sample with mockito

gistfile1.java

import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.spy;
import static org.mockito.Mockito.when;

import org.junit.Test;

public class Sample {

    static class Reader {
        private String fileName;

        public Reader() {
        }

        public void init(String fileName) {
            this.fileName = fileName;
            readHeader();
        }

        public boolean readHeader() {
            return true;
        }
    }

    @Test
    public void sample() throws Exception {
        //Reader orig = new Reader();
        //Reader reader = spy(orig);
        Reader reader = mock(Reader.class);
        when(reader.readHeader()).thenThrow(new RuntimeException());

        reader.init("theFile"); // RuntimeException is not thrown
    }
}