mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-11-24 15:03:19 +03:00
495 lines
16 KiB
Markdown
495 lines
16 KiB
Markdown
---
|
|
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/"]
|
|
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/)
|
|
|
|
```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.
|
|
*/
|
|
|
|
// Import ArrayList class inside of the java.util package
|
|
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.
|
|
public class LearnJava {
|
|
|
|
// A program must have a main method as an entry point.
|
|
public static void main (String[] args) {
|
|
|
|
// Use System.out.println() to print lines.
|
|
System.out.println("Hello World!");
|
|
System.out.println(
|
|
"Integer: " + 10 +
|
|
" Double: " + 3.14 +
|
|
" Boolean: " + true);
|
|
|
|
// To print without a newline, use System.out.print().
|
|
System.out.print("Hello ");
|
|
System.out.print("World");
|
|
|
|
|
|
///////////////////////////////////////
|
|
// 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;
|
|
// anything without is treated as integer by default.
|
|
|
|
// Note: Java has no unsigned types.
|
|
|
|
// 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;
|
|
// otherwise it is treated as double.
|
|
|
|
// 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';
|
|
|
|
// final variables can't be reassigned to another object.
|
|
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!";
|
|
System.out.println(fooString);
|
|
System.out.println(barString);
|
|
System.out.println(bazString);
|
|
|
|
// Arrays
|
|
// The array size must be decided upon instantiation
|
|
// The following formats work for declaring an array
|
|
// <datatype>[] <var name> = new <datatype>[<array size>];
|
|
// <datatype> <var name>[] = new <datatype>[<array size>];
|
|
int[] intArray = new int[10];
|
|
String[] stringArray = new String[1];
|
|
boolean boolArray[] = new boolean[100];
|
|
|
|
// 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};
|
|
|
|
// 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 - 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 maps keys to values. A map cannot
|
|
// contain duplicate keys; each key can map to at most one value.
|
|
// 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.
|
|
|
|
///////////////////////////////////////
|
|
// 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
|
|
System.out.println("1/2 = " + (i1 / i2)); // => 0 (0.5 truncated down)
|
|
|
|
// 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
|
|
|
|
// Bitwise operators!
|
|
/*
|
|
~ Unary bitwise complement
|
|
<< Signed left shift
|
|
>> Signed right shift
|
|
>>> Unsigned right shift
|
|
& Bitwise AND
|
|
^ Bitwise exclusive OR
|
|
| Bitwise inclusive OR
|
|
*/
|
|
|
|
// Incrementations
|
|
int i = 0;
|
|
System.out.println("\n->Inc/Dec-rementation");
|
|
// The ++ and -- operators increment and decrement by 1 respectively.
|
|
// If they are placed before the variable, they increment then return;
|
|
// after the variable they return then increment.
|
|
System.out.println(i++); // i = 1, prints 0 (post-increment)
|
|
System.out.println(++i); // i = 2, prints 2 (pre-increment)
|
|
System.out.println(i--); // i = 1, prints 2 (post-decrement)
|
|
System.out.println(--i); // i = 0, prints 0 (pre-decrement)
|
|
|
|
///////////////////////////////////////
|
|
// Control Structures
|
|
///////////////////////////////////////
|
|
System.out.println("\n->Control Structures");
|
|
|
|
// If statements are c-like
|
|
int j = 10;
|
|
if (j == 10){
|
|
System.out.println("I get printed");
|
|
} else if (j > 10) {
|
|
System.out.println("I don't");
|
|
} else {
|
|
System.out.println("I also don't");
|
|
}
|
|
|
|
// While loop
|
|
int fooWhile = 0;
|
|
while(fooWhile < 100) {
|
|
System.out.println(fooWhile);
|
|
// Increment the counter
|
|
// Iterated 100 times, fooWhile 0,1,2...99
|
|
fooWhile++;
|
|
}
|
|
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
|
|
}
|
|
System.out.println("fooFor Value: " + fooFor);
|
|
|
|
// For Each Loop
|
|
// The for loop is also able to iterate over arrays as well as objects
|
|
// that implement the Iterable interface.
|
|
int[] fooList = {1, 2, 3, 4, 5, 6, 7, 8, 9};
|
|
// 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
|
|
}
|
|
|
|
// Switch Case
|
|
// A switch works with the byte, short, char, and int data types.
|
|
// 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.
|
|
int month = 3;
|
|
String monthString;
|
|
switch (month) {
|
|
case 1: monthString = "January";
|
|
break;
|
|
case 2: monthString = "February";
|
|
break;
|
|
case 3: monthString = "March";
|
|
break;
|
|
default: monthString = "Some other month";
|
|
break;
|
|
}
|
|
System.out.println("Switch Case Result: " + monthString);
|
|
|
|
// 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
|
|
|
|
|
|
////////////////////////////////////////
|
|
// Converting Data Types And Typecasting
|
|
////////////////////////////////////////
|
|
|
|
// 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); // You should always use setter and getter methods
|
|
trek.setCadence(100);
|
|
|
|
// toString returns this Object's string representation.
|
|
System.out.println("trek info: " + trek.toString());
|
|
|
|
} // End main method
|
|
} // End LearnJava class
|
|
|
|
|
|
// You can include other, non-public outer-level 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.
|
|
// }
|
|
|
|
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 subclasses
|
|
String name; // default: Only accessible from within this package
|
|
|
|
// Constructors are a way of creating classes
|
|
// This is a constructor
|
|
public Bicycle() {
|
|
gear = 1;
|
|
cadence = 50;
|
|
speed = 5;
|
|
name = "Bontrager";
|
|
}
|
|
|
|
// This is a constructor that takes arguments
|
|
public Bicycle(int startCadence, int startSpeed, int startGear,
|
|
String name) {
|
|
this.gear = startGear;
|
|
this.cadence = startCadence;
|
|
this.speed = startSpeed;
|
|
this.name = name;
|
|
}
|
|
|
|
// 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(String newName) {
|
|
name = newName;
|
|
}
|
|
|
|
public String getName() {
|
|
return name;
|
|
}
|
|
|
|
//Method to display the attribute values of this Object.
|
|
@Override
|
|
public String toString() {
|
|
return "gear: " + gear + " cadence: " + cadence + " speed: " + speed +
|
|
" name: " + name;
|
|
}
|
|
} // end class Bicycle
|
|
|
|
// PennyFarthing is a subclass of Bicycle
|
|
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");
|
|
}
|
|
|
|
// You should mark a method you're overriding with an @annotation.
|
|
// To learn more about what annotations are and their purpose check this
|
|
// out: http://docs.oracle.com/javase/tutorial/java/annotations/
|
|
@Override
|
|
public void setGear(int gear) {
|
|
gear = 0;
|
|
}
|
|
}
|
|
|
|
// Interfaces
|
|
// Interface declaration syntax
|
|
// <access-level> interface <interface-name> extends <super-interfaces> {
|
|
// // Constants
|
|
// // Method declarations
|
|
// }
|
|
|
|
// Example - Food:
|
|
public interface Edible {
|
|
public void eat(); // Any class that implements this interface, must
|
|
// implement this method.
|
|
}
|
|
|
|
public interface Digestible {
|
|
public void digest();
|
|
}
|
|
|
|
|
|
// We can now create a class that implements both of these interfaces.
|
|
public class Fruit implements Edible, Digestible {
|
|
@Override
|
|
public void eat() {
|
|
// ...
|
|
}
|
|
|
|
@Override
|
|
public void digest() {
|
|
// ...
|
|
}
|
|
}
|
|
|
|
// In Java, you can extend only one class, but you can implement many
|
|
// interfaces. For example:
|
|
public class ExampleClass extends ExampleClassParent implements InterfaceOne,
|
|
InterfaceTwo {
|
|
@Override
|
|
public void InterfaceOneMethod() {
|
|
}
|
|
|
|
@Override
|
|
public void InterfaceTwoMethod() {
|
|
}
|
|
}
|
|
```
|
|
|
|
## Further Reading
|
|
|
|
The links provided here below are just to get an understanding of the topic, feel free to Google and find specific examples.
|
|
|
|
**Official Oracle Guides**:
|
|
|
|
* [Java Tutorial Trail from Sun / Oracle](http://docs.oracle.com/javase/tutorial/index.html)
|
|
|
|
* [Java Access level modifiers](http://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)
|
|
|
|
* [Exceptions](http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html)
|
|
|
|
* [Interfaces](http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html)
|
|
|
|
* [Generics](http://docs.oracle.com/javase/tutorial/java/generics/index.html)
|
|
|
|
* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconv-138413.html)
|
|
|
|
**Online Practice and Tutorials**
|
|
|
|
* [Learneroo.com - Learn Java](http://www.learneroo.com)
|
|
|
|
* [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)
|
|
|
|
* [Java The Complete Reference](http://www.amazon.com/gp/product/0071606300)
|