Conditional calling of a constructor

Question

I've a situation similar to this.

What if I still want to call constructors conditionally? (Though they said creating separate classes is suggestible)

Requirement structure:

Super class:

 public class Super
    {
        public Super(DTO1 dto1Object){
               this.dto1Object = dto1Object;     
          }

        public Super(DTO2 dto2Object)){
               this.dto2Object = dto2Object;     
          }
    }

Derived class:

 public class Derived extends Super
    {
        public Derived(Object obj)
        {
           //some_condition to check if passed object obj is dto1Object                 
           //do something with dto1Object

           //some_condition to check if passed object is dto2Object                               
           //do something with dto2Object


        }
    }

How should I implement it?

EDIT:

Implemented it in this way based on the suggestion below:

Super class:

 public class Super
    {
        protected static DTO1 dto1Obj;
        protected static DTO2 dto2Obj;

        public Super(DTO1 dto1Object){
               this.dto1Object = dto1Object;     
          }

        public Super(DTO2 dto2Object)){
               this.dto2Object = dto2Object;     
          }
    }

Derived class:

    public class Derived extends Super
    {
        public Derived(DTO1 dto1Object){ super(dto1Object); }
        public Derived(DTO2 dto2Object){ super(dto2Object); }

        public static Derived create(Object obj) {

           if (obj.equals(dto1Obj) {
             return new Derived((DTO1) obj);
           }

           if (obj.equals(dto2Obj) {
             return new Derived((DTO2) obj);
           }

           // ...

          private String Function(String str){
            if(create(dto1Obj).equals(dto1Obj) { 
             //do something
            }
            else if(create(dto2Obj).equals(dto2Obj)){
             //do something else 
            }
            return str;
          }

        }
    }

EDIT2:

As per suggestion below, is this the correct way to use instanceof?

if (create(dto1Obj) instanceof DTO1) {
          //something
        }
        else if(create(dto2Obj) instanceof DTO2) {
          //something else
        }

Is shows the following error:

Incompatible conditional operand types Derived and DTO1
Incompatible conditional operand types Derived and DTO2

You can't in the constructor, because the `super(...)` has to be the first statement. — Andy Turner, Jan 30 '18 at 15:45
You can't, basically. Either you need to add a `Super(Object)` constructor, or create a static factory method in `Derived` and two separate private constructors. — Jon Skeet, Jan 30 '18 at 15:45
yes. just use static factory - this is good practice according to the "Effective Java" — Oleksandr Papchenko, Jan 30 '18 at 15:57

score 3 · Accepted Answer · answered Jan 30 '18 at 15:47

3

You can't in the constructor, because the super(...) has to be the first statement.

The only way I can think is to use a static factory method, and invoke class-specific overloads of the constructor:

public class Derived extends Super
{
    private Derived(DTO1 dto1Object){ super(dto1Object); }
    private Derived(DTO2 dto2Object){ super(dto2Object); }

    public static Derived create(Object obj) {
       //some_condition to check if passed object obj is dto1Object                 
       //do something with dto1Object
       if (someCondition) {
         return new Derived((DTO1) obj);
       }

       //some_condition to check if passed object is dto2Object                               
       //do something with dto2Object
       if (someOtherCondition) {
         return new Derived((DTO2) obj);
       }

       // ...?
    }
}

answered Jan 30 '18 at 15:47

Andy Turner

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Perfect! Thanks! I've a quick question (may be a dumb one) Can I do this to check if obj is dto1Obj or dto2Obj? if(create(dto1Obj).equals(dto1Obj)){ //do something } else { //something else } – intruder Jan 30 '18 at 16:40
You can use `instanceof`... But if that's the only check you're trying to do, why not just expose the constructors directly? Then the type is checked at compile time. – Andy Turner Jan 30 '18 at 16:59
Do you mean the way I edited the post? (Can't post code here so I added as an edit in my question) – intruder Jan 30 '18 at 17:20

Conditional calling of a constructor

1 Answers1