How to count number of instances of specific class in arraylist?

Question

I have an array list that looks like:

[new Class1(), new Class2(), new Class1(), new Class1()]

I want to know the most efficient way to extract the number of instances of Class1 in the array list.

For the above example, I want the answer 3.

I am using Java version "1.7.0_79".

Answer 1

if the arraylist is like below,

newClass1 = new Class1();

[newClass1, new Class2(), newClass1, newClass1]

then you can check the frequency like below,

Collections.frequency(arrayList, newClass1);

If you are always adding new instance of Class1 then below will be the solution

[new Class1(), new Class2(), new Class1(), new Class1()]

override equals method in Class1 like below,

 @Override
public boolean equals(Object o){

    if(!(o instanceof Class1 )){
        return false;
    }else{

            return true;
    }
}

then in your test class,

System.out.println(Collections.frequency(array, new Class1()));