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.

还有另一个区别:

不管X是什么，null instanceof X都是假的

null.getClass().isAssignableFrom(X)将抛出NullPointerException

在性能“2”方面(与JMH):

class A{}
class B extends A{}

public class InstanceOfTest {

public static final Object a = new A();
public static final Object b = new B();

@Benchmark
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public boolean testInstanceOf()
{
    return b instanceof A;
}

@Benchmark
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public boolean testIsInstance()
{
    return A.class.isInstance(b);
}

@Benchmark
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public boolean testIsAssignableFrom()
{
    return A.class.isAssignableFrom(b.getClass());
}

public static void main(String[] args) throws RunnerException {
    Options opt = new OptionsBuilder()
            .include(InstanceOfTest.class.getSimpleName())
            .warmupIterations(5)
            .measurementIterations(5)
            .forks(1)
            .build();

    new Runner(opt).run();
}
}

它给:

Benchmark                            Mode  Cnt  Score   Error  Units
InstanceOfTest.testInstanceOf        avgt    5  1,972 ? 0,002  ns/op
InstanceOfTest.testIsAssignableFrom  avgt    5  1,991 ? 0,004  ns/op
InstanceOfTest.testIsInstance        avgt    5  1,972 ? 0,003  ns/op

因此，我们可以得出结论:instanceof as fast as isInstance() and isAssignableFrom() not far(+0.9%的执行时间)。所以无论你选择什么都没有真正的区别

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)。

还有一个不同之处。如果要测试的类型(Class)是动态的，例如，作为方法参数传递，那么instanceof将不会为你切割它。

boolean test(Class clazz) {
   return (this instanceof clazz); // clazz cannot be resolved to a type.
}

但是你可以:

boolean test(Class clazz) {
   return (clazz.isAssignableFrom(this.getClass())); // okidoki
}

哎呀，这个答案已经讲过了。也许这个例子对某人有帮助。

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.