public class ImmutableObject {
    @NotNull
    private final String param1;
    @NotNull
    private final String param2;
    @Nullable
    private final String param3;
    @Nullable
    private final String param1;
}

@SuppressWarnings ({"InnerClassFieldHidesOuterClassField"})
public class ImmutableObject {
    @NotNull
    private final String param1;
    @NotNull
    private final String param2;
    @Nullable
    private final String param3;
    @Nullable
    private final String param4;

    public class Builder {
        @NotNull
        private final String param1;
        @NotNull
        private final String param2;
        @Nullable
        private String param3;
        @Nullable
        private String param4;
        public Builder (String param1, String param2) {
            this.param1 = param1;
            this.param2 = param2;
        }
        public Builder param3  (String param3) {
            this.param3 = param3;
            return this;
        }
        public Builder param4 (String param4) {
            this.param4 = param4;
            return this;
        }

        public ImmutableObject build () {
            return new ImmutableObject (this);
        }
    }

    private ImmutableObject (Builder builder) {
        this.param1 = builder.param1;
        this.param2 = builder.param2;
        this.param3 = builder.param3;
        this.param4 = builder.param4;
    }
}