我们在团队中做的一些测试表明，A.class. isassignablefrom (B. getclass())比B instanceof a工作得更快，如果你需要对大量元素进行检查，这是非常有用的。

Instanceof也不能用于基本类型或泛型类型。如下代码所示:

//Define Class< T > type ... 

Object e = new Object();

if(e instanceof T) {
  // Do something.
}

错误是:不能对类型参数t执行instanceof检查，而是使用它的erasure对象，因为进一步的泛型类型信息将在运行时被擦除。

由于删除运行时引用的类型擦除而无法编译。然而，下面的代码将编译:

if( type.isAssignableFrom(e.getClass())){
  // Do something.
}

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。

用一些例子来证明它怎么样?

@Test
public void isInstanceOf() {
    Exception anEx1 = new Exception("ex");
    Exception anEx2 = new RuntimeException("ex");
    RuntimeException anEx3 = new RuntimeException("ex");

    //Base case, handles inheritance
    Assert.assertTrue(anEx1 instanceof Exception);
    Assert.assertTrue(anEx2 instanceof Exception);
    Assert.assertTrue(anEx3 instanceof Exception);

    //Other cases
    Assert.assertFalse(anEx1 instanceof RuntimeException);
    Assert.assertTrue(anEx2 instanceof RuntimeException);
    Assert.assertTrue(anEx3 instanceof RuntimeException);
}

@Test
public void isAssignableFrom() {
    Exception anEx1 = new Exception("ex");
    Exception anEx2 = new RuntimeException("ex");
    RuntimeException anEx3 = new RuntimeException("ex");

    //Correct usage = The base class goes first
    Assert.assertTrue(Exception.class.isAssignableFrom(anEx1.getClass()));
    Assert.assertTrue(Exception.class.isAssignableFrom(anEx2.getClass()));
    Assert.assertTrue(Exception.class.isAssignableFrom(anEx3.getClass()));

    //Incorrect usage = Method parameter is used in the wrong order
    Assert.assertTrue(anEx1.getClass().isAssignableFrom(Exception.class));
    Assert.assertFalse(anEx2.getClass().isAssignableFrom(Exception.class));
    Assert.assertFalse(anEx3.getClass().isAssignableFrom(Exception.class));
}

Instanceof只能用于引用类型，不能用于基本类型。isAssignableFrom()可以用于任何类对象:

a instanceof int  // syntax error
3 instanceof Foo  // syntax error
int.class.isAssignableFrom(int.class)  // true

看到http://java.sun.com/javase/6/docs/api/java/lang/Class.html isAssignableFrom (. lang . class)。

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.