对于单元测试，使用mockito这样的模拟框架：

import org.hamcrest.Matchers;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import org.restlet.representation.Representation;

import static org.junit.Assert.assertThat;
import static org.mockito.Mockito.when;

@ExtendWith(MockitoExtension.class)
public class RestletTest {
    @Mock
    private Representation representation;

    @Test
    public void demonstrateMock() {
        when(representation.getAvailableSize()).thenReturn(1024l);

        ClassToTest t = new ClassToTest();
        assertThat(t.callRepresentation(representation), Matchers.is(1024l));
    }
}

class ClassToTest {
    public long callRepresentation(Representation representation) {
        return representation.getAvailableSize();
    }
}