Returning child type generic to parent type generic in java-CodePudding

I went through a lot of questions related to this and I couldn't clinch my issue. My question is as bellow.

I have a parent abstract class.

public class abstract Parent{ }

I have another two child classes which are extended from the above parent class.

public class ChildOne extends Parent{}
public class ChildTwo extends Paretn{}

In another class, I am using these three classes as bellow.

public class A{ 
   public List<ExcelRecord<Parent>> getExcelRecords(){
      ChildOne childone = new ChildOne();
      List<ExcelRecord<ChildOne>> list = new ArrayList();
      //few logics

      return list; //**compilation here**
   }
}

compilation issue is: required: List<ExcelRecord> provided: ist<ExcelRecord>

I need to return child type generic to parent type generic. How can I achieve this?

update. The returned value of this method is going to use in the legacy method which can't be altered accordingly.

CodePudding user response：

Use wildcards in your return type. This wildcard will accept any class that extends the Parent class


public List<ExcelRecord<? extends Parent>> getExcelRecords() {
   //...
}

Feel free to check out the Java Generics FAQ for more details

CodePudding user response：

If you cannot alter the return type you can only alter the returned value.

Therefore simply alter the type of your local variable:

List<ExcelRecord<Parent>> list = new ArrayList<>();
                  ^relevant change             ^also add Diamond 
                                               operator to give 
                                               the ArrayList a 
                                               concrete type

The ExcelRecord can only be of the generic type Parent since that's what you declared in the method header.