;
// Strings
String fooString = "My String Is Here!";
// \n is an escaped character that starts a new line
String barString = "Printing on a new line?\nNo Problem!";
// \t is an escaped character that adds a tab character
String bazString = "Do you want to add a tab?\tNo Problem!";
2013-07-01 22:50:18 +00:00
System.out.println(fooString);
System.out.println(barString);
System.out.println(bazString);
2013-07-01 22:50:18 +00:00
// Arrays
//The array size must be decided upon declaration
//The format for declaring an array is follows:
//<datatype> [] <var name> = new <datatype>[<array size>];
int [] intArray = new int[10];
String [] stringArray = new String[1];
boolean [] booleanArray = new boolean[100];
// Another way to declare & initialize an array
int [] y = {9000, 1000, 1337};
// Indexing an array - Accessing an element
System.out.println("intArray @ 0: " + intArray[0]);
// Arrays are zero-indexed and mutable.
intArray[1] = 1;
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
// LinkedLists
// Maps
// HashMaps
///////////////////////////////////////
// Operators
///////////////////////////////////////
System.out.println("\n->Operators");
int i1 = 1, i2 = 2; // Shorthand for multiple declarations
// Arithmetic is straightforward
System.out.println("1+2 = " + (i1 + i2)); // => 3
System.out.println("2-1 = " + (i2 - i1)); // => 1
System.out.println("2*1 = " + (i2 * i1)); // => 2
2013-07-01 22:51:55 +00:00
System.out.println("1/2 = " + (i1 / i2)); // => 0 (0.5 truncated down)
2013-07-01 22:50:18 +00:00
// Modulo
System.out.println("11%3 = "+(11 % 3)); // => 2
// Comparison operators
System.out.println("3 == 2? " + (3 == 2)); // => false
System.out.println("3 != 2? " + (3 != 2)); // => true
System.out.println("3 > 2? " + (3 > 2)); // => true
System.out.println("3 < 2? " + (3 < 2)); // => false
System.out.println("2 <= 2? " + (2 <= 2)); // => true
System.out.println("2 >= 2? " + (2 >= 2)); // => true
2013-07-01 22:50:18 +00:00
// Bitwise operators!
/*
~ Unary bitwise complement
<< Signed left shift
>> Signed right shift
>>> Unsigned right shift
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise inclusive OR
[Java Bitwise Operators](http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op3.html)
2013-07-01 22:50:18 +00:00
*/
// Incrementations
int i = 0;
2013-07-01 22:50:18 +00:00
System.out.println("\n->Inc/Dec-rementation");
System.out.println(i++); //i = 1. Post-Incrementation
System.out.println(++i); //i = 2. Pre-Incrementation
System.out.println(i--); //i = 1. Post-Decrementation
System.out.println(--i); //i = 0. Pre-Decrementation
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
System.out.println("\n->Control Structures");
// If statements are c-like
int j = 10;
if (j == 10){ // Evaluates to true only if value of j is equal to 10.
System.out.println("In this case, I get executed and I print this since j is equal to 10.");
} else if (i > 10) { // Evaluates to true only if value of j is greater than 10.
System.out.println("I never run if j = 10; I will run only if j is greater than 10");
} else { // If none of the above if-conditions match, then control will fall back to else-condition; In this case, if value of j is less than 10.
System.out.println("I also never run if j = 10; I will run only if j is less than 10");
}
2013-06-30 06:13:53 +00:00
2013-07-01 22:50:18 +00:00
// While loop
int fooWhile = 0;
while(fooWhile < 100)
{
//System.out.println(fooWhile);
//Increment the counter
//Iterated 99 times, fooWhile 0->99
fooWhile++;
}
2013-07-01 22:50:18 +00:00
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
fooDoWhile++;
}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
}
2013-07-01 22:50:18 +00:00
System.out.println("fooFor Value: " + fooFor);
// Switch Case
//A switch works with the byte, short, char, and int primitive data types.
//It also works with enumerated types (discussed in Enum Types), the String class, and a few special classes that wrap certain primitive types: Character, Byte, Short, and Integer.
2013-07-01 22:50:18 +00:00
int month = 3;
String monthString;
switch (month){
case 1:
monthString = "January";
2013-07-01 22:50:18 +00:00
break;
case 2:
monthString = "February";
2013-07-01 22:50:18 +00:00
break;
case 3:
monthString = "March";
2013-07-01 22:50:18 +00:00
break;
default: //The default section handles all values that are not explicitly handled by one of the case sections.
monthString = "Some other month";
2013-07-01 22:50:18 +00:00
break;
}
2013-07-01 22:50:18 +00:00
System.out.println("Switch Case Result: " + monthString);
// Effective JDK 7.0, Strings can be used for case-matching in Switch statement.
final String dayOfWeek = "Friday";
switch (dayOfWeek) {
case "Monday":
System.out.println("Aargh!! Start of the work week!");
break;
case "Tuesday":
case "Wednesday":
case "Thursday":
System.out.println("Midweek");
break;
case "Friday":
System.out.println("Awesome! End of work week");
break;
case "Saturday":
case "Sunday":
System.out.println("Weekend");
break;
default:
System.out.println("There is no such weekday, at least on our planet.");
}
2013-07-01 22:50:18 +00:00
///////////////////////////////////////
// Converting Data Types And Typcasting
///////////////////////////////////////
// Converting data
// Convert String To Integer
Integer.parseInt("123");//returns an integer version of "123"
// Convert Integer To String
Integer.toString(123);//returns a string version of 123
// For other conversions check out the following classes:
// Double
// 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
///////////////////////////////////////
System.out.println("\n->Classes & Functions");
// (definition of the Bicycle class follows)
// Use new to instantiate a class
Bicycle trek = new Bicycle();
// Call object methods
trek.speedUp(3); // Should always use setter and getter methods to access the attributes of a class
2013-07-01 22:50:18 +00:00
trek.setCadence(100);
// toString is a convention to display the attribute values of this Object.
// Has to be overridden in the POJO (Plain Old Java Objects) to see the actual values else internal representation of the address will be displayed.
2013-07-01 22:50:18 +00:00
System.out.println("trek info: " + trek.toString());
} // End main method
} // End LearnJava class
// You can include other, non-public classes in a .java file
// Class Declaration Syntax:
// <public/private/protected> class <class name>{
// //data fields, constructors, functions all inside.
// //functions are called as methods in Java.
2013-07-01 22:50:18 +00:00
// }
class Bicycle {
// Bicycle's Fields/Variables
public int cadence; // Public: Can be accessed from anywhere
private int speed; // Private: Only accessible from within the class
protected int gear; // Protected: Accessible from the class and all the subclasses
String name; // default: Only accessible from within this package
2013-07-01 22:50:18 +00:00
// Constructors are a way of creating classes
// This is a default constructor
public Bicycle() {
gear = 1;
cadence = 50;
speed = 5;
name = "Bontrager";
2013-07-01 22:50:18 +00:00
}
// This is a specified constructor (it contains arguments)
public Bicycle(int startCadence, int startSpeed, int startGear, String name) {
this.gear = startGear;
this.cadence = startCadence;
this.speed = startSpeed;
this.name = name;
2013-06-30 06:13:53 +00:00
}
2013-07-01 22:50:18 +00:00
// Function 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>)
public int getCadence() {
return cadence;
}
// void methods require no return statement
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(int newName) {
name = newName;
}
public String getName() {
return name;
}
//Method to display the attribute values of this Object.
@Override
2013-07-01 22:50:18 +00:00
public String toString() {
return "gear: " + gear +
" cadence: " + cadence +
" speed: " + speed +
" name: " + name;
2013-07-01 22:50:18 +00:00
}
} // end class Bicycle
2013-07-02 10:06:22 +00:00
// PennyFarthing is a subclass of Bicycle
2013-07-01 22:50:18 +00:00
class PennyFarthing extends Bicycle {
// (Penny Farthings are those bicycles with the big front wheel.
// They have no gears.)
public PennyFarthing(int startCadence, int startSpeed){
// Call the parent constructor with super
super(startCadence, startSpeed, 0, "PennyFarthing");
2013-07-01 22:50:18 +00:00
}
// You should mark a method you're overriding with an @annotation
2013-07-05 01:18:42 +00:00
// To learn more about what annotations are and their purpose
// check this out: http://docs.oracle.com/javase/tutorial/java/annotations/
2013-07-01 22:50:18 +00:00
@Override
public void setGear(int gear) {
gear = 0;
}
2013-06-30 06:13:53 +00:00
}
2013-06-29 17:12:23 +00:00
```
## Further Reading
The links provided here below are just to get an understanding of the topic, feel free to Google and find specific examples.
2013-06-30 03:52:18 +00:00
Other Topics To Research:
2013-06-30 03:54:53 +00:00
* [Java Tutorial Trail from Sun / Oracle](http://docs.oracle.com/javase/tutorial/index.html)
2013-06-30 06:13:53 +00:00
* [Java Access level modifiers](http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html)
2013-06-30 06:13:53 +00:00
* [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)
2013-06-30 06:13:53 +00:00
2013-06-30 12:50:04 +00:00
* [Exceptions](http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html)
2013-06-30 06:13:53 +00:00
2013-06-30 12:50:04 +00:00
* [Interfaces](http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html)
* [Generics](http://docs.oracle.com/javase/tutorial/java/generics/index.html)
2013-06-30 06:13:53 +00:00
2013-07-01 14:32:19 +00:00
* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconv-138413.html)