What is static? CS340100, NTHU Yoshi

Static? 靜態 ? class Test { static int staticX; int instanceX; public Test(int var1, int var2) { this.staticX = var1; this.instanceX = var2; } Instance members belong to the individual objects Class members belong to the class, shared by all the objects

When we creates many objects… Test obj1 = new Test(1,2); Test obj2 = new Test(3,4); Test obj3 = new Test(5,6); Objects (obj1, obj2, obj3) have their own instanceX – System.out.println(obj1.instanceX); //prints 2 – System.out.println(obj2.instanceX); //prints 4 – System.out.println(obj3.instanceX); //prints 6

Static Variable are Shared Static variables do not belong to an individual object In the code above, the value of staticX are modified to 1, 3, 5, sequentially – They were modifying the same one (they?) – Static variables belong to the class – this.staticX = 3 is equivalent to Test.staticX = 3

When will you declare a field as static? Let’s see an example – You are creating a class for representing the accounts of a bank Each account has the user name, the saving, and the interest rate – Each user has different name – Each user has different saving – The interest rate are identical, shared by all the accounts

Example: Account.java class Account { int money; String name; static double rate = 0.02; //2% public Account(String name, int money) { this.name = name; this.money = money; }

Example2: 數學 class Math { public static final double PI = ; } 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 getRate() { return rate; }

Static method 不存在任何實體即可使用 – System.out.println( Account.getRate() ); // 印出 雖然我們也可以產生實體後再以實體的 reference variable 來存取 – Account acc1 = new Account(“John”, 1000); – System.out.println( acc1.getRate() ); 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.getRate()

So now let’s check Hello World public class HelloWorldApp { public static void main(String[] args) { System.out.println("Hello World!"); } Now you know that why the main method is declared as static?

Overriding in Static Method? No overriding in static method – It is known as hiding – Which is executed is according to the type

Discussion 為什麼我 compile 常看到 compiler 說，你不能 在 static method 存取 non-static variables? – Static method 本來就存在，實體不存在當然不 能存取 – 即使實體存在，也不知道你想存取 ” 哪一個實體 ” 為什麼 main 要宣告成 static? – 一開始總要有個進入點，讓程式開始執行 – 如果不使用 static 修飾字，則一定要 new 了之後 產生實體才能使用，那麼就無從開始了

更 Advance 的問題 ClassLoader? 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.forName(“java.lang.String”); – klass.newInstance(); Why?

Class XXX { Static fields Static methods } XXX.class An instance of class XXX New/instantiate Memory

Homework

References lang/Class.html lang/Class.html