ClassCastException

Overview

• ClassCastException is a runtime exception. when one Object is Casted to another Object of different dataType ClassCastException persists.
• There are Three Rules for Object TypeCasting. ` A b = (C) d`
• RULE-1 Type of d and C must have some relationship either parent to child or child to parent or same type. If this condition is not satisfied it gives us Compiletime error
• RULE-2 C must be derived type of A. If this condition is not satisfied it gives us Compiletime error

• RULE-3 The Underlying original object type of d must be either same or derived type of C. If this condition is not satisfied it gives us Runtime error

• Example

  Class Animal{}
  Class Petanimal Extends Animal{}
  Class Wildanimal Extends Animal{}
  Class Cat Extends Petanimal{}
  Class Dog Extends Petanimal{}
  Class Lion Extends Wildanimal{}
  Class Tiger Extends Wildanimal{}
  public class ClassCastException{
  public static void main(String[] args){
  }
  }
  

• Wildanimal a = new Tiger(); Valid
• Tiger u = new (Tiger) a; Valid
• Petanimal v = (Petanimal)a; CompileTime Error
• Wildanimal w = (Wildanimal)a; Valid
• Animal x = (Lion)a RunTime Error
• Wildanimal y = (Petanimal)a CompileTime Error
• IdtRoundingDto idtRoundingDto = (IdtRoundingDto) mappedGeneralResponse Invalid
• IdtRoundingDto idtRoundingDto = (IdtRoundingDto) payloadableList valid

IdtRoundingDto Class

@Data @AllArgsConstructor @NoArgsConstructor @ToString
public class IdtRoundingDto implements Paylodable{
private List<IdtRounding> patients;

Paylodable Class

public interface Payloadable

MappedGeneralResponse Class

@Data @AllArgsConstructor @NoArgsConstructor @ToString
public class MappedGeneralResponse implements Paylodable{
private List<Payloadable> data;

public Payloadable getReport(map<String, Object> request){
MappedGeneralResponse mappedGeneralResponse = service.getDataFromDb(new IdtRoundingDto());
}

getDataFromDb method

public MappedGeneralResponse getDataFromDb(Payloadable reportData){
}

• getting data to variable idtRoundingDto of type IdtRoundingDto from mappedGeneralResponse of type Payloadable

  IdtRoundingDto idtRoundingDto = mappedGeneralResponse.getData.get(0); 
  

• Its shows an compiletime error “Cast to IdtRoundingDto” since mappedGeneralResponse is of type Payloadable type which cannot be assigned to idtRoundingDto which is of IdtRoundingDto type. Required type is IdtRoundingDto but we provided payloadable.

• casting mappedGeneralResponse to IdtRoundingDto

  IdtRoundingDto idtRoundingDto = (IdtRoundingDto) mappedGeneralResponse.getData.get(0); 
  

• compile error will be fixed but it gives us runtime error because mappedGeneralResponse is not either same or derived type of IdtRoundingDto Which fails rule 3 of object typeCasting.

• explicit casting of mappedGeneralResponse to IdtRoundingDto

  public Payloadable getReport(map<String, Object> request){
  IdtRoundingDto idtRoundingDto = convertToIdtRoundingDto(mappedGeneralResponse);
  return idtRoundingDto;
  }
  

convertToIdtRoundingDto method

private IdtRoundingDto convertToIdtRoundingDto(MappedGeneralResponse mappedGeneralResponse){
List<Payloadable> payloadableList = mappedGeneralResponse.getData();
IdtRoundingDto idtRoundingDto = (IdtRoundingDto) payloadableList.get(0);
return idtRoundingDto;
}

• payloadableList is retrieving data from mappedGeneralResponse which is child of Payloadable.
• idtRoundingDto is retrieving data from payloadableList by casting to IdtRoundingDto which is child of Payloadable which satisfies rule 1,2,3 of Object TypeCasting.