Refactor multiple If' statements in Java-8

Question

I need to validate mandatory fields in my class

For example, 9 fields must not be null.

I need to check if they are all null but I

Answer 1

A little bit complicated, but I have a good solution because it's generic and can be used with any objects:

Excess excess = new Excess(new Limit());

Checker checker = new Checker<>(
    identity(),
    List.of(
        new CheckerValue<>("excess date is null", Excess::getAsOfDate),
        new CheckerValue<>("limit is null", Excess::getLimit)
    ),
    List.of(new Checker<>(Excess::getLimit, List.of(new CheckerValue<>("limit id is null", Limit::getId))))
);

System.out.println(checker.validate(excess));

This code will print:

excess date is null
    limit id is null

The first class Checker contains:

• sourceFunction - for getting the object
• values - for checking each field from object obtained from sourceFunction

• children - a list of Checker

  class Checker {
  
  Function sourceFunction;
  List> values;
  List> children = emptyList();
  
  /*All args constructor; 2 args constructor*/
  
  public String validate(S object) {
      T value = sourceFunction.apply(object);
      if(value != null) {
          String valueString = values.stream().map(v -> v.validate(value)).filter(Optional::isPresent).map(Optional::get).collect(joining("\n"));
          valueString += "\n\t";
          valueString += children.stream().map(c -> c.validate(value)).collect(Collectors.joining("\n"));
          return valueString;
      }
      return "";
  }
  }
  

and CheckerValue class:

class CheckerValue {

    String validationString;
    Function fun;

    /*all args constructor*/

    public Optional validate(T object) {
        return fun.apply(object) != null ? Optional.empty() : Optional.of(validationString);
    }
 }