public interface FileProccessor {
    public <RT> RT setHeader(RT header);
}

public class AProcessor implements FileProccessor {

    @Override
    public Header setHeader(Header header) {
        return null;
    }
}

public <RT> RT setHeader(RT header);

public interface FileProccessor<T> {
    public T setHeader(T header);
}

public class AProcessor implements FileProccessor<Header> {
}

public interface FileProccessor<RT, RX> {
    public RT setHeader(RT header);
    public RX setFooter(RX footer);
}

public class AProcessor implements FileProccessor<Header, Footer> {

    @Override
    public Header setHeader(Header header) {
        return null;
    }

    @Override
    public Footer setFooter(Footer footer) {
        return null;
    }
}

public interface FileProccessor<RT> {
    public RT setHeader(RT header);
}

public class AProcessor implements FileProccessor<Header> {
    @Override
    public Header setHeader(Header header) {
        return null;
    }
}

FileProcessor f = new AProcessor();
String s =  f.setHeader("Bah");

public interface FileProccessor<RT> {
    public RT setHeader(RT header);
}

public class AProcessor implements FileProccessor<Header> {

    @Override
    public Header setHeader(Header header) {
        return null;
    }
}

FileProcessor<Header> f = new AProcessor();

@Override
    public Object setHeader(Object header) {
        return null;
    }

public interface FileProccessor<T> {
    public T setHeader(T header);
 }

public class AProcessor implements FileProccessor<Header> {
    @Override
    public Header setHeader(Header header) {
        return null;
    }
}