如何将对象转换为长数据类型Java

Question

I have this code which gives me an error?

public int part(Object key) {   
    long clientId = (long) key;
    ...
}

Below is the error:

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Long

Not sure why it throws an exception.

Answer 1

As has been explained in the comments, Java does not allow casting of one primitive wrapper type into another type of primitive, even if the casting is allowed on the primitives themselves.

Your exception stacktrace is showing that the key parameter is an Integer object. If so, then simply use Integer's method created specifically for this type of conversion:

long clientId = ((Number) key).longValue();

You'd better be very sure that the key is always an Number object and is not null for this to work. You may need to test for null prior to this method being called.

Answer 2

You can't cast Integer to Long , even though you can convert from int to long . This would work for you:

Long clientId = new Long(key)

NPE is thrown if the Integer is null. I will leave the error handling up to you though. :)

Alternatively, you can use:

Long clientId = Long.valueOf(key.longValue());