How to use an existing ArrayList inside Lambda Expression/block? - arraylist

Question

I am new to Java 8 and was trying to rewrite an existing code snippet logic using the Java 8 features.
However I am not sure how to use an existing arrayList defined outside the block to get values from it when it is placed inside the lambda block. It complains that it has to be final or effectively final.
I started with converting the inner traditional for loop and encountered the same issues with a counter variable which I was able to sort using AtomicInteger but am not sure how to do that for arrayList as well as I cannot also define the arrayList inside the lambda block since it has a dependency of an i variable that is present in the outer while loop.
Any help will be much appreciated !!! Thanks in advance.
Below is my code snippet :-
public String somemethod(ArrayList someValues){
int i=0;
String status="Failed";
ArrayList someOtherValues = new ArrayList();
try
{
while ( i < (someValues.size()))
{
someOtherValues = (ArrayList) someValues.get(i);
someOtherValues.replaceAll(t -> Objects.isNull(t) ? "" : t); //converting every null to "" within the arrayList someOtherValues
int count=4;
AtomicInteger counter=new AtomicInteger(5);
if(!someOtherValues.get(0).toString().equals(""))
{
while ( count < (someOtherValues.size()))
{
IntStream.range(0, 3).forEach(k -> {
someObjectClass someObject=new someObjectClass();
someOtherObjectClass id = new someOtherObjectClass(someOtherValues.get(0).toString(),someOtherValues.get(count).toString()) //Line where the error is
someObject=new someObjectClass(id);
someObject.setId(id);
if(someCondition)
{
try
{
someObject.setSomething(someValue);
counter.incrementAndGet()
}
}
someObject.setsomeOtherValues1(someOtherValues.get(1).toString());
someObject.setsomeOtherValues2(someOtherValues.get(3).toString())
}
count=counter.get();
counter.incrementAndGet();
}
}
i++;
}
catch(Exception e)
{
return status;
}
}
Right now where it is pending is it complains that someOtherValues, which is an existing arrayList defined outside the lambda block needs to be final or effectively final in order to fetch elements.
Is it literally not possible to change/optimize the above function into a fewer lines of code using Java 8 streams/lambdas/forEach ?

Answer 1

As a general rule it is not a good idea to try and change outside variables inside a lambda definition. But since Java's very loose concept of final fields and variables only applies to assigning values, it can still be done.
The only thing you cannot do in a lambda expression with variable defined outside is assigning new values.
So this does not compile:
List<String> lst = new ArrayList<>();
myLambda = e -> {
lst = new ArrayList<>(); //this violates the 'final' rule
lst.add(e);
}
It is however possible to call any method of the outside variable, even if it changes the state of the variable. This will work:
myLambda = e -> {
lst.add(e);
}
Even though you're still changed the state of the variable lst, this is legal code.
But I strongly advise against it. Lambdas are meant to used in a functional matter and should not change any variables defined elsewhere. It's a better choice to create a list inside the lambda, return it from the lambda and then add it to the outside list.