What is static Static class Test static int

  • Slides: 11
Download presentation
What is static?

What is static?

Static? • 靜態? class Test { static int static. X; int instance. X; public

Static? • 靜態? class Test { static int static. X; int instance. X; public Test(int var 1, int var 2) { this. static. X = var 1; this. instance. X = var 2; } } • 差別在於，instance. X是一個”instance member”，而 static. X是一個”class member”

當我們產生很多物件時… • • Test obj 1 = new Test(1, 2); Test obj 2 =

當我們產生很多物件時… • • Test obj 1 = new Test(1, 2); Test obj 2 = new Test(3, 4); Test obj 3 = new Test(5, 6); 物件obj 1, obj 2, obj 3所指向的實體，會各自 有各自的instance. X – System. out. println(obj 1. instance. X); //印出 2 – System. out. println(obj 2. instance. X); //印出 4 – System. out. println(obj 3. instance. X); //印出 6

Example: Account. java class Account { int money; String name; static double rate =

Example: Account. java class Account { int money; String name; static double rate = 0. 002; //0. 2% public Account(String name, int money) { this. name = name; this. money = money; } }

Example 2: 數學 class Math { public static final double PI = 3. 1415926;

Example 2: 數學 class Math { public static final double PI = 3. 1415926; } • 我們會使用 Math. PI 來取得這個公定的常數

用在 Method 時呢? • 不需要有instance，即可以使用 • 再看Account的例子 class Account { private int money; private

用在 Method 時呢? • 不需要有instance，即可以使用 • 再看Account的例子 class Account { private int money; private String name; private static double rate = 0. 002; //0. 2% public Account(String name, int money) { this. name = name; this. money = money; } public static double get. Rate() { return rate; } }

Static method • 不存在任何實體即可使用，則 • System. out. println( Account. get. Rate() ); //印出 0.

Static method • 不存在任何實體即可使用，則 • System. out. println( Account. get. Rate() ); //印出 0. 002 • 雖然我們也可以產生實體後再以實體的 reference variable來存取 – Account acc 1 = new Account(“John”, 1000); – System. out. println( acc 1. get. Rate() ); • 但是這樣的寫法是”不被建議的”，造成誤解， 雖然compile可以過，但Eclipse會警告，要你改 成Account. get. Rate()

更Advance的問題 • Class. Loader? • 有一個class，名字叫做java. lang. Class (注意 大小寫) – 前者是說”有一個類別” – 後者是說”這個類別名字叫java.

更Advance的問題 • Class. Loader? • 有一個class，名字叫做java. lang. Class (注意 大小寫) – 前者是說”有一個類別” – 後者是說”這個類別名字叫java. lang. Class” • 有一種寫法 – Class klass = Class. for. Name(“java. lang. String”); – klass. new. Instance(); • Why?

7 n int factorial int n int factorialint7 n int factorial int n int factorialint
int Sequential Search int k in A intint Sequential Search int k in A int
n int max int a int b returnn int max int a int b return
3 int sumint a int b int c3 int sumint a int b int c
void polygonint a int b int c intvoid polygonint a int b int c int
Windows SOCKET WSASocket int af int type intWindows SOCKET WSASocket int af int type int
void mulint a int b int c intvoid mulint a int b int c int
int led 3 int brightness 0 int incrementint led 3 int brightness 0 int increment
typedef struct int xsize int ysize int leveltypedef struct int xsize int ysize int level
1 int cadbank4 int ojtkorea ojtkorea cadbank Int1 int cadbank4 int ojtkorea ojtkorea cadbank Int
3 2 int dec int a int b3 2 int dec int a int b
Trie struct node int next26 int fail intTrie struct node int next26 int fail int
int main int x 2 int y xint main int x 2 int y x
dowhile int calculatorint a int b do intdowhile int calculatorint a int b do int
typedef struct int xsize int ysize int leveltypedef struct int xsize int ysize int level
include stdio h int main int x intinclude stdio h int main int x int
7 n int factorial int n int factorialint7 n int factorial int n int factorialint
Thought experiment static int y 0 int mainintThought experiment static int y 0 int mainint
1 void Xint lines int static blank int1 void Xint lines int static blank int
static int cubicint x int y y xstatic int cubicint x int y y x
Public static int Bigint x int y BigPublic static int Bigint x int y Big
1 public static int divideint a int b1 public static int divideint a int b
4 static void func static int counter 04 static void func static int counter 0
Robot Class name int street int avenue DirectionRobot Class name int street int avenue Direction
class CInt Ptr int mp Int public CIntclass CInt Ptr int mp Int public CInt
public class Television int channel int volume booleanpublic class Television int channel int volume boolean
class Employee int salary int bonus public voidclass Employee int salary int bonus public void
o class Point int x y public into class Point int x y public int
class AVLNode public int dann public int balanceclass AVLNode public int dann public int balance
1 1 class Car int colornumber int doornumber1 1 class Car int colornumber int doornumber
class Geometric public Geometricint base int length intclass Geometric public Geometricint base int length int
class Car Array private int position private intclass Car Array private int position private int
int int main coord o 110 10 classint int main coord o 110 10 class
class Tdate public void Tdate int int voidclass Tdate public void Tdate int int void