Java中的RTTI

RTTI可以帮助我们在运行时识别对象和类的信息。

一般传统的RTTI有三种实现方式：

1. 向上转型或向下转型（upcasting and downcasting），在java中，向下转型（父类转成子类）需要强制类型转换
2. Class对象（用了Class对象，不代表就是反射，如果只是用Class对象cast成指定的类，那就还是传统的RTTI）
3. instanceof或isInstance()

 

通过Class对象实现RTTI  

interface HasBatteries{}

interface Waterproof{}

interface Shoots{}



class Toy{

	Toy() {}

	Toy(int i) {}

}

class FancyToy extends Toy implements HasBatteries, Waterproof, Shoots {

	public FancyToy() {

		super(1);

	}

}





public class ToyTest {



	public static void printInfo(Class c) {

		System.out.println("Class name: " + c.getName() + "\n"

				+ "is interface: " + c.isInterface() + "\n"

				+ "Simple name: " + c.getSimpleName() + "\n"

				+ "Cannonical name: " + c.getCanonicalName()

		);

	}

	public static void main(String[] args) {

		Class c = null;

		try {

			c = Class.forName("FancyToy"); //通过Class提供的forName方法得到FancyToy的Class对象

		} catch (ClassNotFoundException e) {

			// TODO Auto-generated catch block

			e.printStackTrace();

		}

		System.out.println("FancyToy类的基本信息：");

		printInfo(c);

		System.out.println("FancyToy实现的接口的信息：");

		for (Class interf : c.getInterfaces()) {

			printInfo(interf);

		}

		

		Class up = c.getSuperclass();

		System.out.println("FancyToy的父类的信息：");

		printInfo(up);

		Toy toy = null;

		try {

			toy = (Toy)up.newInstance();

		} catch (InstantiationException e) {

			// TODO Auto-generated catch block

			e.printStackTrace();

		} catch (IllegalAccessException e) {

			// TODO Auto-generated catch block

			e.printStackTrace();

		}

		printInfo(Toy.class); //通过类字面值常量获得Class对象

	}



}



输出：

FancyToy类的基本信息：

Class name: FancyToy

is interface: false

Simple name: FancyToy

Cannonical name: FancyToy

FancyToy实现的接口的信息：

Class name: HasBatteries

is interface: true

Simple name: HasBatteries

Cannonical name: HasBatteries

Class name: Waterproof

is interface: true

Simple name: Waterproof

Cannonical name: Waterproof

Class name: Shoots

is interface: true

Simple name: Shoots

Cannonical name: Shoots

FancyToy的父类的信息：

Class name: Toy

is interface: false

Simple name: Toy

Cannonical name: Toy

Class name: Toy

is interface: false

Simple name: Toy

Cannonical name: Toy

　　

Class还有泛化的，例如： Class<? extends Base>表示所有继承自Base的类的Class对象。

 

instanceof /Class.isInstance  

class Base{}

class Derived extends Base{}



public class ToyTest {

	public static void test(Object x) {

		/*

		 * instanceof /Class.isInstance   表示左边的是右边的这个类吗？或者是右边这个类的派生类吗？

		 * == 或者 equal 比较的是确切的类型

		 */

		System.out.println("Testing x of type : " + x.getClass()); 

		System.out.println("x instance of Base : " + (x instanceof Base));

		System.out.println("x instanceof Derived : " + (x instanceof Derived));

		System.out.println("Base.isInstance : " + Base.class.isInstance(x));

		System.out.println("Derived.isInstance : " + Derived.class.isInstance(x));

		System.out.println("x.getClass() == Base.class" + (x.getClass() == Base.class));

		System.out.println("x.getClass() == Derived.class" + (x.getClass() == Derived.class));

		System.out.println("x.getClass().equals(Base.class)" + (x.getClass().equals(Base.class)));

		System.out.println("x.getClass().equals(Derived.class)" + (x.getClass().equals(Derived.class)));

	}

	

	public static void main(String[] args) {

		test(new Base());

		test(new Derived());

	}

}

　　

参考： 《Java编程思想》14章