Custom converter throws java.lang.ClassCastException when calling setBean on binder for field type of textfield

Question

Trying to implement a custom converter for a textfield to a custom data type and bonded the same field with the binder when I call binder.setBean(vo) it throws the given exception

java.lang.ClassCastException: class com.demo.vo.DemoVO cannot be cast to class java.lang.String (com.demo.vo.DemoVO is in unnamed module of loader 'deployment.demo.ear' @7cc3d734; java.lang.String is in module java.base of loader 'bootstrap') at deployment.demo.ear.demo.war//com.vaadin.flow.component.textfield.TextField.setValue(TextField.java:34) at deployment.demo.ear.demo.war//com.vaadin.flow.data.binder.Binder$BindingImpl.convertAndSetFieldValue(Binder.java:1275) at deployment.demo.ear.demo.war//com.vaadin.flow.data.binder.Binder$BindingImpl.initFieldValue(Binder.java:1196) at deployment.demo.ear.demo.war//com.vaadin.flow.data.binder.Binder$BindingImpl.access$200(Binder.java:1032) at deployment.demo.ear.demo.war//com.vaadin.flow.data.binder.Binder.lambda$setBean$1(Binder.java:1772) at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)

DemoVO Class

public class DemoVO {
    private int id;
    private String code;
    private String details;
    // getters
    // setters
}

Converter Class

public class DemoVOToStringConverter implements Converter<String, DemoVO> {

    private DemoVO demoVO;

    @Override
    public Result<DemoVO> convertToModel(String value, ValueContext context) {

        if (null == value)) {
            return Result.ok(null);
        }

        demoVO.setCode(value);
        return Result.ok(demoVO);
    }

    @Override
    public String convertToPresentation(DemoVO value, ValueContext context) {

        if (null == value)) {
            return null;
        }

        demoVO = value;
        return value.getCode();
    }
}

HeaderVO Class

public class HeaderVO {
        private DemoVO demo = new DemoVO();
        // getter
        // setter
}

DemoView Class

public class DemoView extends VerticalLayout implements BeforeEnterListener {
            
            private Binder<HeaderVO> binder = null; 
    
            public DemoView() {
                   binder = new Binder<>(HeaderVO.class);
    
                   TextField demoField = new TextField();
                   demoField.setMaxLength(10);
                   demoField.setLabel("Demo");
                   binder.forField(demoField).withConverter(new DemoVOToStringConverter()).bind("demo");
                   add(demoField);
            }
    
        @Override
        public void beforeEnter(BeforeEnterEvent beforeEnterEvent) {
               HeaderVO headerVO = new HeaderVO();
               binder.setBean(headerVO);
        }
   }

Answer 1

While testing your MRE I didn't got a ClassCastException, but I got this exception: 'java.lang.NullPointerException: Null value is not supported'. The TextField does not support a null value, the empty value is a empty string. Note that on your on your convertToPresentation method: never return null.

I would use the bind() method with ValueProvider and Setter:

binder.forField(demoField)
        .bind(
            header -> header.getDemo().getCode(),
            (header, code) -> header.getDemo().setCode(code)
        );

Watch out for null values.

(Futher I am not sure about that instance of the DemoVO in the DemoVOToStringConverter... not sure if the Converter could be reused. But looks like it could work.)