How to replace loop in loop in functional way?

Question

I have code like this:

 List<Account> myAccounts = new ArrayList<>();

 List<String> numbers = new ArrayList<>();

 List<Account> resultList = new ArrayList<>();

        for (String number : numbers) {
            for (Account account : myAccounts) {
                if(number.equals(account.getNumber())){
                    resultList.add(account);
                }
            }
        }

I tried get speciefied accounts with specified numbers, but I do this via loop all numbers and compare to accounts numbers. How can I do this in functional style? Im not asking how to make it into a function, but how to get the same results without having to run two loops every time.

Are you asking how to make it into a function or how to get the same results without having to run two loops every time you want to check? — Nathan Walsh
– Nathan Walsh, Commented Oct 11, 2020 at 14:41
you can use the streams api. Example: List<Integer> list = new ArrayList<>(); list.stream().filter( i -> true ).forEach( i -> list.add(i)); Or you can even replace forEach with simply collect — Omar Abdel Bari
– Omar Abdel Bari, Commented Oct 11, 2020 at 14:46
@merc-angel What do you mean by "without having to run two loops every time"? Do you mean that you don't need to iterate over all elements for every element? Because Naga's answer is almost no different than your nested loop in terms of performance. The only difference is that it doesn't continue after a match is found, but you can easily put a break in your if to accomplish that. — Ivar
– Ivar, Commented Oct 11, 2020 at 14:51

Naga Srinu Kapusetti · Accepted Answer · 2020-10-11 14:46:05Z

3

List<Account> myAccounts = new ArrayList<>();    
List<String> numbers = new ArrayList<>();    
List<Account> resultList = myAccounts.stream()
                .filter(account -> numbers.contains(account.getNumber()))
                .collect(Collectors.toList());

You can use this. only requirement is that you should be using at least Java8

answered Oct 11, 2020 at 14:46

Naga Srinu Kapusetti

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2 Comments

Pshemo Over a year ago

@merc-angel numbers should probably be a Set<String> since sets usually optimize contains method the most. For instance HashSet#contiains complexity is close ot O(1).

Naga Srinu Kapusetti Over a year ago

100% agreed with @Pshemo

WJS · Accepted Answer · 2020-10-11 15:02:47Z

2

Try it like this.

List<Account> results = numbers.stream().flatMap(numb -> myAccounts
                  .stream()
                  .filter(acct->numb.equals(acct.getNumber()))
                  .collect(Collectors.toList());

edited Oct 11, 2020 at 15:02

answered Oct 11, 2020 at 14:47

WJS

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Comments

Nathan Walsh · Accepted Answer · 2020-10-11 14:46:59Z

-1

The best way to accomplish this would be to load your Account objects into a sql lite database. ( Or add it to your current database if you already have one )

https://developer.android.com/training/data-storage/sqlite

Then once you have all of the Account objects store in the database you could just write a query to list the accounts with matching numbers.

Here is a more complete example on how to build everything from creating the database to creating the objects and eventually to querying for them.

https://www.androidauthority.com/creating-sqlite-databases-in-your-app-719366/

answered Oct 11, 2020 at 14:46

Nathan Walsh

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Comments

koleon03 · Accepted Answer · 2020-10-11 14:40:32Z

-2

In a functional approach you would construct a method out of your for loop as in:

public void compareNumbers(List<String> numbers, List<Account> resultList){
  for (String number : numbers) {
        for (Account account : myAccounts) {
            if(number.equals(account.getNumber())){
                resultList.add(account);
            }
   }
}

answered Oct 11, 2020 at 14:40

koleon03

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6 Comments

merc-angel Over a year ago

Ok, but I mean how to replace loop in loop

koleon03 Over a year ago

You have to use loops, otherwise how are you going to compare the values?

merc-angel Over a year ago

I think it is possible to do :D

koleon03 Over a year ago

You could read into lambda, if you're interested. But I'm not sure if you can iterate through arrays with lambda.

Omar Abdel Bari Over a year ago

Streams api that's how, look at the other answers. He asked for a functional approach and this is not functional.

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