[java] Create an ArrayList with multiple object types?

How do I create an ArrayList with integer and string input types? If I create one as:

List<Integer> sections = new ArrayList <Integer>();

that will be an Integer type ArrayList. If I create one as:

List<String> sections = new ArrayList <String>();

that will be of String type. How can I create an ArrayList which can take both integer and string input types? Thank you.

You can just add objects of diffefent "Types" to an instance of ArrayList. No need create an ArrayList. Have a look at the below example, You will get below output:

Beginning....
Contents of array: [String, 1]
Size of the list: 2
This is not an Integer String
This is an Integer 1

package com.viswa.examples.programs;

import java.util.ArrayList;
import java.util.Arrays;

public class VarArrayListDemo {

    @SuppressWarnings({ "rawtypes", "unchecked" })
    public static void main(String[] args) {
        System.out.println(" Beginning....");

        ArrayList varTypeArray = new ArrayList();
        varTypeArray.add("String");
        varTypeArray.add(1); //Stored as Integer

        System.out.println(" Contents of array: " + varTypeArray + "\n Size of the list: " + varTypeArray.size());

        Arrays.stream(varTypeArray.toArray()).forEach(VarArrayListDemo::checkType);
    }

    private static <T> void checkType(T t) {

        if (Integer.class.isInstance(t)) {
            System.out.println(" This is an Integer " + t);
        } else {
            System.out.println(" This is not an Integer" + t);
        }
    }

}

You can make it like :

List<Object> sections = new ArrayList <Object>();

(Recommended) Another possible solution would be to make a custom model class with two parameters one Integer and other String. Then using an ArrayList of that object.

You don't know the type is Integer or String then you no need Generic. Go With old style.

List list= new ArrayList ();

list.add(1);
list.add("myname");

for(Object o = list){

} 

You could create a List<Object>, but you really don't want to do this. Mixed lists that abstract to Object are not very useful and are a potential source of bugs. In fact the fact that your code requires such a construct gives your code a bad code smell and suggests that its design may be off. Consider redesigning your program so you aren't forced to collect oranges with orangutans.

Instead -- do what G V recommends and I was about to recommend, create a custom class that holds both int and String and create an ArrayList of it. 1+ to his answer!

(1)

   ArrayList<Object> list = new ArrayList <>();`     
   list.add("ddd");
   list.add(2);
   list.add(11122.33);    
   System.out.println(list);

(2)

 ArrayList arraylist = new ArrayList();
 arraylist.add(5);        
 arraylist.add("saman");     
 arraylist.add(4.3);        
 System.out.println(arraylist);

You can use Object for storing any type of value for e.g. int, float, String, class objects, or any other java objects, since it is the root of all the class. For e.g.

1. Declaring a class

   class Person {
   public int personId;
   public String personName;
   
   public int getPersonId() {
       return personId;
   }
   
   public void setPersonId(int personId) {
       this.personId = personId;
   }
   
   public String getPersonName() {
       return personName;
   }
   
   public void setPersonName(String personName) {
       this.personName = personName;
   }}
   

2. main function code, which creates the new person object, int, float, and string type, and then is added to the List, and iterated using for loop. Each object is identified, and then the value is printed.

       Person p = new Person();
   p.setPersonId(1);
   p.setPersonName("Tom");
   
   List<Object> lstObject = new ArrayList<Object>();
   lstObject.add(1232);
   lstObject.add("String");
   lstObject.add(122.212f);
   lstObject.add(p);
   
   for (Object obj : lstObject) {
       if (obj.getClass() == String.class) {
           System.out.println("I found a string :- " + obj);
       }
       if (obj.getClass() == Integer.class) {
           System.out.println("I found an int :- " + obj);
       }
       if (obj.getClass() == Float.class) {
           System.out.println("I found a float :- " + obj);
       }
       if (obj.getClass() == Person.class) {
           Person person = (Person) obj;
           System.out.println("I found a person object");
           System.out.println("Person Id :- " + person.getPersonId());
           System.out.println("Person Name :- " + person.getPersonName());
       }
   }
   

You can find more information on the object class on this link Object in java

Just use Entry (as in java.util.Map.Entry) as the list type, and populate it using (java.util.AbstractMap’s) SimpleImmutableEntry:

List<Entry<Integer, String>> sections = new ArrayList<>();
sections.add(new SimpleImmutableEntry<>(anInteger, orString)):

You can always create an ArrayList of Objects. But it will not be very useful to you. Suppose you have created the Arraylist like this:

List<Object> myList = new ArrayList<Object>();

and add objects to this list like this:

myList.add(new Integer("5"));

myList.add("object");

myList.add(new Object());

You won't face any problem while adding and retrieving the object but it won't be very useful. You have to remember at what location each type of object is it in order to use it. In this case after retrieving, all you can do is calling the methods of Object on them.

It depends on the use case. Can you, please, describe it more?

• If you want to be able to add both at one time, than you can do the which is nicely described by @Sanket Parikh. Put Integer and String into a new class and use that.

• If you want to add the list either a String or an int, but only one of these at a time, then sure it is the List<Object>

  which looks good but only for first sight! This is not a good pattern. You'll have to check what type of object you have each time you get an object from your list. Also This type of list can contain any other types as well.. So no, not a nice solution. Although maybe for a beginner it can be used. If you choose this, i would recommend to check what is "instanceof" in Java.

• I would strongly advise to reconsider your needs and think about maybe your real nead is to encapsulate Integers to a List<Integer> and Strings to a separate List<String>

Can i tell you a metaphor for what you want to do now? I would say you want to make a List wich can contain coffee beans and coffee shops. These to type of objects are totally different! Why are these put onto the same shelf? :)

Or do you have maybe data which can be a word or a number? Yepp! This would make sense, both of them is data! Then try to use one object for that which contains the data as String and if needed, can be translated to integer value.

public class MyDataObj {
String info;
boolean isNumeric;

public MyDataObj(String info){
    setInfo(info);
}

public MyDataObj(Integer info){
    setInfo(info);
}

public String getInfo() {
    return info;
}

public void setInfo(String info) {
    this.info = info;
    this.isNumeric = false;
}

public void setInfo(Integer info) {
    this.info = Integer.toString(info);
    this.isNumeric = true;
}

public boolean isNumeric() {
    return isNumeric;
}
}

This way you can use List<MyDataObj> for your needs. Again, this depends on your needs! :)

Some edition: What about using inharitance? This is better then then List<Object> solution, because you can not have other types in the list then Strings or Integers: Interface:

public interface IMyDataObj {
public String getInfo();
}

For String:

public class MyStringDataObj implements IMyDataObj {

final String info;

public MyStringDataObj(String info){
    this.info = info;
}

@Override
public String getInfo() {
    return info;
}
}

For Integer:

public class MyIntegerDataObj implements IMyDataObj {

final Integer info;

public MyIntegerDataObj(Integer info) {
    this.info = info;
}

@Override
public String getInfo() {
    return Integer.toString(info);
}
}

Finally the list will be: List<IMyDataObj>

List<Object> list = new ArrayList<>();
          list.add(1);
          list.add("1");

As the return type of ArrayList is object, you can add any type of data to ArrayList but it is not a good practice to use ArrayList because there is unnecessary boxing and unboxing.

I am also newish to Java and just figured this out. You should create your own class which stores the string and integer, and then make a list of these objects. For instance (I am sure this code is imperfect, but better than arrayList):

class Stuff {
    private String label;
    private Integer value;

    // Constructor or setter
    public void Stuff(String label, Integer value) {
        if (label == null || value == null) {
            return;
        }
        this.label = label;
        this.value = value;
    }

    // getters

    public String getLabel() {
        return this.label;
    }

    public Integer getValue() {
        return this.value;
    }
}

Then in your code:

private ArrayList<Stuff> items = new ArrayList<Stuff>();
items.add(new Stuff(label, value));

for (Stuff item: items) {
     doSomething(item.getLabel()); // returns String
     doSomething(item.getValue()); // returns Integer
}