Skip to content

extract list inside a list as a single list

I’ve a two POJOs, Sample code below

class A {
    String name;
    Object entries; // this can be a List<String> or a string - hence Object datatype
   //getters and setter here

class B {
    int snumber;
    List<A> values;
   //getters and setters here

Controller class

class ControllerA {
    public getList(B b) {
        List<String> list = b.getValues() -> e.getEntries()).collect(Collectors.toList()));

This returns me a list of the list:

[[12345, 09876], [567890, 43215]]

but what I want is a single list like

[12345,09876,567890, 43215]

How can I do that with Java 8 streams?

I’ve tried flatMap also, but that doesn’t go well with the Object datatype of entries.


Consider a List<String> as a field entries in the A class.

As @Eugene mentioned in the comments,

If it’s a single String make it a List of a single element; if it’s a list of multiple strings make it as such.

Working with a collection of a single type can simplify the process:

b.getValues()                           // -> List<A>
        .stream()                       // -> Stream<A>
        .map(A::getEntries)             // -> Stream<List<String>>
        .flatMap(Collection::stream)    // -> Stream<String>
        .collect(Collectors.toList());  // -> List<String>