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