mirror of
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412 lines
13 KiB
Markdown
412 lines
13 KiB
Markdown
---
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language: java
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contributors:
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- ["Jake Prather", "http://github.com/JakeHP"]
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filename: LearnJava.java
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---
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Java is a general-purpose, concurrent, class-based, object-oriented computer programming language.
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[Read more here.](http://docs.oracle.com/javase/tutorial/java/index.html)
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```java
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// Single-line comments start with //
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/*
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Multi-line comments look like this.
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*/
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/**
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JavaDoc comments look like this. Used to describe the Class or various
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attributes of a Class.
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*/
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// Import ArrayList class inside of the java.util package
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import java.util.ArrayList;
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// Import all classes inside of java.security package
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import java.security.*;
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// Each .java file contains one public class, with the same name as the file.
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public class LearnJava {
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// A program must have a main method as an entry point
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public static void main (String[] args) {
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// Use System.out.println to print lines
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System.out.println("Hello World!");
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System.out.println(
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"Integer: " + 10 +
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" Double: " + 3.14 +
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" Boolean: " + true);
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// To print without a newline, use System.out.print
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System.out.print("Hello ");
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System.out.print("World");
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///////////////////////////////////////
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// Types & Variables
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///////////////////////////////////////
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// Declare a variable using <type> <name> [
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// Byte - 8-bit signed two's complement integer
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// (-128 <= byte <= 127)
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byte fooByte = 100;
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// Short - 16-bit signed two's complement integer
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// (-32,768 <= short <= 32,767)
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short fooShort = 10000;
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// Integer - 32-bit signed two's complement integer
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// (-2,147,483,648 <= int <= 2,147,483,647)
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int fooInt = 1;
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// Long - 64-bit signed two's complement integer
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// (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
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long fooLong = 100000L;
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// L is used to denote that this variable value is of type Long;
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// anything without is treated as integer by default.
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// Note: Java has no unsigned types
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// Float - Single-precision 32-bit IEEE 754 Floating Point
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float fooFloat = 234.5f;
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// f is used to denote that this variable value is of type float;
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// otherwise it is treated as double.
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// Double - Double-precision 64-bit IEEE 754 Floating Point
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double fooDouble = 123.4;
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// Boolean - true & false
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boolean fooBoolean = true;
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boolean barBoolean = false;
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// Char - A single 16-bit Unicode character
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char fooChar = 'A';
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// Use final to make a variable immutable
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final int HOURS_I_WORK_PER_WEEK = 9001;
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// Strings
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String fooString = "My String Is Here!";
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// \n is an escaped character that starts a new line
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String barString = "Printing on a new line?\nNo Problem!";
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// \t is an escaped character that adds a tab character
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String bazString = "Do you want to add a tab?\tNo Problem!";
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System.out.println(fooString);
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System.out.println(barString);
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System.out.println(bazString);
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// Arrays
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//The array size must be decided upon declaration
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//The format for declaring an array is follows:
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//<datatype> [] <var name> = new <datatype>[<array size>];
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int [] intArray = new int[10];
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String [] stringArray = new String[1];
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boolean [] booleanArray = new boolean[100];
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// Another way to declare & initialize an array
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int [] y = {9000, 1000, 1337};
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// Indexing an array - Accessing an element
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System.out.println("intArray @ 0: " + intArray[0]);
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// Arrays are zero-indexed and mutable.
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intArray[1] = 1;
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System.out.println("intArray @ 1: " + intArray[1]); // => 1
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// Others to check out
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// ArrayLists - Like arrays except more functionality is offered,
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// and the size is mutable
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// LinkedLists
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// Maps
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// HashMaps
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///////////////////////////////////////
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// Operators
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///////////////////////////////////////
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System.out.println("\n->Operators");
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int i1 = 1, i2 = 2; // Shorthand for multiple declarations
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// Arithmetic is straightforward
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System.out.println("1+2 = " + (i1 + i2)); // => 3
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System.out.println("2-1 = " + (i2 - i1)); // => 1
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System.out.println("2*1 = " + (i2 * i1)); // => 2
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System.out.println("1/2 = " + (i1 / i2)); // => 0 (0.5 truncated down)
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// Modulo
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System.out.println("11%3 = "+(11 % 3)); // => 2
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// Comparison operators
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System.out.println("3 == 2? " + (3 == 2)); // => false
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System.out.println("3 != 2? " + (3 != 2)); // => true
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System.out.println("3 > 2? " + (3 > 2)); // => true
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System.out.println("3 < 2? " + (3 < 2)); // => false
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System.out.println("2 <= 2? " + (2 <= 2)); // => true
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System.out.println("2 >= 2? " + (2 >= 2)); // => true
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// Bitwise operators!
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/*
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~ Unary bitwise complement
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<< Signed left shift
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>> Signed right shift
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>>> Unsigned right shift
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& Bitwise AND
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^ Bitwise exclusive OR
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| Bitwise inclusive OR
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*/
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// Incrementations
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int i = 0;
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System.out.println("\n->Inc/Dec-rementation");
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System.out.println(i++); //i = 1. Post-Incrementation
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System.out.println(++i); //i = 2. Pre-Incrementation
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System.out.println(i--); //i = 1. Post-Decrementation
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System.out.println(--i); //i = 0. Pre-Decrementation
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///////////////////////////////////////
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// Control Structures
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///////////////////////////////////////
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System.out.println("\n->Control Structures");
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// If statements are c-like
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int j = 10;
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if (j == 10){
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System.out.println("I get printed");
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} else if (j > 10) {
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System.out.println("I don't");
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} else {
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System.out.println("I also don't");
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}
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// While loop
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int fooWhile = 0;
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while(fooWhile < 100)
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{
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//System.out.println(fooWhile);
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//Increment the counter
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//Iterated 99 times, fooWhile 0->99
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fooWhile++;
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}
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System.out.println("fooWhile Value: " + fooWhile);
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// Do While Loop
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int fooDoWhile = 0;
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do
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{
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//System.out.println(fooDoWhile);
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//Increment the counter
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//Iterated 99 times, fooDoWhile 0->99
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fooDoWhile++;
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}while(fooDoWhile < 100);
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System.out.println("fooDoWhile Value: " + fooDoWhile);
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// For Loop
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int fooFor;
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//for loop structure => for(<start_statement>; <conditional>; <step>)
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for(fooFor=0; fooFor<10; fooFor++){
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//System.out.println(fooFor);
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//Iterated 10 times, fooFor 0->9
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}
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System.out.println("fooFor Value: " + fooFor);
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// Switch Case
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// A switch works with the byte, short, char, and int data types.
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// It also works with enumerated types (discussed in Enum Types),
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// the String class, and a few special classes that wrap
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// primitive types: Character, Byte, Short, and Integer.
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int month = 3;
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String monthString;
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switch (month){
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case 1:
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monthString = "January";
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break;
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case 2:
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monthString = "February";
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break;
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case 3:
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monthString = "March";
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break;
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default:
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monthString = "Some other month";
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break;
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}
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System.out.println("Switch Case Result: " + monthString);
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///////////////////////////////////////
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// Converting Data Types And Typcasting
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///////////////////////////////////////
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// Converting data
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// Convert String To Integer
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Integer.parseInt("123");//returns an integer version of "123"
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// Convert Integer To String
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Integer.toString(123);//returns a string version of 123
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// For other conversions check out the following classes:
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// Double
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// Long
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// String
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// Typecasting
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// You can also cast java objects, there's a lot of details and
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// deals with some more intermediate concepts.
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// Feel free to check it out here:
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// http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
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///////////////////////////////////////
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// Classes And Functions
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///////////////////////////////////////
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System.out.println("\n->Classes & Functions");
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// (definition of the Bicycle class follows)
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// Use new to instantiate a class
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Bicycle trek = new Bicycle();
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// Call object methods
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trek.speedUp(3); // You should always use setter and getter methods
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trek.setCadence(100);
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// toString is a convention to display the value of this Object.
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System.out.println("trek info: " + trek.toString());
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} // End main method
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} // End LearnJava class
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// You can include other, non-public classes in a .java file
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// Class Declaration Syntax:
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// <public/private/protected> class <class name>{
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// //data fields, constructors, functions all inside.
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// //functions are called as methods in Java.
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// }
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class Bicycle {
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// Bicycle's Fields/Variables
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public int cadence; // Public: Can be accessed from anywhere
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private int speed; // Private: Only accessible from within the class
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protected int gear; // Protected: Accessible from the class and subclasses
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String name; // default: Only accessible from within this package
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// Constructors are a way of creating classes
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// This is a default constructor
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public Bicycle() {
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gear = 1;
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cadence = 50;
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speed = 5;
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name = "Bontrager";
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}
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// This is a specified constructor (it contains arguments)
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public Bicycle(int startCadence, int startSpeed, int startGear, String name) {
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this.gear = startGear;
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this.cadence = startCadence;
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this.speed = startSpeed;
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this.name = name;
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}
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// Function Syntax:
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// <public/private/protected> <return type> <function name>(<args>)
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// Java classes often implement getters and setters for their fields
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// Method declaration syntax:
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// <scope> <return type> <method name>(<args>)
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public int getCadence() {
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return cadence;
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}
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// void methods require no return statement
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public void setCadence(int newValue) {
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cadence = newValue;
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}
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public void setGear(int newValue) {
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gear = newValue;
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}
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public void speedUp(int increment) {
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speed += increment;
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}
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public void slowDown(int decrement) {
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speed -= decrement;
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}
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public void setName(String newName) {
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name = newName;
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}
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public String getName() {
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return name;
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}
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//Method to display the attribute values of this Object.
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@Override
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public String toString() {
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return "gear: " + gear +
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" cadence: " + cadence +
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" speed: " + speed +
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" name: " + name;
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}
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} // end class Bicycle
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// PennyFarthing is a subclass of Bicycle
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class PennyFarthing extends Bicycle {
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// (Penny Farthings are those bicycles with the big front wheel.
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// They have no gears.)
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public PennyFarthing(int startCadence, int startSpeed){
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// Call the parent constructor with super
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super(startCadence, startSpeed, 0, "PennyFarthing");
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}
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// You should mark a method you're overriding with an @annotation
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// To learn more about what annotations are and their purpose
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// check this out: http://docs.oracle.com/javase/tutorial/java/annotations/
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@Override
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public void setGear(int gear) {
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gear = 0;
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}
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}
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```
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## Further Reading
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The links provided here below are just to get an understanding of the topic, feel free to Google and find specific examples.
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Other Topics To Research:
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* [Java Tutorial Trail from Sun / Oracle](http://docs.oracle.com/javase/tutorial/index.html)
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* [Java Access level modifiers](http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html)
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* [Object-Oriented Programming Concepts](http://docs.oracle.com/javase/tutorial/java/concepts/index.html):
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* [Inheritance](http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html)
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* [Polymorphism](http://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html)
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* [Abstraction](http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html)
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* [Exceptions](http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html)
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* [Interfaces](http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html)
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* [Generics](http://docs.oracle.com/javase/tutorial/java/generics/index.html)
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* [Java Code Conventions](http://www.oracle.com/technetwork/java/codeconv-138413.html)
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Books:
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* [Head First Java] (http://www.headfirstlabs.com/books/hfjava/)
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