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I don't understand why my generic type declaration doesn't match

Here are the classes declarations:
public interface IPoint<N extends Number> {
...
}
public abstract class PointP<N extends Number> implements IPoint<N> {
...
}
public class Pointf extends PointP<Float> {
...
}
public interface ISegment<T extends Number, P extends IPoint<T>> {
...
}
public abstract class SegmentP<N extends Number, P extends IPoint<N>> implements ISegment<N, P> {
...
}
public class Segmentf extends SegmentP<Float, Pointf> {
...
}
public abstract class LinesPIterator<N extends Number, S extends ISegment<N, IPoint<N>>> implements Iterable<S>, Iterator<S> {
...
}
public class LinesfIterator extends LinesPIterator<Float, Segmentf> {
...
}
The compiler refuses the Segmentf type in the generic declaration of the LinesfIterator class with the error message:
Bound mismatch: The type Segmentf is not a valid substitute for the bounded parameter <S extends ISegment<N,IPoint<N>>> of the type LinesPIterator<N,S>
However for me everything seems correct. The declaration of the LinesfIterator class seems to me to have the same hierarchical schema as the Segmentf class which compiles without problem.
Is there a solution to this way of doing things?

As already said, your hierarchy seems to be unnecessarily complex and shall be simplified. For example, I see no meaning in Pointf -> PointP -> IPoint the hierarchy.
If you want to fix your issue, you have to allow a subtype ? extends IPoint<N> in the LinesPIterator class, so:
public abstract class LinesPIterator<N extends Number, S extends ISegment<N, ? extends IPoint<N>>>
implements Iterable<S>, Iterator<S>
{
// ...
}
Moreover, there would be better to implement only Iterable as long as it provides an Iterator and you might end up with duplicated implementation.
public static class LinesfIterator extends LinesPIterator<Segmentf, Pointf, Float> {
#Override
public Iterator<Segmentf> iterator() {
return new Iterator<Segmentf>() {
#Override
public boolean hasNext() { /* TO DO */ }
#Override
public Segmentf next() { /* TO DO */ }
};
}
}

This remark on the use of an anonymous class rather than a direct use really caught my attention because intuitively, when I can avoid going through an anonymous class I do. On the one hand because it is an additional instantiation and on the other hand because it is more difficult to identify at debug (when they are several in the same class).
And I can't see the reasons why I should prefer the use of an anonymous class for this case.
Maybe with my classes as an example the explanation will be easier.
for(Segmentf segment : new LinesfIterator(cube.getPoints(), cube.getIndices())) {
System.out.println(segment);
}
public abstract class LinesPIterator<N extends Number, S extends ISegment<N, ? extends IPoint<N>>> implements Iterable<S>, Iterator<S> {
private N[][] points;
private int[] indices;
private int count;
public LinesPIterator(N[][] points, int[] indices) {
super();
this.points = points;
this.indices = indices;
}
protected abstract S instanciateIteration(final N[] pointDeb, final N[] pointFin);
#Override
public Iterator<S> iterator() {
return this;
}
#Override
public boolean hasNext() {
return count < (indices.length - 1);
}
#Override
public S next() {
return instanciateIteration(points[indices[count++]], points[indices[count++]]);
}
}
public class LinesfIterator extends LinesPIterator<Float, Segmentf> {
public LinesfIterator(Float[][] points, int[] indices) {
super(points, indices);
}
#Override
protected Segmentf instanciateIteration(Float[] point1, Float[] point2) {
return new Segmentf(point1, point2);
}
}

Java generics self-reference: is it safe?

I have this simple interface:
public interface Node<E extends Node<E>>
{
public E getParent();
public List<E> getChildren();
default List<E> listNodes()
{
List<E> result = new ArrayList<>();
// ------> is this always safe? <-----
#SuppressWarnings("unchecked")
E root = (E) this;
Queue<E> queue = new ArrayDeque<>();
queue.add(root);
while(!queue.isEmpty())
{
E node = queue.remove();
result.add(node);
queue.addAll(node.getChildren());
}
return result;
}
}
I see that this is always an instance of Node<E> (by definition).
But I can't imagine a case where this is not an instance of E...
Since E extends Node<E>, shouldn't Node<E> also be equivalent to E by definition??
Can you give an example of an object that's an instance of Node<E>, but it's not an instance of E??
Meanwhile, my brain is melting...
The previous class was a simplified example.
To show why I need a self-bound, I'm adding a bit of complexity:
public interface Node<E extends Node<E, R>, R extends NodeRelation<E>>
{
public List<R> getParents();
public List<R> getChildren();
default List<E> listDescendants()
{
List<E> result = new ArrayList<>();
#SuppressWarnings("unchecked")
E root = (E) this;
Queue<E> queue = new ArrayDeque<>();
queue.add(root);
while(!queue.isEmpty())
{
E node = queue.remove();
result.add(node);
node.getChildren()
.stream()
.map(NodeRelation::getChild)
.forEach(queue::add);
}
return result;
}
}
public interface NodeRelation<E>
{
public E getParent();
public E getChild();
}

An easy example to illustrate the problem: a node of a different type of node:
class NodeA implements Node<NodeA> {
...
}
And:
class NodeB implements Node<NodeA> {
...
}
In this case, E root = (E) this would resolve to NodeA root = (NodeA) this, where this is a NodeB. And that's incompatible.

Without <E extends Node<E>>, you could have either of these cases:
Node<Integer>
where the generic type isn't a Node at all, or
Node<DifferentNode>
where the generic bounds don't match.
That said, it's not typical to see a bound this way, as Node<E> is expected to be a node that contains some value of type E, and children would be a List<Node<E>>, not a List<E>.

The problem is not in E root = (E) this. It might work well until you start iterating through result of listNodes().
That example demonstrates where exactly ClassCastException will be thrown:
public interface Node<E extends Node<E>> {
List<E> getRelatedNodes();
default List<E> getAllNodes() {
List<E> result = new ArrayList<>();
result.add((E) this); //<--that cast is not a problem because of type erasure
return result;
}
}
class NodeA implements Node<NodeA> {
public NodeA() {
}
#Override
public List<NodeA> getRelatedNodes() {
return null;
}
}
class NodeB implements Node<NodeA> {
private List<NodeA> relatedNodes;
public NodeB(List<NodeA> relatedNodes) {
this.relatedNodes = relatedNodes;
}
#Override
public List<NodeA> getRelatedNodes() {
return relatedNodes;
}
}
Execute:
List<NodeA> nodes = new NodeB(Arrays.asList(new NodeA())).getAllNodes(); //according to generic it is list of NodeA objects
for (NodeA node : nodes) { //ClassCastException will be thrown
System.out.println(node);
}

With this sort of situation it is often useful to have a getThis method that (by convention) returns this.
I would do the following
public interface Node<E extends Node<E, R>,
R extends NodeRelation<E, R>>
{
public List<R> getParents();
public List<R> getChildren();
public List<E> listDescendants() ;
}
public interface NodeRelation<E extends Node<E, R>,
R extends NodeRelation<E, R>>
{
public E getParent();
public E getChild();
}
abstract class ANode<E extends ANode<E,R>,
R extends ARelation<E,R>>
implements Node<E,R> {
abstract protected E getThis() ;
public List<E> listDescendants()
{
List<E> result = new ArrayList<>();
E root = getThis() ;
...
return result;
}
}
abstract class ARelation<E extends ANode<E,R>,
R extends ARelation<E,R>>
implements NodeRelation<E,R> {
}
class CNode extends ANode<CNode, CRelation> {
public CNode getThis() { return this ; }
...
}
class CRelation extends ARelation<CNode, CRelation> {
...
}
Although I might not bother with having both abstract class and interface layers.

How create multi constructor by same parameter in Java

How resolve this
how change code to get same result
public class myClass
{
List<Drawable> d;
List<Bitmap> b;
public myClass(Integer[] resIDsList)
{
...
}
public myClass(List<? extends Drawable> drawableList) // error occure here
{
d = drawableList;
}
public myClass(List<? extends Bitmap> bitmapList) // and here too
{
b = bitmapList;
}
}
if constructors are same in above?

Define static factory methods, and make the constructor private:
static myClass fromDrawables(List<? extends Drawable> list) {
return new myClass(list, null);
}
static myClass fromBitmaps(List<? extends Bitmap> list) {
return new myClass(null, list);
}
private myClass(List<? extends Drawable> drawables, List<? extends Bitmap> bitmaps) {
// ...
}
(You probably want to add another factory method for the myClass(Integer[]) case; but I hope you get the idea of how to extend the above code for this).
Now you would invoke the factory methods, rather than the constructor:
// Not new myClass(...)
myClass a = myClass.fromDrawables(drawablesList);
myClass b = myClass.fromBitmaps(bitmapsList);
I'd recommend reading Effective Java 2nd Ed Item 1: "Consider static factory methods instead of constructors" for a thorough discussion of this approach.

Due to type erasure, this will not compile. One solution would be to pass the elements to the constructor and simply add them to each respective List.
public myClass(Drawable... drawables) {
d = new ArrayList<>(Arrays.asList(drawables));
}
public myClass(Bitmap... bitmaps) {
b = new ArrayList<>(Arrays.asList(bitmaps));
}

You will need to change your List<Drawable> d; to List<? extends Drawable> d; outside of your method. Do this for your List<Bitmap> b; as well. Afterwards, your class should look like this:
public class myClass
{
List<? extends Drawable> d;
List<? extends Bitmap> b;
public myClass(Integer[] resIDsList)
{
...
}
public myClass(List<? extends Drawable> drawableList) // error occurs here
{
d = drawableList;
}
public myClass(List<? extends Bitmap> bitmapList) // and here too
{
b = bitmapList;
}
}
You could also merge both of those constructors into one if you wanted, with this:
public myClass(List<? extends Drawable> drawableList, List<? extends Bitmap> bitmapList)
{
d = drawableList;
b = bitmapList;
}

Avoid cast in a generics hierarchy

I have some difficulty to simplify more the problem. Sorry if they are too many code here.
I try to improve the architecture of the code above because I hate warning and cast and I feel something wrong.
Now, the code.
I have a util class with these two parametrized methods (same signature as OpenJPA's CriteriaBuilder...)
public class MyUtil {
public void equal(List<?> l, Object value) {
// do something (see CriteriaBuilder.equal method)
}
public <Y extends Comparable<? super Y>> void greaterThan(List<? extends Y> l, Y value) {
// do something (see CriteriaBuilder.greaterThan method)
}
}
Then, I want to be able to abstract them to call it via an interface.
public interface IOperation<T> {
// maybe make this method generic ? but how ?
public abstract void doOp(List<T> l, T value);
}
public abstract class AbstractOperation<T> implements IOperation<T> {
protected MyUtil myUtil;
}
public class EqualOp extends AbstractOperation<Object> {
#Override
public void doOp(List<Object> path, Object value) {
myUtil.equal(path, value);
}
}
public class GreaterThanOp<T extends Comparable<? super T>> extends AbstractOperation<T> {
#Override
public void doOp(List<T> path, T value) {
myUtil.greaterThan(path, value);
}
}
I create a factory
public class OperationFactory {
private static OperationFactory instance;
public static OperationFactory getInstance() {...}
public IOperation<?> get(String op) {
if ("=".equals(op)) {
return new EqualOp();
} else if (">".equals(op)) {
return new GreaterThanOp<Comparable<? super Object>>();
}
throw new InvalidParameterException();
}
}
Then I use it :
public class Client {
public void needOp(String op) {
IOperation<String> operation = (IOperation<String>) OperationFactory.getInstance().get(op); // How to avoid this cast ?
List<String> l = null;
operation.doOp(l, "a string");
}
}
My question is : is it possible to avoid this cast in the Client class ? How ? Is there a way to have a better architecture ?
Thanks for reading

I'm assuming you can require your type to be Comparable.
Parameterize EqualOp like GreaterThanOp:
public class EqualOp<T extends Comparable<T>> extends AbstractOperation<T> {
#Override public void doOp(List<T> path, T value) ...
And define get() like this:
public <T extends Comparable<T>> IOperation<T> get(String op) {
if ("=".equals(op)) {
return new EqualOp<T>();
} else if (">".equals(op)) {
return new GreaterThanOp<T>();
}
...

Inheritance: Incompatible Generic Types with Same Bound

So, I have this public interface NodeLevel<E extends NodeLevelEnum>, which has methods public E getParent() and public E getEnum(). An implementing abstract public class NodeLevelAbstract<E extends NodeLevelEnum> implements NodeLevel<E> delegates to instances of class A that implements public interface NodeLevelEnum<E extends NodeLevelEnum>. The last declaration should mean that instances of classes that implement NodeLevelEnum can do something with instances of classes that implement NodeLevelEnum.
Now, this is NodeLevelAbstract#getParent():
#Override
public E getParent() {
return this.getEnum().getParent();
}
Results in a compiler error (I use NetBeans 7.2, btw):
incompatible types
required: E
found: NodeLevelEnum
where E is a type-variable:
E extends NodeLevelEnum declared in class NodeLevelAbstract
To me, this means that a descendant of NodeLevelEnum is required, but NodeLevelEnum is found, which, given that the bound in Java Generics includes itself, sounds like absolute bollocks.
Why are these types incompatible? And is there any graceful way to do what I mean to do?
Thanks in advance. =)
Update 1
Btw, NodeLevelEnum#getParent() returns <E extends NodeLevelEnum>, and not NodeLevelEnum, which the error says it does.
Update 2
abstract public class NodeLevelAbstract<E extends NodeLevelEnum> implements
NodeLevel<E> {
protected E _enum;
#Override
public E getEnum() {
return this._enum;
}
#Override
public E getParent() {
return this.getEnum().getParent();
}
public static <E extends NodeLevelEnum<E>> E[] getEnumLineage(E _enum) {
ArrayList<E> ancestors = new ArrayList<>();
E currentEnum = _enum;
do {
ancestors.add(currentEnum);
currentEnum = currentEnum.getParent();
} while (currentEnum != null);
return (E[]) ancestors.toArray();
}
public static <E extends NodeLevelEnum<E>> HashMap<String, String>
getEnumLineageValueMap(
E _enum) {
HashMap<String, String> map = new HashMap<>();
for (E e : getEnumLineage(_enum)) {
map.put(e.getCode(), e.getValue());
}
return map;
}
}
public interface NodeLevel<E extends NodeLevelEnum> {
public E getEnum();
public E getParent();
}
public interface NodeLevelEnum<E extends NodeLevelEnum> {
public E getParent();
}
public interface FilestructureLevel<E extends NodeLevelEnum<E>> extends
NodeLevel<E> {
public String getPathPrefix();
}
public class FileLevel<E extends NodeLevelEnum<E>> extends NodeLevelAbstract<E>
implements FilestructureLevel<E> {
protected String _pathPrefix;
#Override
public String getPathPrefix() {
return this._pathPrefix;
}
public HashMap<String, String> getValueMap(Boolean withPath) {
return getEnumLineageValueMap(this.getEnum(), withPath);
}
public static <E extends NodeLevelEnum<E>> HashMap<String, String>
getEnumLineageValueMap(
E _enum) {
return getEnumLineageValueMap(_enum, false);
}
public static <E extends NodeLevelEnum<E>> HashMap<String, String>
getEnumLineageValueMap(
E _enum, Boolean withPath) {
HashMap<String, String> map = new HashMap<>();
FileLevelEnum[] lineage = (FileLevelEnum[]) getEnumLineage(_enum);
for (FileLevelEnum e : lineage) {
String value = !withPath ? e.getValue() : e.getPathPrefix()
+ e.getValue();
map.put(e.getCode(), value);
}
return map;
}
}