@Path("/test")
    public class TestResource {

        @GET
        @Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
        @XmlElementWrapper( name= "tests")
        @XmlElements({
            @XmlElement(name= "coolTest", type=CoolTest.class)
            /**Add other type of test that are managed by this resource **/})
        public List<Test> getTests(){
            List<Test> tests = new ArrayList<Test>();
            tests.add(new CoolTest("1", "Cool"));
            return tests;
        }
    }

@XmlTransient
    public abstract class Test {

        private String id;

        public Test(){}

        public Test(String id){
            this.id = id;
        }

        @XmlElement
        public String getId() {
        return id;
        }

        public void setId(String id) {
            this.id = id;
        }
    }

@XmlRootElement
    public class CoolTest extends Test{

        private String coolMsg;

        public CoolTest(){}

        public CoolTest(String id, String coolTest){
            super(id);
            this.coolMsg = coolTest;
        }

        @XmlElement
        public String getCoolMsg() {
            return coolMsg;
        }

        public void setCoolMsg(String coolTest) {
            this.coolMsg = coolTest;
        }
    }

SEVERE: A message body writer for Java class java.util.ArrayList, and Java type java.util.List<org.arrowhead.wp5.core.model.Test>, and MIME media type application/xml was not found.

@XmlRootElement
    @XmlSeeAlso({CoolTest.class})

@Path("/test")
    @XmlRootElement
    public class TestResource {

        @XmlElementWrapper(name = "tests")
        @XmlElements({@XmlElement(name="coolTestPotato", type=CoolTest.class)})
        public List<Test> getTestList(){
            List<Test> tests = new ArrayList<Test>();
            tests.add(new CoolTest("1", "Cool"));
            return tests;
        }

        @GET
        @Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
        public TestResource getTests(){
            return this;
        }
    }