Static

can be used without calling obj

public class RoundStuff
{
	public static final double PI = 3.14
    public static final double area(double radius)
    {
    	return ( PI * ~~~)
    }
}

public class RoundStuffDemo
{
	public static void main(String[] args)
    {
    	System.out.println( RoundStuff.area(radius) + "square inches."); 
}

static method :

-cannot refer to an instance variable of the class
-cannot invoke a non-static method of the class
-a static method can invoke another static method
-a static method has no this (static method belongs to class area, so no instance created)

static variables:

-belongs to the class as a whole, and not just to one obj
-there's only one copy of a static variable per class (cf. each obj has its own copy of instance variables)
-all obj of the class can read and change a static variable
-a static method can access static variable

-static variables can be used in static, non static both
-non static variables cannot be used in static

private static int myStaticVariable;

-declared and initialized at the same time (recommended) / if not, explicitly initialized
private static int myStaticVariable = 0;

-a static variable should always be defined private
exception: constant ( pi=3.14, BIRTH_YEAR = 1954 ...)

math class

no import needed
all methods & data are static
pow a b a 의 b승
abs 절대값
min
max
round 반올림
ceil 올림
floor 버림
sqrt 제곱

Wrapper Classes

boxing

Byte bObj = 5;
Short sObj = 15;

unboxing

byte b = bObj.byteValue();
short s = sObj.shortValue();
short s = sObj; //automatic unboxing

Integer.MAX_VALUE, MIN
Double.MAX_VALUE, MIN
Boolean.TRUE
parseInt
parseDouble
toString
char toUpperCase
char toLowerCase
boolean isUpperCase, Lower
boolean isWhitespace
boolean isLetter
boolean isDigit
boolean isLetterOrDigit

References

main memory consists of a long list of num address called byte
memory location: entire chunk of memory that holds the data
reference: memory address where an obj is located
class type = reference type

variable1과 variable2가 같은 주소를 가짐, 같은 변수를 갖게됨

call by value - parameters in java

any change to the value of the parameter cannot change the value of its argument

call by reference - class type parameters

any change to the value of parameter can change the value of its argument

Pitfall: use of == with variables of a class type

constant null

special constant that may be assigned to a variable of any class type
method cannot be invoked using a variable that is initialized to null

anonymous obj

usingMisusingReferences (중요)

public Person(Person original)
    {
        if (original == null)
        {
            System.out.println("Fatal error.");
            System.exit(0);
        }

        name = original.name;
        born = new Date(original.born);

        if (original.died == null)
            died = null;
        else
            died = new Date(original.died);
    }

Person class:
obj of the class Person has a date of birth(not null)
date of death is equal to or later than the date of birth

copy constructors

should create an obj that is a separate, independent
values are same as original

Privacy Leaks

string은 copy constructors 안써도댐

Mutable & Immutable Classes

class String contains no mutator methods that can change any of the data in a String obj
String is immutable class

Deep copy vs Shallow copy

Deep copy: copy that has no references in common with the original (예외: immutable obj)
Shallow copy: not a deep copy, can cause dangerous privacy leak

Packages and Javadoc

1. import A.B.C;
2. import A.B.*;
   2번이 1번을 포함하지 않음

java.lang

contains fundamental classes (Math, String, wrapper ...)
import automatically

javadoc

automatically produce documentation

/**
@param
@return
@throws
@deprecated
@see
@author
@version 
*/ 

to run javadoc, give the following command in console
javadoc -d Documentation_Directory Package_Name
javadoc ClassName.java
javadoc *.java