CODEWITHSUNDEEP
java
gumba
gumba

Reputation: 72

Check if an object is of the same type as the passed one

I have class A and two classes B and C which extend A. I have an array list which contains multiple instances of all of these classes. Can I have a method which takes in an object and returns all objects in the array list which is of the same type as the one given to the method and how would I go about doing this?

Upvotes: 0

Views: 700

Answers (3)

singe3
singe3

Reputation: 2105

Assuming you have that object somewhere to point to your list List<A> yourOriginalList

If you are using Java 8 you can write a reusable filter method not just for B or C elements but for anything you want to compare. This is using the new lamba expressions introduced in Java 8.

public List<A> getFilteredList(Predicate<A> predicate){
        List<A> filteredList = new ArrayList<A>();
        for(A value : yourOriginalList){
            if(predicate.test(value)){
                filteredList.add(value);
            }
        }
        return filteredList;
    }

To filter for B objects, use :

List<A> filteredListOfB = getFilteredList((value) -> value instanceof B);

To filter for C objects, use :

List<A> filteredListOfC = getFilteredList((value) -> value instanceof C);

As you can seee you can write anything you want to test in the parameter of getFilteredList

(not tested for syntax error, but the idea is here)

Upvotes: 0

Marco13
Marco13

Reputation: 54639

Although it's already answered, I had started creating an example, as an MCVE, so I'll just drop this here:

import java.util.ArrayList;
import java.util.List;

public class TypeFilterTest 
{
    public static void main(String[] args) {

        List<Object> list = new ArrayList<Object>();
        list.add(1);
        list.add(2.0f);
        list.add(3.0);
        list.add(4);
        list.add(5.0f);
        list.add(6.0);
        list.add("seven");
        list.add(null);

        System.out.println("Integers    : "+findSameType(list, Integer.class));
        System.out.println("Floats      : "+findSameType(list, Float.class));
        System.out.println("Doubles     : "+findSameType(list, Double.class));

        System.out.println("Comparables : "+findSubType(list, Comparable.class));

    }

    private static <T> List<T> findSameType(List<? super T> list, Class<T> type)
    {
        List<T> result = new ArrayList<T>();
        for (int i=0; i<list.size(); i++)
        {
            Object element = list.get(i);
            if (element != null && element.getClass().equals(type))
            {
                result.add(type.cast(element));
            }
        }
        return result;
    }

    private static <T> List<T> findSubType(List<? super T> list, Class<T> type)
    {
        List<T> result = new ArrayList<T>();
        for (int i=0; i<list.size(); i++)
        {
            Object element = list.get(i);
            if (type.isInstance(element))
            {
                result.add(type.cast(element));
            }
        }
        return result;
    }   

}

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1500235

Sure - you'd use getClass() to find the execution-time type:

private final List<A> allKnownValues = ...;

List<A> getAllValuesOfTheSameType(A sample) {
    ArrayList<A> results = new ArrayList<>();
    for (A candidate : allKnownValues) {
        if (candidate.getClass() == sample.getClass()) {
            results.add(candidate);
        }
    }
    return results;
}

If you want to include subtypes, you could use:

if (sample.getClass().isInstance(candidate))

Upvotes: 2

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