How to Iterate over an ArrayList of Objects for a specific type in Java

Question

I am trying to iterate through an ArrayList myWaste, of waste objects.

Some of these waste objects are of type "packaging", from the Packaging class, and the objects of type "packaging" need to be counted, and the count needs to be returned.

I have tried with a for each loop but I am not having much luck:

int count = 0;
for (Waste packaging : myWaste){
    count += 1;
}
return count;

Do you mean that Packaging is a class that extends the Waste class? If so, you could just make a for loop iterating through every waste object in your for loop and check if the waste object is an instance of packaging with the instanceof keyword. — Raheel138
– Raheel138, Commented Mar 28, 2017 at 3:18

Eduardo · Accepted Answer · 2017-03-28 03:34:39Z

2

Try this:

int count = 0;
for(myWaste mw : myWaste)
   if(mw instanceof Packaging)
      count++;

Or if you wanna try lambdas with java8+ :

int count = myWaste.stream().filter(myWaste -> myWaste instanceof Packaging).count();

edited Mar 28, 2017 at 3:34

answered Mar 28, 2017 at 3:18

Eduardo

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1 Comment

Raheel138 Over a year ago

Since the myWaste ArrayList is consisted of objects that are of the Waste class, can't you use Waste w: myWaste instead of Object o: myWaste?

Vince · Accepted Answer · 2017-03-28 03:32:17Z

1

If I'm not mistake, try this:

int count = 0;
for (Waste packaging : myWaste){
    if (packaging instanceof Packaging) {
        count += 1;
    }
}
return count;

edited Mar 28, 2017 at 3:32

Vince

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answered Mar 28, 2017 at 3:20

wangyuntao

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