Java中的access modifier及instanceof运算符

访问修饰符（access modifier）
1） public（公共的）：被public所修饰的属性和方法可以被所有类访问。
2） protected（受保护的）：被protected所修饰的属性和方法可以在类内部、相同包以及该类的子类所访问。
3） private（私有的）：被private所修饰的属性和方法只能在该类内部使用
4） 默认的（不加任何访问修饰符）：在类内部以及相同包下面的类所使用。

 

instanceof运算符

http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html 写道

 

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

The following program, InstanceofDemo , defines a parent class (named Parent ), a simple interface (named MyInterface ), and a child class (named Child ) that inherits from the parent and implements the interface.

class InstanceofDemo {
    public static void main(String[] args) {

        Parent obj1 = new Parent();
        Parent obj2 = new Child();

        System.out.println("obj1 instanceof Parent: "
            + (obj1 instanceof Parent));
        System.out.println("obj1 instanceof Child: "
            + (obj1 instanceof Child));
        System.out.println("obj1 instanceof MyInterface: "
            + (obj1 instanceof MyInterface));
        System.out.println("obj2 instanceof Parent: "
            + (obj2 instanceof Parent));
        System.out.println("obj2 instanceof Child: "
            + (obj2 instanceof Child));
        System.out.println("obj2 instanceof MyInterface: "
            + (obj2 instanceof MyInterface));
    }
}

class Parent {}
class Child extends Parent implements MyInterface {}
interface MyInterface {}

 Output:

obj1 instanceof Parent: true
obj1 instanceof Child: false
obj1 instanceof MyInterface: false
obj2 instanceof Parent: true
obj2 instanceof Child: true
obj2 instanceof MyInterface: true

When using the instanceof operator, keep in mind that null is not an instance of anything.