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I have two ArrayList of types ArrayList<ObjectType1> and ArrayList<ObjectType2>.

Then I have created another ArrayList<Object> as object is super class of every object type.

ArrayList<Object1> obj1 = new ArrayList<Object1>();

ArrayList<Object2> obj2 = new ArrayList<Object2>();

These two object class has its own specific getters and setters method.

ArrayList<Object> obj3 = new ArrayList<Object>();


obj3.add(obj1);
obj3.add(obj2);

Now when I am accessing obj3 I want to fetch obj1 and obj2's getters and setters.??

Can any one please help for a solution..???

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  • what you have tried ? Commented May 27, 2014 at 11:33
  • what do you mean by "I want to fetch obj1 and obj2's getters and setters"? Commented May 27, 2014 at 11:33

3 Answers 3

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3

You can do this things:

ArrayList<MyClass1> obj1 = new ArrayList<MyClass1>();
ArrayList<MyClass2> obj2 = new ArrayList<MyClass2>();

ArrayList<Object> obj3 = new ArrayList<Object>();

obj3.add(obj1); 
obj3.add(obj2);

for (int = 0 ; i < obj3.size() ; i++) {
    Object obj = obj3.get(i);
    if (obj instanceof MyClass1) {
        MyClass1 cls1 = (MyClass1)obj;
        cls1.get()..//You getters and setters
    }
    if (obj instanceof MyClass2) {
        MyClass2 cls2 = (MyClass2)obj;
        cls2.get()..//You getters and setters
    }
}

Or you can do something smarter and create an interface or Abstract Class/Base Class

public Interface Getable {
    void getAttrbiute();
}

And than just implement in MyClass1, MyClass2. In this case you can run over the list without the casting an the instaceof check

ArrayList<MyClass1> obj1 = new ArrayList<MyClass1>();
ArrayList<MyClass2> obj2 = new ArrayList<MyClass2>();

ArrayList<Getable> obj3 = new ArrayList<Getable>();

obj3.add(obj1); 
obj3.add(obj2);

for (int = 0 ; i < obj3.size() ; i++) {
   Getable objGetable = obj3.get(i);
   objGetable.getAttrbiute();
 }

Hope that helps..

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2 Comments

when I am doing obj3.size, I am getting a definite size but when I am doing obj3.get(i) in the next step its returning null, Why is this so?
maybe in the obj1 , obj2 you inserted null in one of the nodes?
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if it is a simple object of few extensions scope u can use instanceof for example

class ObjectType1 {
    String getData1() {
        return "data 1";
    }
}

class ObjectType2 {
    String getData2() {
        return "data 2";
    }
}
private String getData(Object obj) {
    if (obj instanceof ObjectType1)
        return ((ObjectType1) obj).getData1();
    else
        return ((ObjectType2) obj).getData2();
}

But for good programming practise try using interfaces or abstraction a basic interface implementation might look like 

interface DataHolder{
    String getData();
}
class ObjectType1 implements DataHolder {
    String getData1() {
        return "data 1";
    }
    public String getData(){
        return getData1();
    }
}

class ObjectType2  implements DataHolder {
    String getData2() {
        return "data 2";
    }
    public String getData(){
        return getData2();
    }
}
List<DataHolder> dataSet = new ArrayList<DataHolder>();

private void print() {
    dataSet.add(new ObjectType1());
    dataSet.add(new ObjectType2());
    for (DataHolder iHolder : dataSet){
        System.out.println(iHolder.getData());
    }
}
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0

You'll have identified that you pull objects from obj3 as Object, rather than their individual types.

You can either:

  1. check the type of each object extracted via instanceof and cast appropriately before accessing the getters. This is very nasty.
  2. Or, (much better) implement a common interface containing the appropriate getters. This interface would be implemented by every object in your obj3 list, and that list would be defined as ArrayList<MyInterface>, where MyInterface defines your getters.

Fundamentally this is a modelling issue. By putting everything into a list of Objects, you're saying that all you know about these objects is that they're Objects. Instead you need to leverage polymorphism around your solution.

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1 Comment

can you give an example for your second option?

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