What is the difference between isAssignableFrom and instanceof in this example? [duplicate]

Question

Taking the code below into consideration:

@RestControllerAdvice public class GlobalExceptionHandler extends ResponseEntityExceptionHandler { @Resource private MessageSource messageSource; private HttpHeaders headers(){ HttpHeaders headers = new HttpHeaders(); headers.setContentType(MediaType.APPLICATION_JSON); return headers; } private ResponseError responseError(String message,HttpStatus statusCode){ ResponseError responseError = new ResponseError(); responseError.setStatus("error"); responseError.setError(message); responseError.setStatusCode(statusCode.value()); return responseError; } @ExceptionHandler(Exception.class) private ResponseEntity<Object> handleGeneral(Exception e, WebRequest request) { if (e.getClass().isAssignableFrom(UndeclaredThrowableException.class)) { UndeclaredThrowableException exception = (UndeclaredThrowableException) e; return handleBusinessException((BusinessException) exception.getUndeclaredThrowable(), request); } else { String message = messageSource.getMessage("error.server", new Object[]{e.getMessage()}, null); ResponseError error = responseError(message,HttpStatus.INTERNAL_SERVER_ERROR); return handleExceptionInternal(e, error, headers(), HttpStatus.INTERNAL_SERVER_ERROR, request); } } @ExceptionHandler({BusinessException.class}) private ResponseEntity<Object> handleBusinessException(BusinessException e, WebRequest request) { ResponseError error = responseError(e.getMessage(),HttpStatus.CONFLICT); return handleExceptionInternal(e, error, headers(), HttpStatus.CONFLICT, request); } }

What would be the biggest difference between replacing

if (e.getClass().isAssignableFrom(UndeclaredThrowableException.class)) {

with

if (e instanceof UndeclaredThrowableException) {

Line 19.

I found some explanations but none detailed enough. I wanted to understand the situations in which I should use isAssignableFrom or instanceof.

At the very least, they will be have differently when e is null. — Sweeper
– Sweeper, Commented Apr 27, 2024 at 1:49
Java: class.isInstance() vs. class.isAssignableFrom() This link partially responds. I understand the difference between the concepts "instanceof" and "isAssignableFrom". My question is more in the practice of this code that I brought. I found some tutorials that specifically used isAssignableFrom to check the Exception.class within a GlobalExceptionHandler. I'm sorry if the title was too generic. — cator
– cator, Commented Apr 27, 2024 at 1:58

meriton · Accepted Answer · 2024-04-27 01:56:50Z

2

e.getClass().isAssignableFrom(Whatever.class)

tests whether the runtime class of e is a superclass of Whatever,

e instanceof Whatever

tests whether the runtime class of e is a subclass of Whatever.

In your code, the latter makes more sense, but in modern Java, there is an even shorter and clearer way of writing that:

if (e instanceof Whatever w) { // w is now a variable of type Whatever }

answered Apr 27, 2024 at 1:56

meriton

70.9k14 gold badges117 silver badges190 bronze badges

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

2 Comments

cator Over a year ago

So isAssignableFrom is an older way of checking the instance of a class? Is it currently better to use instanceof?

Slaw Over a year ago

@cator No. As mentioned, it's the difference between checking if e is a supertype versus checking if e is a subtype. The two test opposite things. As for the pattern matching at the end of the answer, that is a newer way to do instanceof, not isAssignableFrom, and does not change the semantics of the code. It requires Java 16+.

Ahmet Emin Saglik · Accepted Answer · 2024-04-27 01:58:07Z

isAssignableFrom: This method checks if the class represented by one Class object is the same as, or is a superclass or superinterface of, the class represented by another Class object. It is used to determine if one class is assignable from another class or interface.
instanceof: This operator checks if an object is an instance of a particular class or interface, including instances of subclasses. It is used to test if an object is of a particular type.

Collectives™ on Stack Overflow

What is the difference between isAssignableFrom and instanceof in this example? [duplicate]

2 Answers 2

2 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Linked

Related