CODEWITHSUNDEEP
javagenericsreflectionjava-8
Michael Langhammer
Michael Langhammer

Reputation: 730

Obtain generic type of a Field using reflection

Is there a way to obtain the generic type of a field in Java?

I have following object variables:

protected ScheduleView<WantedClass> scheduleLine1;
protected ScheduleView<SomeOtherClass> scheduleLine2;

Now I try using reflection to obtain all the object variables who have ScheduleView as type with WantedClass as generic type:

Arrays.asList(this.getClass().getDeclaredFields()).stream().map(field -> {
    ScheduleView<WantedClass> retValue = null;

    System.out.println(field.getGenericType()); // prints control.ScheduleView<dto.WantedClass>

    try {
        if (field.getType() == ScheduleView.class) { // here I also want to check if the generic type is WantedClass
            retValue = (ScheduleView<WantedClass>) field.get(this);
        } else {
            retValue = null;
        }
    } catch (IllegalAccessException e) {
        retValue = null;
    } finally {
        return retValue;
    }
}).filter(scheduleView -> scheduleView != null).forEach(scheduleView -> {
        /* some more code */
});

The thing is I also want to check in the if-Statement if the generic type is WantedClass. I also tried using the method getGenericType() but it seems something like this is not possible:

field.getGenericType() == ScheduleView<WantedClass>.class

So is there a way to get the generic type of a field?

Upvotes: 3

Views: 401

Answers (2)

srborlongan
srborlongan

Reputation: 4579

An implementation of @Andy Turner's answer using streams:

// For each declared field in the class...
Arrays.stream(Ideone.class.getDeclaredFields())
  // select fields...
  .filter(
    // that have a generic type...
    field -> Stream.of(field.getGenericType())
      // that is an instance of ParametrizedType...
      .filter(ParameterizedType.class::isInstance)
      .map(ParameterizedType.class::cast)
      // whose raw type is ScheduledView...
      .filter(ptype -> ptype.getRawType().equals(ScheduledView.class))
      // and whose actual type arguments...
      .map(ParameterizedType::getActualTypeArguments)
      .flatMap(Arrays::stream)
      // include WantedClass...
      .anyMatch(WantedClass.class::equals)
  )
  // then get their names...
  .map(Field::getName)
  // and print them.
  .forEach(System.out::println)
;

Upvotes: 2

Andy Turner
Andy Turner

Reputation: 140494

You have to check if the field is a ParameterizedType, and then inspect the contents of getActualTypeArguments, e.g.

    for (Field field : Ideone.class.getDeclaredFields()) {
        Type type = field.getGenericType();
        if (type instanceof ParameterizedType) {
            ParameterizedType ptype = (ParameterizedType) type;
            if (ptype.getRawType() == ScheduledView.class) {
                if (ptype.getActualTypeArguments().length == 1
                    && ptype.getActualTypeArguments()[0] == WantedClass.class) {
                  // Do whatever with the field.
                  System.out.println(field.getName());
                }
            }
        }
    }

Ideone demo

Upvotes: 4

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