如何使用Java 8 Stream API过滤集合中的列表

Question

class A {
    List<B> b;
}

class B{
    boolean flag;

    B(boolean flag) {
        this.flag = flag;
    }
    //getter setter
}

B b1 = new B(true);
B b2 = new B(true);
B b3 = new B(false);

List<B> list1 = new ArrayList<>();
list1.add(b1);
list1.add(b2);
list1.add(b3);

Set<A> a = new HashSet<>();
a.add(list1);

There is a set of object A.

Object A contains a list of object B. B has a boolean property name 'flag' Using java 8 streams API, how to filter the set above such that it contains the list of B with boolean flag as true only. ie b3 should be removed from list1. To keep it simple just added the single value.

Answer 1

You have a class A that contains a list of B ; the goal is to remove, in each A , all the B that matches a condition.

The first solution is make a stream of the set of A s and map each of them to a new A where the list of B was filtered. Assuming that there is a constructor A(List<B> b) and the appropriate getters ( List<B> getB() in class A ; boolean isFlag() in class B ), you could have:

Set<A> set = new HashSet<>();

Set<A> filtered =
    set.stream()
       .map(a -> new A(a.getB().stream().filter(B::isFlag).collect(Collectors.toList())))
       .collect(Collectors.toSet());

A second solution is possible if you can modify the list of B s in-place, instead of creating a new one. In such a case, you could have:

set.forEach(a -> a.getB().removeIf(b -> !b.isFlag()));

which will remove all B where the flag is false for each A .

Answer 2

    @org.junit.Test
    public void test() {
        B b1 = new B(true);
        B b2 = new B(true);
        B b3 = new B(false);

        List<B> list1 = new ArrayList();
        list1.add(b1);
        list1.add(b2);
        list1.add(b3);

        Set<A> a = new HashSet();
        a.add(new A(list1));

        a.stream().forEach(aa -> aa.setB(aa.getB().stream().filter(B::isFlag).collect(Collectors.toList())));

        System.out.println(a);
    }

output:

[A{b=[B{flag=true}, B{flag=true}]}]

Answer 3

a.stream().forEach(l -> l.removeIf(b -> !b.getFlag()));