How can I cast Generics in Java?

Question

I have this code:

FVDTO.setStatus("fail"); List<String[]> invalidFields = new ArrayList<String[]>(); Iterator<ConstraintViolation<HazardSubmission>> iterator = cv.iterator(); while(iterator.hasNext()) { ConstraintViolation<HazardSubmission> i = iterator.next(); String property = i.getPropertyPath().toString(); String message = i.getMessage(); invalidFields.add(new String[] { property, message }); } FVDTO.setInvalidFields(invalidFields); return new JsonResolution(FVDTO);

I've taken some out to keep things DRY so I can then use it with other classes, i.e HazardSubmission is one class, and there will be others. The below code shows my attempt, obviously manually casting <HazardSubmission> here won't work it needs to be like o.getClass();

public static List<String[]> GetInvalidProperties(Set<ConstraintViolation<Object>> cv, Object o) { List<String[]> invalidFields = new ArrayList<String[]>(); Iterator<ConstraintViolation<HazardSubmission>> iterator = cv.iterator(); while(iterator.hasNext()) { ConstraintViolation<HazardSubmission> i = iterator.next(); String property = i.getPropertyPath().toString(); String message = i.getMessage(); invalidFields.add(new String[] { property, message }); }

}

The second code block fails because I don't really know what I'm doing, I want to pass in the cv for param 1, with a general object type, then somehow pass in the type as a second paramter.

Could you someone please explain how to do this?

I haven't used hibernate, but the whole point of using generics so you don't have to cast — edthethird
– edthethird, Commented Mar 23, 2012 at 2:26
Why do you want Object in the parameter when your method is only working on HazardSubmission? — Bhesh Gurung
– Bhesh Gurung, Commented Mar 23, 2012 at 2:33

Edwin Dalorzo · Accepted Answer · 2012-03-23 02:38:39Z

I think you might be looking for a generic method

public static <T> List<String[]> GetInvalidProperties(Set<ConstraintViolation<T>> cv){ Iterator<ConstraintViolation<T>> iterator = cv.iterator(); while(iterator.hasNext()) { ConstraintViolation<T> i = iterator.next(); String property = i.getPropertyPath().toString(); String message = i.getMessage(); invalidFields.add(new String[] { property, message }); } }

If all T extends a given class or interface you could even say

public static <T extends MyClassOrInterface> List<String[]> GetInvalidProperties(Set<ConstraintViolation<T>> cv){ //... }

Luiggi Mendoza · Accepted Answer · 2012-03-23 02:29:51Z

cv.iterator() will return you an Iterator<ConstraintViolation<Object>>, and you need Iterator<ConstraintViolation<HazardSubmission>>. This is done because cv is defined as Set<ConstraintViolation<Object>>. If you want a generic way for this, you can change

Set<ConstraintViolation<Object>> cv

to

Set<ConstraintViolation<? extends Object>> cv

in that way your code will compile.

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