GenericsTest.java

package io.katharsis.utils;

import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

import java.util.List;

import static org.assertj.core.api.Assertions.assertThat;

public class GenericsTest {

    @Rule
    public ExpectedException expectedException = ExpectedException.none();

    @Test
    public void onGenericClassShouldReturnFirstParameter() throws Exception {
        // WHEN
        Class<?> clazz = Generics
            .getResourceClass(SampleGenericClass.class.getDeclaredField("strings").getGenericType(), List.class);

        // THEN
        assertThat(clazz).isEqualTo(String.class);
    }

    @Test
    public void onGenericWildcardClassShouldThrowException() throws Exception {
        // THEN
        expectedException.expect(RuntimeException.class);

        // WHEN
        Generics.getResourceClass(SampleGenericClass.class.getDeclaredField("stringsWildcard").getGenericType(),
            List.class);
    }

    private static class SampleGenericClass {
        private List<String> strings;
        private List<? extends String> stringsWildcard;
    }
}