class Foo {
    private Map<String, Object> data = new HashMap<>();

    String build() {
        StringBuilder builder = new StringBuilder();
        builder.append("{");

        Iterator<Map.Entry<String, Object>> it =  data.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry entry = it.next();
            builder.append(String.format(
                ""%s": "%s"", entry.getKey(), entry.getValue()
            ));

            if (it.hasNext()) {
                builder.append(", ");
            }
        }
        builder.append("}");

        return builder.toString();
    }

    Foo a(Object value) {
        data.put("a", value);
        return this;
    }
    Foo a() {
        return a(null);
    }

    Foo b(Object value) {
        data.put("b", value);
        return this;
    }

    Foo c(Object value) {
        data.put("c", value);
        return this;
    }
}

public static void main(String[] args) {
    System.out.println("text: " + new Foo().a(null).b(1).c("123").build());
    // text: {"a": "null", "b": "1", "c": "123"}

    System.out.println("text: " + new Foo().build());
    // text: {}

    System.out.println("text: " + new Foo().a().build());
    // text: {"a": "null"}
}