Java – how to calculate the number of instances of a specific class in ArrayList?

I have an array list that looks like:

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

I want to know the most effective way to extract the number of Class1 instances from the array list

For the above example, I want answer 3

I use the Java version of "1.7.0_79"

Solution

If ArrayList is as follows,

newClass1 = new Class1();

[newClass1,newClass1,newClass1]

Then you can check the following frequencies,

Collections.frequency(arrayList,newClass1);

If you always add new instances of Class1, here is the solution

[new Class1(),new Class1()]

Override is equal to the method in Class1, as shown 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()));