Java Basics 1.01

Main Class

public static void main(String[] args) {

 

Required by Class:

• public - can be called anywhere
• static - no instance required to run
• void - nothing returned by class
• String[] args
  • [] - an array
  • args - default variable to hold passed data
  • Passed by Java’s JVM

 

 

Passing Args

Command Line

java Main arg1 arg2

In Eclipse:

Documentation

• http://docs.oracle.com/javase/6/docs/api/
• Mouseover, F2
• Windows > View > Help

Garbage Collection

• Objects referenced created in heap memory
• As long as variable referenced, it’s retained
• When referenced expire, they’re eligible to be garbage collected
• Garbage Collector runs own thread
• Can’t force garbage collection
• OutOfMemoryError thrown if memory runs out

Expiration

• Variable to local functions or blocks expire when function is complete
• Set value to null

Tips for Managing Memory

• Minimize number of object created
• Runtime.*
  • Runtime.maxMemory()
  • Runtime.totalMemory()
  • Runtime.freeMemory()
• java -Xms256s HelloWorld - Initial heap size
• java -Xms256m HelloWorld - Max heap size
• java -Xms256n HelloWorld - Heap size for young generation objects

 

Classes and Objects

//Starting class has main() method

public class SimpleApplication {

     //

     public static void main(String[] args) {

           // Welcomer = datatype

           // welcomer is new instance

           Welcomer welcomer = new Welcomer();

           welcomer.sayHello();

     }

}

 

public class Welcomer {

// instance variable = welcome - private to the class

private String welcome = “Hello”;

     public void sayHello() {

           System.out.println(welcome);

     }

}

 

String variables objects

String is class

String welcome = “Hello!”;

String welcome = new String(“Hello!”);

String = array[H,e,l,l,o,!]

 

Same as:

char[] chars = {‘H’,’e’,’l’,’l’,’o’,’!’};

String s = new String(chars);

 

 

Types of Variables

• Primitives - stored directly in memory
  • Numerics - ints, floating point decimals
  • Single characters
  • Boolean (true/false)
• Complex objects
  • Strings
  • Dates
  • Everything else

 

Declaring a Primitive Varialbe

• Data Type - Required
• Variable name - Required
• Initial value - option
  • int newVariable = 10;
  • [Data type] [Variable name] = [Initial value];
  • variable name MUST start with lowercase
  • Numeric default = 0
  • Boolean default = false

Declaring a Complex Variable

• instance of classes
• declared in 3 parts
• Init uses new keyword and class constructor
• Inital value - optional, built from class constructor
• Date newDate = new Date();
• [Data type] [Variable name] = [Initial value from constructor]
• Variable name alpha char or _
• Date newDate = null

 

Scope

• local vars - declared inside function (de-referenced)
  • void doSomething()
• public vars -
  • public doSomethingElse()

 

Class variables = Field variable

• Declared outside a class method

 

public class MyClass {

     String sayHello = new String(“Hello”);

     void doSomething() {

           System.out.println(sayHello);

     }

}