理解 Java 中的静态方法

Question

Can someone help me understand what does the following statement mean?

"Like any method, a static method can create or use named objects of its type , so a static method is often used as a “shepherd” for a flock of instances of its own type . "

Source: http://www.codeguru.com/java/tij/tij0037.shtml#Heading79

Answer 1

Here's an example: say you have a class Person that looks like this:

public class Person {
    static ArrayList<Person> people = new ArrayList<>();
    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
        people.add(this);
    }

    public void display() {
        System.out.println(name + ", age " + age);
    }

    public static void displayAll() {
        for (int i=0; i<people.size(); i++) {
            people.get(i).display();
        }
    }
}

In this example, people belongs to the Person class itself because it is static, whereas name and age are non-static and belong to each instance of Person . Similarly, because displayAll() is static, it can only be called by Person , whereas the non-static display() can only be called by individual instances of Person .

To illustrate this, say you have this in your main class:

Person john = new Person("John", 25);
Person amy = new Person("Amy", 27);
System.out.println(john.name + " is " + john.age);
System.out.println(amy.name + " is " + amy.age);

This would create two instances of Person , john and amy , and would give the following output:

John is 25
Amy is 27

The following code would also work (assume for all examples from here on that john and amy have already been created as in the previous example):

john.display();
amy.display();

This would give this output:

John, age 25
Amy, age 27

Now, because john and amy are specific instances of Person , they cannot reference static variables or call static methods, so both of these next lines of code would be uncompilable:

System.out.println(john.people.size());
amy.displayAll();

However, the following would work:

System.out.println(Person.people.size());
Person.displayAll();

This would give this output:

2
John, age 25
Amy, age 27

However, the following would NOT work:

Person.display();
System.out.println(Person.name);
System.out.println(Person.age);

Person.display() does not work because display() is not a static method. The next two lines don't work because the variables name and age belong to specific instances of Person and do not apply to the Person class in general.