CODEWITHSUNDEEP
javaconstructorinner-classesnested-classouter-classes
kmecpp
kmecpp

Reputation: 2479

Create Instance of a Class that is Used in the Constructor of its Outer Class

I'm trying to create an instance of a class that is used in the constructor of its outer class. Referring to the code below, I need a UserData object but I also need a TimeOnlineInfo object to create it, and I don't see a way of getting TimeOnlineInfo object without first having an instance of UserData because TimeOnlineInfo is not static. I can't make it static though because it needs to access a method from its outer class. Is there anyway I could get this to work or get the most similar effect? I did realize that I could just make the classes static and not save the data directly in the addTime method but i was already halfway through this question and I'm curious to see if there is a way of doing this.

Here is a very simplified version of my code:

class UserData {
    TimeOnlineInfo timeOnline;

    public UserData(Object data1, Object data2, Object data3, Object data4, Object data5, TimeOnlineInfo timeOnlineInfo){
        this.timeOnlineInfo = timeOnlineInfo;
    }

    public class TimeOnlineInfo {
        private int time;

        public TimeOnlineInfo(int time){
           this.time = time;
        }

        public void addTime(int time){
            this.time += time;
            UserData.this.saveData();
        }
    }
}


UserData userData = new UserData(new UserData.TimeOnlineInfo());//Doesn't work because PlayInfo is not a static class
UserData userData = new UserData(userData.new TimeOnlineInfo());//This is just a stupid because i'm using the uncreated object in its own constructor

Upvotes: 0

Views: 84

Answers (3)

biziclop
biziclop

Reputation: 49724

You could change your constructor to this:

public void UserData(int time){
    this.timeOnlineInfo = new TimeOnlineInfo(time);
}

You can also make TimeOnlineInfos constructor private to stop others instantiating rogue TimeOnlineInfo instances.

But based on the snippet you posted, I'd probably get rid of TimeOnlineInfo completely.

I've been warned that what I thought was a constructor isn't one. So first things first, never create regular methods with the same name as the class.

The rest still stands though, the problem with this arrangement is that anyone can create a TimeOnlineInfo object, which will trigger UserData.this.saveData() even if it isn't referenced by the UserData it was created with.

So either:

  1. Make the TimeOnlineInfo instantiation a job for UserData.
  2. Get rid of TimeOnlineInfo. (Not feasible if your class is already big, but then again if your class is that big, you probably need to do other things too.)

  3. Make your inner class static and manage the connection explicitly: 

    public void setTimeOnlineInfo(TimeOnlineInfo timeOnlineInfo){
if (timeOnlineInfo.userData != null) {
    throw new IllegalArgumentException( "TOI already belongs to other UserData" );
}
 if (this.timeOnlineInfo != null) {
     this.timeOnlineInfo.userData = null;
 }
 timeOnlineInfo.userData = this;
 this.timeOnlineInfo = timeOnlineInfo;

    }

    public static class TimeOnlineInfo { private int time; private UserData userData;

     public TimeOnlineInfo(int time){
    this.time = time;
 }

 public void addTime(int time){
     this.time += time;
     userData.saveData();
 }

    }

Far from ideal, but it would solve a couple of other problems too.

Upvotes: 0

Makoto
Makoto

Reputation: 106390

You've got some general confusion about a few things. Let's start at the beginning.

First, this isn't a constructor.

public void UserData(TimeOnlineInfo timeOnlineInfo){
    this.timeOnlineInfo = timeOnlineInfo;
}

You meant to drop the void declaration.

public UserData(TimeOnlineInfo timeOnlineInfo){
    this.timeOnlineInfo = timeOnlineInfo;
}

Second, it doesn't make structural sense to instantiate the outer class with an instance of the inner class, since the only way to get an instance of the inner class is to use the outer class to begin with.

For example, you would have to use new UserData().new TimeOnlineInfo(int) for an instance of TimeOnlineInfo, but that's only:

  • if you elected to create a no-arg constructor for UserData
  • if you really didn't care about the instance of UserData you were getting back

If you really want to keep this design, then consider passing in an int to your construtor instead, so it can be supplied to the instance of TimeOnlineInfo.

class UserData {
    TimeOnlineInfo timeOnlineInfo;

    public void saveData() {
        // stub
    }

    public UserData(int value) {
        this.timeOnlineInfo = new TimeOnlineInfo(value);
    }


    public class TimeOnlineInfo {
        private int time;

        public TimeOnlineInfo(int time){
            this.time = time;
        }

        public void addTime(int time){
            this.time += time;
            UserData.this.saveData();
        }
    }
}

Upvotes: 2

Mena
Mena

Reputation: 48404

First of all, your UserData constructor isn't one.

You've declared it as a void method, so no UserData constructor would compile, parametrized with a TimeOnlineInfo instance.

But this might only be a typo.

Then, if you can implement a parameterless constructor for UserData on top of the existing one, you could use the following idiom:

UserData ud = new UserData(new UserData().new TimeOnlineInfo(42));

However, this seems to indicate a more general problem within your design, as you're basically having to initialize UserData twice to get a usable instance.

The idea here is, you either inject TimeOnlineInfo in a constructor, in which case TimeOnlineInfo is likely to be used by broader scopes than just UserData, or you only use TimeOnlineInfo within UserData, in which case use an empty constructor for UserData and initialize your internal TimeOnlineInfo within it.

Upvotes: 1

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