第7天第4节: static关键字

==static on properties: all obj shared the same variable which is on the globe data area . one obj change it, all objt changes
class Person
{
    private int age;
    static String city = "Beijing";

    public Person(int age) {
        this.age = age;
    }
    public String getInfo() {
        return "age=" + this.age + ",city=" + this.city;
    }
}

public class testb
{
    public static void main ( String args[] ) {
        Person per1 = new Person(30);
        Person per2 = new Person(20);   
        per1.city = "Shanghai";
        Person.city = "Shanghai";
        System.out.println(per1.getInfo());   //==> shanghai
        System.out.println(per2.getInfo());   //==> shanghai
    }
}

===the static properties and method can called from the class directly

===the static method can only use static properties and static mehtod;

===the static method can be called without any objects

===public static void main (String args[])  must exactly
       main is reserved.

===memeoty
1. stack: reference to obj
2. heap: properties of objs
3. global code: method of class
4. global data: static properties of class