CODEWITHSUNDEEP
javaarraylist
JustAnotherDoomer
JustAnotherDoomer

Reputation: 105

Counting the number of references to certain type in ArrayList

Let's say I've made the following.

ArrayList<Object> happylist = new ArrayList<Object>();
happylist.add("cat");
happylist.add(98);
...

And so on, adding different types of elements. What sort of method would help me count how many times there was a reference to certain type in my ArrayList?

Upvotes: 3

Views: 156

Answers (5)

JustAnotherDoomer
JustAnotherDoomer

Reputation: 105

Thank you for your answers, they helped a lot. For my particular problem in real life, I was able to ease my problem by knowing beforehand what type I was looking for. In my case, I did the following method to call in main method.

int counter = 0;
for (int i = 0; i < happylist.size(); i++) {
   if (happylist.get(i) instanceof WantedType) {
      counter++;
   }
} return counter;

Upvotes: 0

TongChen
TongChen

Reputation: 1430

In Java 8 or other high version you can use Stream Group API to do, a simple code like this:

    ArrayList<Object> happylist = new ArrayList<Object>();
    happylist.add("cat");
    happylist.add(98);
    happylist.add(198);
    happylist.add(1L);
    Map<Object,IntSummaryStatistics> result = happylist.stream()
            .collect(Collectors.groupingBy(o -> o.getClass(),Collectors.summarizingInt(value -> 1)));

    // output result
    result.entrySet().stream()
            .forEach(entry -> System.out.println(String.format("class name = %s\t sum count = %d",entry.getKey(),entry.getValue().getSum())));

IntSummaryStatistics is state object for collecting statistics such as count, min, max, sum, and average.

Upvotes: 0

Farruh Habibullaev
Farruh Habibullaev

Reputation: 2392

It could easily be done counting the number of different types of reference in the list using reflections. I have coded the following method:

public Map<String, Integer> countReferences(List<Object> happyList) {
    Map<String, Integer> referenceCounter = new HashMap<>();

    for (Object object : happyList) {
        String className = object.getClass().getName();
        referenceCounter.put(className, referenceCounter.getOrDefault(className, 0) + 1);
    }
    return referenceCounter;
}

Basically, each class with difference name gives difference reference. By counting reference to each type, and storing them in map gives you what you are looking for.

But I am not quite sure the usefulness of such particular problems.

Upvotes: 1

devmind
devmind

Reputation: 364

static long countByTypeJava8(Collection col, Class clazz) {
    return col.stream()
            .filter(clazz::isInstance)
            .count();
}

static long countByType(Collection col, Class clazz) {
    long count = 0;
    for (Object o : col) {
        if (clazz.isInstance(o)) {
            count++;
        }
    }
    return count;
}

Upvotes: 0

Hazim
Hazim

Reputation: 381

You can use the getClass() method to determine some object's class.

Take a look at the documentation for Object.

Upvotes: 1

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