Java Class.isAssignableFrom confusion

Question

I find primitive type problem

System.out.println("Integer.class.isAssignableFrom(int.class) = " + Integer.class.isAssignableFrom(int.class)); System.out.println("int.class.isAssignableFrom(Integer.class) = "+int.class.isAssignableFrom(Integer.class));

both of the statements return false to the caller. so that seems like boxing is not applicable here. My question is if my observation is correct or there are other API who can do this testing correctly?

--------------------------------following up---------------------------------------------

As I said, I mainly want to check if a Object is assignable to a Field when using reflection. I hope the mechanism could be more precise at run time so I made a implementation like this.

 public static boolean isAssignableFrom(final Field field, final Object obj) { if (field.getType().equals(Integer.class) || field.getType().equals(int.class)) { return obj.getClass().equals(Integer.class) || field.getType().equals(int.class); } else if (field.getType().equals(Float.class) || field.getType().equals(float.class)) { return obj.getClass().equals(Float.class) || field.getType().equals(float.class); } else if (field.getType().equals(Double.class) || field.getType().equals(double.class)) { return obj.getClass().equals(Double.class) || field.getType().equals(double.class); } else if (field.getType().equals(Character.class) || field.getType().equals(char.class)) { return obj.getClass().equals(Character.class) || field.getType().equals(char.class); } else if (field.getType().equals(Long.class) || field.getType().equals(long.class)) { return obj.getClass().equals(Long.class) || field.getType().equals(long.class); } else if (field.getType().equals(Short.class) || field.getType().equals(short.class)) { return obj.getClass().equals(Short.class) || field.getType().equals(short.class); } else if (field.getType().equals(Boolean.class) || field.getType().equals(boolean.class)) { return obj.getClass().equals(Boolean.class) || field.getType().equals(boolean.class); } else if (field.getType().equals(Byte.class) || field.getType().equals(byte.class)) { return obj.getClass().equals(Byte.class) || field.getType().equals(byte.class); } return field.getType().isAssignableFrom(obj.getClass()); } }

That seems the best I can do -_-! thanks

I am not sure, I mean statement Integer a = 1 is compilable. — Korben
– Korben, Commented Jan 4, 2013 at 6:09
possible duplicate of Does int.class equal Integer.class or Integer.TYPE in Java? — Amit Deshpande
– Amit Deshpande, Commented Jan 4, 2013 at 6:10
@Korben - autoboxing is a compile time process that boxes or unboxes instances. It doesn't apply to classes at all, which is what the isAssignableFrom method operates on. Key thing to keep in mind is - instances can be autoboxed, classes cannot. — Perception
– Perception, Commented Jan 4, 2013 at 6:15
@Korben - I'm unsure as to what you are trying to test. Are you trying to verify that autoboxing is available in the version of Java you are using? Or something else? — Perception
– Perception, Commented Jan 4, 2013 at 6:22

Miro Hudak · Accepted Answer · 2013-05-11 14:35:37Z

11

I suppose, ClassUtils.isAssignable(Class, Class, boolean) from Apache commons-lang is the one to help.

JavaDoc

answered May 11, 2013 at 14:35

Miro Hudak

2,2032 gold badges22 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

sparkym3 Over a year ago

The Apache method will work but will also allow for long is assignable from int to be true as well. You could just attempt to cast the object to the type yourself and then catch the classcastexception if it fails.

Community · Accepted Answer · 2017-05-23 10:33:51Z

int.class and Integer.class are two separate class objects. check this answer for more details

From Java doc Class#isAssignableFrom

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter. It returns true if so; otherwise it returns false. If this Class object represents a primitive type, this method returns true if the specified Class parameter is exactly this Class object; otherwise it returns false.

Krease · Accepted Answer · 2013-01-04 06:06:43Z

From the documentation on isAssignableFrom:

this method tests whether the type represented by the specified Class parameter can be converted to the type represented by this Class object via an identity conversion or via a widening reference conversion. See The Java Language Specification, sections 5.1.1 and 5.1.4 , for details.

Integer cannot be assigned to an int (or vice versa) through this way, so your method will return false - boxing and unboxing are done at compile time, not at runtime - see this article for more info on it

but final Integer a = 1; final int b = new Integer(2); are OK
This is allowed only as a convenience - the compiler will basically turn that into final Integer a = Integer.valueOf(1); and final int b = new Integer(2).intValue() (there may be some optimization in there, but that's the gist of how autoboxing works).

user3525375 · Accepted Answer · 2020-11-16 09:13:31Z

Just a hint, the code you pasted above has a bug in it. If you call the method with "field= int.class", it does not matter what type object is, the method will always return true. Seems to be a copy and paster misstake ;)

if (field.getType().equals(Integer.class) || field.getType().equals(int.class)) { return obj.getClass().equals(Integer.class) || field.getType().equals(int.class);

better would be:

if (field.getType().equals(Integer.class) || field.getType().equals(int.class)) { return obj.getClass().equals(Integer.class) || obj.getType().equals(int.class);

Collectives™ on Stack Overflow

Java Class.isAssignableFrom confusion

4 Answers 4

1 Comment

Comments

2 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

2 Comments

Comments

Linked

Related