Fetch List of object from another list using java 8

Question

List<Customer> customers = findAllCustomer();   

public class Customer implements Serializable {

    private State state;

    //getter and setter

I have below approached using jdk 7

List<State> states = new ArrayList<>();

for (Customer customer : customers) {
    states.add(customer.getState());
}

How can I achieve the same thing using jdk 8?

Also, the original code is not terrible and still works perfectly fine with JDK8+ ... — Thilo, Jan 25 '19 at 06:39

score 7 · Accepted Answer · answered Jan 25 '19 at 06:35

Stream the contents, map to get the state and collect it as a List.

customers.stream()
    .map(Customer::getState)
    .collect(Collectors.toList());

If you need an ArrayList as the resultant list

customers.stream()
    .map(Customer::getState)
    .collect(Collectors.toCollection(ArrayList::new));

score 0 · Answer 2 · answered Jan 25 '19 at 06:37

0

List<State> states = new ArrayList<>();

customers
    .stream()
    .map(Customer::getState)
    .forEach(states::add);

answered Jan 25 '19 at 06:37

Khalid Shah

3,132
3
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"List states = new ArrayList<>(); " why are you declaring the arraylist if you are using java 8? You should never do it like this. Always use collect and Collectors. This is bad. – Abhishek Honey Jan 25 '19 at 06:56
1

@ABHISHEKHONEY this is not bad. That was just another way. – Khalid Shah Jan 25 '19 at 06:57
Kindly refer this :) https://stackoverflow.com/questions/28319064/java-8-best-way-to-transform-a-list-map-or-foreach – Abhishek Honey Jan 25 '19 at 07:03
@ABHISHEKHONEY they said that " that was preferable" not bad :) because it doesn't require mutating a collection and it is readable. – Khalid Shah Jan 25 '19 at 07:04

score 0 · Answer 3 · answered Jan 25 '19 at 06:40

0

Using Lambda and forEach

customers.forEach(p -> {
        states.add(p.getState())  
      }
    );

answered Jan 25 '19 at 06:40

DirtyMind

2,353
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Oleg Cherednik · Answer 4 · 2019-01-25T07:51:09.237

List<State> states = customers.stream()
                              .map(Customer::getState)
                              .collect(Collectors.toList());

Additionally you can wrap this issue to the static method:

public static List<State> getCustomerStates(List<Customer> customers) {
   return customers.stream()
                   .map(Customer::getState)
                   .collect(Collectors.toList());
}

...or to the function:

private static final Function<List<Customer>, List<State>> getCustomerStates =
        customers -> customers.stream()
                              .map(Customer::getState)
                              .collect(Collectors.toList());

There's a syntax error in your **function** declaration: Replace the lambda operator after `getCustomerStates` with assignment operator. — Andy Sug, Jan 25 '19 at 07:47

Dang Nguyen · Answer 5 · 2019-01-25T08:15:05.257

0

Worth to mentions that if the state is actually a List<state> (states)

public class Customer implements Serializable {
private List<State> states;//Important note: I changed State to a List<State> here

//getter and setter

It will be a little bit tricky to get a List of states here

List<State> states = customers.stream()
    .map(Customer::getStates)
    .filter(Objects::nonNull)//null check
    .flatMap(Collection::stream)
    .collect(Collectors.toList());

edited Jan 25 '19 at 08:15

answered Jan 25 '19 at 06:52

Dang Nguyen

1,209
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If state is already a List, then `states.add` wouldn't have worked. The OP should have used `addAll` – Thiyagu Jan 25 '19 at 08:00
@user7 Sorry but I don't get your comment? My answer mentions another case OP might need. And I didn't say anything about `add` and `addAll`? – Dang Nguyen Jan 25 '19 at 08:07
If it is another case, then it is fine. But in OP `state` cannot be a List. So, this doesn't answer the **current** question – Thiyagu Jan 25 '19 at 08:08
@user7 did you read my answer or not? I specific said `Worth to mentions that if...` and I actually did define a `List states` in the first code part? Is it not enough? – Dang Nguyen Jan 25 '19 at 08:10
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/187310/discussion-between-dang-nguyen-and-user7). – Dang Nguyen Jan 25 '19 at 09:30

Fetch List of object from another list using java 8

5 Answers5