How do you filter nested loops using Java 8 streams and filters?

Question

How do you filter nested loops using java8 streams and filters?

Suppose I have a list of cars (List<Car>), each car having a list of Engines (List<Engine>), each engine having a List<Parts>. In regular Java this structure can be described as:

for(Car car : cars) {
    for (Engine engine : car.getEngines()) {
        for (Part part : engine.getParts()) {
            // ...
        }
    }
}

Suppose I initialise the list of cars as:

List<Car> cars =  new ArrayList<Car>(Arrays.asList(new Car(), new Car(), new Car()));
cars.get(0).setEngines(null);
cars.get(1).setEngines(new ArrayList<Engine>());
cars.get(2).setEngines(new ArrayList<Engine>() {{
    add(new Engine());
    add(null);
    add(new Engine());  
}});

If I want to filter out nulls of List<Engine>, then I would do

cars.stream().filter(p -> p.getEngines() != null).forEach(System.out::println);

If I want to filter out empty arraylists of List, then I would do

cars.stream().filter(p -> !p.getEngines().isEmpty()).forEach(System.out::println);

But how do I remove the null Engine in a 3rd car and yet keep two other engines attached to the original list structure? In other words, can we go into the 2nd, 3rd, nth level of hierarchy with Java 8 filters or do filters only work on the top-most layer? I also tried to use .anyMatch(), without much luck.

just to further clarify, consider the following example: I have 3 cars in my garage. Each car has 3 placeholders for engine. Each engine has 3 placeholders for parts that make up the engine:

Car #1: 
 Engine#1: part1, part2, part3
 Engine#2: null, part2, empty
 Engine#3: part1, null, part3
Car #2: 
 Engine#1: part1, part2, part3
 empty:    null, null, null
 null:     null, null, null
Car #3: 
 Engine#1: null, empty, part3
 Engine#2: null, part2, empty
 Engine#3: part1, null, part3

Question: how do we use Java 8 .filter, such that when after filtering I get the following:

Car #1: 
 Engine#1: part1, part2, part3
 Engine#2: part2, 
 Engine#3: part1, part3
Car #2: 
 Engine#1: part1, part2, part3
Car #1: 
 Engine#1: part3
 Engine#2: part2,
 Engine#3: part1,part3

=======================

Another example

Guys I hope this example that I just made up is clearer:.. Essentially it is the same as above only it is more verbose and instead of cars we can think of banks to minimize abstraction. For conciseness I make all fields public, I hope you don't mind.

Suppose I am affiliated with 4 banks in my 'bank wallet'
Bank#1:
I physically bank here. I am forced to have 3 accounts, but only are 2 filled with some cash and 3rd is yet to opened (ie null)
Bank #2:
I plan to bank here. Account support structure is created (empty ArrayList), but no accounts are added
Bank #3:
I filled out some marketing form. They have me in their CRM but no accounts will ever be opened
Bank #4:
This bank burned down, there is an artifact placeholder in the wallet, which is null.

The following code describes this:

public class Bank_Wallet {

    public static void main(String[] args) {
        List<Bank> banks = new ArrayList<Bank>(Arrays.asList(new Bank(), new Bank(), new Bank(), null));
        // 1st bank with physical accounts, but one of them is null
        banks.get(0).accounts = Arrays.asList(new Account(), null, new Account());
        // 2nd bank with empty accounts  
        banks.get(1).accounts = new ArrayList<Account>();

        System.out.println("RAW original");
        banks.stream().forEach(System.out::println);

        System.out.println("\nFiltered result...   ");
        banks.stream()// get stream
                .filter(p -> p != null) // get rid of null banks
                .filter(p -> p.accounts != null) // get rid of null accounts
                .filter(p -> !p.accounts.isEmpty()) // get rid of empty accounts
                // .filter(p->p.accounts. ?????? ??) ?? how do I remove null account from the remaining bank entry?
                .forEach(System.out::println);

    }// main
}

The support classes are here:

public class Bank {
    public String name;
    public static int counter = 0;
    public List<Account> accounts;

    public Bank() {
        this.name = "Bank: #" + Bank.counter++;
    }

    @Override
    public String toString() {
        return "Bank [name=" + this.name + ", accounts=" + this.accounts + "]";
    }

public class Account {
    public String name;
    public static int counter;

    public Account() {
        this.name = "Account: " + Account.counter++;
    }

    @Override
    public String toString() {
        return "Account [name=" + this.name + "]";
    }

}

when you run this code you will see that after suggested filtering all I am left with is

Bank [name=Bank: #0, accounts=[Account [name=Account: 0], null, Account [name=Account: 1]]]

Question: What other filter do I need do add to the code to get the above result not show null in the accounts and yet retain the overall structure (Bank->Account->etc->etc)

Bank [name=Bank: #0, accounts=[Account [name=Account: 0], Account [name=Account: 1]]]

Answer 1

The stream equivalent of

for(Car car : cars) {
    for (Engine engine : car.getEngines()) {
        for (Part part : engine.getParts()) {
            // ...
        }
    }
}

is

cars.stream()
    .flatMap(car -> car.getEngines().stream())
    .flatMap(engine -> engine.getParts().stream())
    .forEach(part -> { /* ... */ });

The ... code will however not have access to car and engine.

To check for null, you can check in two places:

cars.stream()
    .flatMap(car -> car.getEngines().stream())
    .filter(engine -> engine != null)
    .flatMap(engine -> engine.getParts().stream())
    .forEach(part -> { /* ... */ });

or

cars.stream()
    .flatMap(car -> car.getEngines()
                       .stream()
                       .filter(engine -> engine != null))
    .flatMap(engine -> engine.getParts().stream())
    .forEach(part -> { /* ... */ });

Answer 2

And why do you not simply write this ?

cars.stream()
    .filter(car -> notEmpty(car.getEngines()))
    .filter(car -> car.getEngines().stream().allMatch(engine -> notEmpty(engine.getParts())))
    .forEach(System.out::println);

public static <T> boolean notEmpty(Collection<T> collection) {
   return collection != null && !collection.isEmpty();
}