Java Generics questions for multiple type parameters

Question

I have this code:

public interface IDimension<S extends IDimension<S,T>, T extends Number> extends Comparable<S> {
  T toNumber();

  default T toBaseNumber() {
    return toNumber();
  }

  S fromNumber( T units );

  Class<T> numberType();
}

When I implement IDimension like below(Implementation1), I get 'Type parameter 'TestBaseSample' is not within its bound; should implement 'IDimension<TestBaseSample<Float,Integer>, java.lang.Long>' error:

Implementation1 :

class TestBaseSample<Integer, Float> 
    implements IDimension<TestBaseSample<Float, Integer>, Long> { 
}

I understand why 'Implementation1' gives error, but not able to understand why 'Implementation2' and 'Implementation3' works?

Implementation2:

class TestBaseSample2<Integer, Float> 
    implements IDimension<TestBaseSample2<Float, Float>, Long> {
}

Implementation3 :

class TestBaseSample3<Integer, Float> 
    implements IDimension<TestBaseSample3<Integer, Integer>, Long> {
}

Do you realize that you are making Integer and Float into variable type names? eg. class TestBaseSample3<T, S> where T and S are generic types. ideone.com/Zq4AVt for example. Maybe you should change those to T & S so it is clear, or make them concrete. class TestBaseSample3 implements ... — matt
– matt, Commented Oct 19, 2016 at 18:38

matt · Accepted Answer · 2016-10-19 18:46:06Z

1

It looks like you are attempting to create a concrete type, but you are actually creating generic parameters and giving them the names of java classes.

Consider your first example TestBaseSample2 if we change the generic parameters to T and S

class TestBaseSample2<T, S> 
    implements IDimension<TestBaseSample2<S, S>, Long> {
}

Now, you should be able to make an instance provided S is a Long and T can be anything else.

Here is a simpler example.

static class OtherList<Integer> extends ArrayList<Integer>{}

public static void main (String[] args) throws java.lang.Exception
{
    OtherList<String> list = new OtherList<>();
    list.add("this");
    System.out.println(list.size());
}

answered Oct 19, 2016 at 18:46

matt

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Comments

Kiskae · Accepted Answer · 2016-10-20 00:36:34Z

0

Remove all types from your implementations (and never name generic parameters using class names, especially classes from the JDK - use single letters):

class TestBaseSample implements IDimension<TestBaseSample, Long> { 
}

class TestBaseSample2 implements IDimension<TestBaseSample2, Long> {
}

class TestBaseSample3 implements IDimension<TestBaseSample3, Long> {
}

edited Oct 20, 2016 at 0:36

Kiskae

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answered Oct 19, 2016 at 18:54

Bohemian♦

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