user414967 user414967 - 1 year ago 75
Java Question

issue with Generics while designing

I am facing some issues in Generics while designing a filter framework.
I am getting compilation error. I dont understand and how to ressolve.
I tried giving but I think that is not a perfect way.
Following the compilation error

The method filter(String, Filter<Product>) in the type
CollectionMapper3<Product> is not applicable for the arguments
(String, Filter<Object>)


public interface Filter<T> {
public abstract boolean matches(T object);

public abstract class AttributeFilter3<T,C extends Comparable<C>> implements Filter<T> {
String property;
Comparable<C> comparable;

public AttributeFilter3(Comparable<C> comparable) {
this.comparable = comparable ;

public void setProperty(String property) { = property;

protected abstract boolean compare(int value);

public boolean matches(T t) {
return false;

public static <T,C extends Comparable<C>> Filter<T> EQUALS(Comparable<C> comparable) {
return new AttributeFilter4<T,C>(comparable) {
protected boolean compare(int result) {
return result == 0;


public class CollectionMapper3<T> {

private Collection<T> collection;

public CollectionMapper3(Collection<T> collection) {
this.collection = collection;


public final CollectionMapper3<T> filter(String propertyName, Filter<T> filter) {
Collection<T> result = new ArrayList<>();
for (T t : collection) {
if (filter instanceof AttributeFilter3) {
((AttributeFilter3) filter).setProperty(propertyName);
if (filter.matches(t)) {
collection = result;
return this;

public Collection<T> getResult() {
return collection;


public class Client3 {
public static void main(String[] args) {
Collection<Product> allProducts = MockUtil.getAllProducts();
// Here is the compilation error I am getting
Collection<Product> result = new CollectionMapper3<Product>(allProducts)
.filter("name", AttributeFilter3.EQUALS("sss")).filter("quantityInStock", AttributeFilter3.EQUALS(8))


public class Product {

private String name;

public void setName(String name) { = name;
public String getName() {
return name;

public String toString() {
return name ;


Answer Source

You don't provide anything from which to infer T when you invoke AttributeFilter3.EQUALS("sss"), so Java will use Object.

You can either provide type parameters:

AttributeFilter3.<String, String>EQUALS("sss")

or you can update the method signature to accept a type token:

public static <T,C extends Comparable<C>> Filter<T> EQUALS(Comparable<C> comparable, Class<T> ignored) {


AttributeFilter3.EQUALS("sss", String.class)