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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 = 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 getRate() {
return rate;
}
}
Static method
• 不存在任何實體即可使用
– System.out.println( Account.getRate() ); //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 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?
Memory
An instance of
class XXX
Class XXX
{ Static fields
Static methods }
New/instantiate
XXX.class
An instance of
class XXX
An instance of
class XXX
Homework
References
• http://java.sun.com/j2se/1.4.2/docs/api/java/
lang/Class.html