What is static Static class Test static int

What is static?

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 = 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 = 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; } • 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 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. 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. lang. Class” • 有一種寫法 – Class klass = Class. for. Name(“java. lang. String”); – klass. new. Instance(); • Why?