public static class A {
        private Integer x;

        public Integer getX() {
            return x;
        }

        void setX(Integer x) {
            this.x = x;
        }
    }
    
    public static class B {
        private Integer x;
        
        public Integer getX() {
            return x;
        }
        
        void setX(Integer x) {
            this.x = x;
        }
    }
    
    @Mapper
    public interface Mapping {
        
        Mapping INSTANCE = Mappers.getMapper(Mapping.class);
        
        A toA(B b);
        
        B toB(A a);
    }

public class Main$MappingImpl implements Main.Mapping {

    @Override
    public Main.A toA(Main.B b) {
        if ( b == null ) {
            return null;
        }

        Main.A a = new Main.A();

        return a;
    }

    @Override
    public Main.B toB(Main.A a) {
        if ( a == null ) {
            return null;
        }

        Main.B b = new Main.B();

        return b;
    }
}

Main.A a = new Main.A();
a.setX(b.getX());
return a;