isAssignableFrom方法浅析

源码

 /**
     * Determines if the class or interface represented by this
     * {@code Class} object is either the same as, or is a superclass or
     * superinterface of, the class or interface represented by the specified
     * {@code Class} parameter. It returns {@code true} if so;
     * otherwise it returns {@code false}. If this {@code Class}
     * object represents a primitive type, this method returns
     * {@code true} if the specified {@code Class} parameter is
     * exactly this {@code Class} object; otherwise it returns
     * {@code false}.
     *
     * 
 Specifically, this method tests whether the type represented by the
     * specified {@code Class} parameter can be converted to the type
     * represented by this {@code Class} object via an identity conversion
     * or via a widening reference conversion. See The Java Language
     * Specification, sections 5.1.1 and 5.1.4 , for details.
     *
     * @param cls the {@code Class} object to be checked
     * @return the {@code boolean} value indicating whether objects of the
     * type {@code cls} can be assigned to objects of this class
     * @exception NullPointerException if the specified Class parameter is
     *            null.
     * @since JDK1.1
     */
    public native boolean isAssignableFrom(Class cls);

翻译

Class a = A.class;
Class b = B.class;
boolean c = a.isAssignableFrom(b)

如果当前类对象(对应a)表示的类或者接口 是 参数类(对应b)表示的类或者接口 相同类型或者父类或者父接口。则返回true；否则返回false。当前类对象(对应a)表示的原始类型(原始类型一共有8种，它们分别是char,boolean,byte,short,int,long,float,double),如果参数类(对应b)和当前类对象(对应a)类型一样则返回true；否则返回false。

确切的说，这个方法用于判断参数类(对应b)表示的类型能否转换为当前类对象(对应a)表示的类型通过标识转换或者拓宽和窄化简单类型转换。详情请参阅 Java语言规范第5.1.1和5.1.4节。

实例 

public class ChinaMadeCar extends ChinaMadeCarA implements DeviceA {

}

public class ChinaMadeCarA extends Test{

}

public class Test   {

}

public interface DeviceA {

}


 

public class Chery extends ChinaMadeCar implements Car,Device{
    public static void main(String[] args) {
        //相同类  返回true
        boolean b0 = Chery.class.isAssignableFrom(Chery.class);
        //当前类对象直接父类ChinaMadeCar 返回true
        boolean b1 = ChinaMadeCar.class.isAssignableFrom(Chery.class);
        //当前类对象直接实现的接口Car 返回true
        boolean b2 = Car.class.isAssignableFrom(Chery.class);
        //当前类对象直接实现的接口Device 返回true
        boolean b3 = Device.class.isAssignableFrom(Chery.class);
        //父类ChinaMadeCar直接实现的接口DeviceA 返回true
        boolean b4 = DeviceA.class.isAssignableFrom(Chery.class);

        //间接父类ChinaMadeCarA(父类ChinaMadeCar的直接父类) 返回false
        boolean b5 = ChinaMadeCarA.class.isAssignableFrom(Chery.class);
        boolean b6 = Test.class.isAssignableFrom(Chery.class);
        System.out.println("b0="+b0+"\n"+"b1="+b1+"\n"
                +"b2="+b2+"\n"+"b3="+b3+"\n"+"b4="+b4+"\n"
                +"b5="+b5+"\n"
                +"b6="+b6);
    }
}
//        控制台输出结果：
//        b0=true
//        b1=true
//        b2=true
//        b3=true
//        b4=true
//        b5=true
//        b6=true
//        b7=false