检查Object是否是String，HashMap或HashMap []的实例

Question

I have an Object in java. Is there a way to check if an object is an instance of a String, HashMap, or HashMap[ ] before actually casting it to those objects?

If not, as it seems counterintuitive that the above would work, is there a way to cast it into each object, and test something about the newly casted object to see if its in fact the type of object into which it was casted?

Answer 1

Yes:

 if(obj instanceof String)
 {
     String str = (String) obj;
     .
     .
     .
 }

By the way, to clarify regarding this:

[…] test something about the newly casted object to see if its in fact the type of object into which it was casted?

You cannot cast something into an invalid type. If obj has type String , then ((Integer)obj) will cause a ClassCastException to be raised at run-time.

Answer 2

You are looking for the instanceof operator.

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

Example: "Hello" instanceof String would return true while new Integer(5) instanceof String would return false .