Java类型检查

类型检查有三种：

1. instanceof

instanceof是运算符只被用于对象引用变量，检查左边的被测试对象是不是右边类或接口的实例化。如果被测对象是null值，则测试结果总是false。

用法：result = object instanceof class

2. isInstance(Object obj)

这个方法与instanceof等价，其中obj是被测试的对象，如果obj是调用这个方法的class或接口的实例，则返回true。

用法：result = class.instanceof(object)

3.isAssignableFrom(Class cls)

如果调用这个方法的class或接口与参数cls表示的类或接口相同，或者是参数cls表示的类或接口的父类，则返回true。

用法：result = SuperClass.isAssignableFrom(SubClass)

class SuperClass {

}
class SubClass extends SuperClass {

}

public class Test {

    public static void main(String[] args) {
        SuperClass superClass = null;
        SubClass subClass = null;
        boolean res;

        System.out.println("instanceof/isinstance test case 1: ------------------");
        //res = superClass instanceof SuperClass;
        res = SuperClass.class.isInstance(superClass);
        System.out.println("superClass instanceof SuperClass: " + res);
        
        System.out.println("instanceoftest/isinstance test case 2: ------------------");
        superClass = new SuperClass();
        subClass = new SubClass();

        //res = superClass instanceof SuperClass;
        res = SuperClass.class.isInstance(superClass);
        System.out.println("superClass instanceof SuperClass: " + res);

        //res = superClass instanceof SubClass;
        res = SubClass.class.isInstance(superClass);
        System.out.println("superClass instanceof SubClass: " + res);
        
        //res = subClass instanceof SubClass;
        res = SubClass.class.isInstance(subClass);
        System.out.println("subClass instanceof SubClass: " + res);

        System.out.println("instanceof/isinstance test case 3: ------------------");
        superClass = new SubClass();
        subClass = new SubClass();

        //res = superClass instanceof SuperClass;
        res = SuperClass.class.isInstance(superClass);
        System.out.println("superClass instanceof SuperClass: " + res);

        //res = superClass instanceof SubClass;
        res = SubClass.class.isInstance(superClass);
        System.out.println("superClass instanceof SubClass: " + res);

        //res = subClass instanceof SuperClass;
        res = SuperClass.class.isInstance(subClass);
        System.out.println("subClass instanceof SuperClass: " + res);

        System.out.println(" isAssignableFrom case : ------------------");
        res = SuperClass.class.isAssignableFrom(SubClass.class);
        System.out.println("SuperClass isAssignableFrom SubClass: " + res);
        
        res = SubClass.class.isAssignableFrom(SuperClass.class);
        System.out.println("SubClass isAssignableFrom SuperClass: " + res);
    }
}