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".
You could iterate through the ArrayList and check using the instanceof operator.
for (Object e : lst)
{
if (e instanceof Car)
{
carCount++;
}
}
instanceof will be true for objects of classes that extend Car, as well as Car itself. If that isn't what you want, then make it if (e.getClass() == Car.class).
Collections.frequency isn't any more efficient than if you or I wrote it. grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/…equals methods, which is surely slower than an instanceof operation.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()));
equals to make all instances of the class to be equal.You can need to check the class at any given position in the array by using the keyword instanceof
public static void main(String[] args) {
Number[] myN = new Number[5];
//populate... ignore this if want
for (int i = 0; i < myN.length; i++) {
if (i%2==0) {
myN[i]= new Integer(i);
}else{
myN[i]= new Double(i);
}
}
int classACounter=0;
int classBCounter=0;
//check
for (int i = 0; i < myN.length; i++) {
if (myN[i] instanceof Integer){
System.out.println(" is an int");
classACounter++;
}
if (myN[i] instanceof Double){
System.out.println(" is a double");
classBCounter++;
}
}
System.out.println("There are "+classACounter+" elements of the class A");
System.out.println("There are "+classBCounter+" elements of the class B");
}
Well, You can easily filter them by
result = Iterables.filter(collection, YourClass.java);
then you can apply .size() on the result.
