I was transfering an objected oriented code which was written previously on JAVA to dart to test it visually on my device through flutter framework.
I have a class called Person that has two child classes which are SuperPerson(That has two child classes which are SuperHero and Villian and has an Attack Method) and Civil. and i made a method for the SuperHero class which is called protect that is targeted to protect objects from the Civil class.

Right now, i'm trying to make that when the Villian call the Attack method several times on a SuperHero's object to the point where that object's health get to Zero, it makes the SuperHero object no longer protecting the Civil. and I'm not really sure on how to access Civil's object so that i can make them unprotected by the SuperHero object after death.

Person Class

class Person {
 //civil side
   String protector = '';
   bool protection = false;
  //hero side
   String protectedTargetName = '';
   bool protecting = false;

  //setters
    //Civil side
      String setProtector(String protector) {
        return this.protector = protector;
      }
    
      bool setCivilProtection(bool protection) {
        return this.protection = protection;
      }
    
    //Hero Side
      String setProtectedTargetName(String protectedTargetName) {
        return this.protectedTargetName = protectedTargetName;
      }
    
      bool setHeroProtection(bool protecting) {
        return this.protecting = protecting;
      }

    //getters
    //civil side
      String getProtector() {
        return protector;
      }
    
      bool isProtected() {
        return protection;
      }
    
    //hero side
      String getProtectedTargetName() {
        return protectedTargetName;
      }
    
      bool isProtecting() {
        return protecting;
      }
  }

SuperHero Class

class SuperHero extends SuperPerson
{      
  SuperHero(String n, int a, String s, String p) : super(n, a, s, p) {
        setProtectedTargetName('none');
        setHeroProtection(false);
      }

 void toProtect(Person target, BuildContext context) {
    String tName = target.getName();
    String pName = getName();
    setProtectedTargetName(target.getName());//Hero's side
    setHeroProtection(true);//Hero's side
    target.setCivilProtection(true);//Civil's side
    target.setProtector(getName());//Civil's side
    final snackBar = SnackBar(
      duration: const Duration(milliseconds: 500),
      content: Text('$tName is under $pName\'s protection.'),
    );
    ScaffoldMessenger.of(context).showSnackBar(snackBar);
  }

  void toUnProtect(Person target, BuildContext context) {
    String tName = target.getName();
    String pName = getName();
    setProtectedTargetName('none');//Hero's side
    setHeroProtection(false);//Hero's side
    target.setCivilProtection(false);//Civil's side
    target.setProtector('none');//Civil's side
    final snackBar = SnackBar(
      duration: const Duration(milliseconds: 500),
      content: Text('$tName is no longer under $pName\'s protection'),
    );
    ScaffoldMessenger.of(context).showSnackBar(snackBar);
  }
}

Civil Class

class Civil extends Person {
  //Constructor
  Civil(String n, int a, String s) : super(n, a, s) {
    setProtector('None');
    setCivilProtection(false);
  }
}

Villian Class: Here it only remove the protection from the SuperHero's side but there still protection on the Civil's side which i don't know how to access it from here.

void attack(Person target, BuildContext context) {
    String tName = target.getName();
    String tPronouns = target.pronouns();
    String tProtector = target.getProtectorName();
    if (!(target.isProtected())) {
      super.attack(target, context);
      if (target.isProtecting()) {
        if (target.getHealth() == 0) {
          final snackBar = SnackBar(
            duration: const Duration(milliseconds: 500),
            content:
                Text('$tName is no longer under $tProtector\'s protection'),
          );
          ScaffoldMessenger.of(context).showSnackBar(snackBar);
          //Hero side
          target.setProtectedName('none');
          target.setHeroProtection(false);
          //Supposed to be Civil side but idk how to do it properly
          setCivilProtection(false);
          setProtectorName('none');
        }
      }
    } else {
      final snackBar = SnackBar(
        duration: const Duration(milliseconds: 500),
        content: Text(
            'You can\'t attack $tName, $tPronouns is already under $tProtector\'s protection'),
      );
      ScaffoldMessenger.of(context).showSnackBar(snackBar);
    }
  }

UPFATE
Declaring a variable which is responsible for storing the protected Civil worked fine and without errors in Java. however, when i tried to transfer it to dart i recieved an error since that i'm not quite familiar with dart.
Person class

Person protectedPerson; //Error at this line

  Person getProtectedPerson() {
    return protectedPerson;
  }

  Person setProtectedPerson(Person protectedPerson) {
    return this.protectedPerson = protectedPerson;
  }

The error:

    Non-nullable instance field 'protectedPerson' must be initialized.
Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.

In addition to recieving another error when i'm trying to unprotect the Civil. I don't know if there's any additional way to make this function accepts Null value.

setProtectedPerson(null);

The error:

The argument type 'Null' can't be assigned to the parameter type 'Person'.

UPDATE - Solution

adding question mark after the type helped to solve all the errors.

Person? protectedPerson;

  Person? getProtectedPerson() {
    return protectedPerson;
  }

  Person? setProtectedPerson(Person? protectedPerson) {
    return this.protectedPerson = protectedPerson;
  }`

CodePudding user response：

Can a SuperHero protect multiple Civils? If so, store a list of Civils that the SuperHero is currently protecting (List<Civil>) as an attribute. Whenever a SuperHero dies, remove the protector of all of the members of the list of the corresponding SuperHero.

List<Civil> protectedCivils;

// Whenever you want to remove all of the SuperHero's (target) protection of its Civils.
target.removeProtectedCivils();

// What the method would look like
void removeProtectedCivils() {
   for(Civil civil : protectedCivils) civil.removeProtection();
}

Otherwise, if the SuperHero can only protect one Civil, store the currently protected Civil as an attribute in SuperHero and then you can access it whenever you want to remove the protection relationship. Overall, your code looks could be improved by just referencing the objects in question instead of using Strings and Booleans.