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16 KiB
| language | contributors | filename | ||||||
|---|---|---|---|---|---|---|---|---|
| java |
|
LearnJava.java |
Java is a general-purpose, concurrent, class-based, object-oriented computer programming language. Read more here.
// 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 - An 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
// An automatic iteration through an array or list of objects.
int[] fooList = {1,2,3,4,5,6,7,8,9};
//for each loop structure => for(<object> : <array_object>)
//reads as: for each object in the array
//note: the object type must match the array.
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 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); // 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 {
public void eat() {
//...
}
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 {
public void InterfaceOneMethod() {
}
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:
Online Practice and Tutorials
Books: