Introduction to Java
Java is a high-level, general-purpose, compiled programming language. Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of computer architecture. It is intended to let application developers “write once, run anywhere” (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation.
History
Java was originally developed by James Gosling at Sun Microsystems (which has since been acquired by Oracle Corporation) and released in 1995 as a core component of Sun Microsystems’ Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them. The original and reference implementation Java compilers, virtual machines, and class libraries were originally released by Sun under proprietary licenses. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of its Java technologies under the GNU General Public License.
Java Resources
- Getting Started: Language Download
- Getting Started: Development Tools
- Getting Started: Testing
- Getting Started: Deployment
- Beginner Tutorials
- Intermediate Tutorials
- Advanced Tutorials
- Good Beginner Blog
- Good Intermediate Blog
- Good Advanced Blog
- Official Documentation
Quick Reference – Java
Here is a quick reference for the basics of Java…
Hello World in Java
App.java import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; public class App { public static void main(String[] args) { System.out.println("What is your name?"); try { BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in)); String yourName = bufferedReader.readLine(); System.out.println("Hello " + yourName + "!"); } catch(IOException e) { e.printStackTrace(); } } }
Java: Basic Syntax
Variables & Data Types
Wrapper classes provide methods like .parseInt() or .toString(). Primitives are more efficient.
Primitives Wrapper classes boolean Boolean byte Byte short Short int Integer long Long float Float double Double char Character, String
Branching
Note: == applies only to numbers, so to compare strings (or other objects), use string1.equals(string2)
if (condition) { // do something } else if (condition) { // do something else } else { // do something else }
Switch statements:
int var = 0; switch(var) { case 1: // do first thing break; case 2: // do second thing break; default: // what to do if nothing matches... break; }
Arrays
Note: You cannot change the length of an array after it is declared.
int[] primeNumbers = { 2, 3, 5, 7, 11 };
Array Lists
import java.util.List; import java.util.ArrayList; List primeNumbers = new ArrayList {2, 3, 5, 7, 13}; primeNumbers.add(17); primeNumbers.get(1); //returns 3 in this case...
Hash Maps
import java.util.Map; import java.util.HashMap; Map<String, String> mapName = new HashMap<string, string>();
Loops
You can loop through all the items in an array:
string[] myList = {"one", "two", "three", "four", "five"}; for (String item : myList) { System.out.println(item); }
You can loop a fixed number of times:
for (int i = 0; i < 5; i++) { System.out.println(i); }
You can loop while a condition is true:
while (true) { // do something that may change the condition }
You can loop until a condition is false (always loops once):
do { // do something that may change the condition } while (true);
Java: Object-Oriented Syntax
Classes
Classes are defined in a file with the same name. All names must be uppercase:
Animal.java class Animal { public string Species; public string Name; public string Sound; }
Objects
Animal myPet = new Animal(); myPet.Species = "dog"; myPet.Name = "Fido"; myPet.Sound = "Bow-wow";
Methods
MyCalculator.java class MyCalculator { public int Add(int num1, int num2) { return num1 + num2; } }
Constructors
Animal.java class Animal { public string Species; public string Name; public string Sound; public Animal(string species, string name, string sound) { Species = species; Name = name; Sound = sound; } } Animal myPet = new Animal("dog", "Fido", "Bow-wow"); Animal yourPet = new Animal("cat", "Garfield", "Screech! Hiss!");