Make method of class using generics be able to return both its own and its subclasses instances

Question

I implemented the following Java classes:

public class Data<T> { private List<T> data; public List<T> getData() { return this.data; } public Data<T> setData(List<T> data) { this.data = data; return this; } public Data<T> getAsNullIfEmpty() { if (this.data == null || this.data.isEmpty()) return null; return this; } }

public class IntegerData extends Data<Integer> {}

I would like the getAsNullIfEmpty() method to be able to be called by its subclasses instances.

The line IntegerData integerData = new IntegerData().getAsNullIfEmpty(); throws the following compilation error: incompatible types: Data<java.lang.Integer> cannot be converted to IntegerData

I tried changing the body of the method getAsNullIfEmpty() to this:

public <E extends Data<T>> E getAsNullIfEmpty() { if (this.data == null || this.data.isEmpty()) return null; return this; }

This doesn't compile though because Data<T> does not extend itself. Is there a way to accomplish this without recurring to overriding the method in each of the child classes or using an explicit cast?

Louis Wasserman · Accepted Answer · 2019-10-04 21:41:20Z

2

You need the "usual" trick for having a class's generics refer to itself, plus an unsafe cast.

public class Data<T, D extends Data<T, D>> { private List<T> data; public List<T> getData() { return this.data; } public D setData(List<T> data) { this.data = data; return (D) this; } public D getAsNullIfEmpty() { if (this.data == null || this.data.isEmpty()) return null; return (D) this; } } public class IntegerData extends Data<Integer, IntegerData> { ... }

answered Oct 4, 2019 at 21:41

Louis Wasserman

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5 Comments

Pshemo Over a year ago

Similar version of this idea can be found in Effective Java in Item 2 which discusses builder pattern. There instead of return (D) this; author uses return self(); which is calling protected abstract D self(); with is required to be overridden in subclass as protected [SubclassName] self(){ return this;} (we assume that D is defined to hold subclass name).

mentallurg Over a year ago

This solution solves the problem with casting. But means also, that you will not allow direct usage of Data class like 'd = new Data<Integer>()', because you will have to implement a subclass for each parameter type. It is not much work, but still some overhead.

Louis Wasserman Over a year ago

@mentallurg I think you could actually get away with Data<Integer, ?> d = new Data<Integer, Data<Integer>>() in this version?

mentallurg Over a year ago

No :) Data<Integer> will not compile because here you have one type parameter instead of two.

Louis Wasserman Over a year ago

Hmmm. And Data<Integer, ?> d = new Data<Integer, Data<Integer, ?>>()?

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