isAssignableFrom(A, B) =

if (A == B) return true
else if (B == java.lang.Object) return false
else return isAssignableFrom(A, getSuperClass(B))

The pseudo code above is a definition of, if references of type/class A is assignable from references of type/class B. It is a recursive definition. To some it may be helpful, for others it may be confusing. I add it in case somebody should find it useful. This is just an attempt to capture my understanding, it is not the official definition. It is used in a certain Java VM implementation and works for many example programs, so while I cannot guarentee that it captures all aspects of isAssignableFrom, it is not completely off.

考虑以下情况。假设你想检查类型A是否是obj类型的超类，你也可以这样做

... A.class.isAssignableFrom（obj.getClass（）） ...

OR

.．. A . obj instanceof .．.

但是isAssignableFrom解决方案要求obj的类型在这里是可见的。如果不是这样(例如，obj的类型可能是一个私有内部类)，则该选项无效。然而，instanceof solution总是可以工作的。

在使用instanceof时，您需要在编译时知道B的类。当使用isAssignableFrom()时，它可以是动态的，并在运行时更改。

更直接的等价于B的实例是

B.class.isInstance(a)

当a为空时，这也可以工作(返回false)。

除了上面提到的基本区别之外，在类中的instanceof操作符和isAssignableFrom方法之间还有一个核心的微妙区别。

Read instanceof as “is this (the left part) the instance of this or any subclass of this (the right part)” and read x.getClass().isAssignableFrom(Y.class) as “Can I write X x = new Y()”. In other words, instanceof operator checks if the left object is same or subclass of right class, while isAssignableFrom checks if we can assign object of the parameter class (from) to the reference of the class on which the method is called. Note that both of these consider the actual instance not the reference type.

考虑一个A、B和C三个类的例子，其中C扩展了B, B扩展了A。

B b = new C();

System.out.println(b instanceof A); //is b (which is actually class C object) instance of A, yes. This will return true.  
System.out.println(b instanceof B); // is b (which is actually class C object) instance of B, yes. This will return true.  
System.out.println(b instanceof C); // is b (which is actually class C object) instance of C, yes. This will return true. If the first statement would be B b = new B(), this would have been false.
System.out.println(b.getClass().isAssignableFrom(A.class));//Can I write C c = new A(), no. So this is false.
System.out.println(b.getClass().isAssignableFrom(B.class)); //Can I write C c = new B(), no. So this is false.
System.out.println(b.getClass().isAssignableFrom(C.class)); //Can I write C c = new C(), Yes. So this is true.

这个帖子让我了解了instanceof与isAssignableFrom的不同之处，所以我想分享一下我自己的一些内容。

我发现使用isAssignableFrom是唯一的(可能不是唯一的，但可能是最简单的)方法来询问自己一个类的引用是否可以接受另一个类的实例，当一个类的实例都没有时进行比较。

因此，当我只有类时，我不认为使用instanceof操作符比较可赋值性是一个好主意，除非我打算从其中一个类创建一个实例;我觉得这样会很草率。