Java 8 Stream filtering value of list in a list

问题

I have a an object which looks like the following

class MyObject {

    String type;
    List<String> subTypes;

}

Is it possible, given a list of MyObject's to use Java 8 streams to filter on both the type and then the subtype?

So far I have

myObjects.stream()
    .filter(t -> t.getType().equals(someotherType)
    .collect(Collections.toList());

but within this I also want another filter on each of the subTypes filtering those on a particular subtype too. I can't figure out how to do this.

An example would be

myObject { type: A, subTypes [ { X, Y, Z } ] }
myObject { type: B, subTypes [ { W, X, Y } ] }
myObject { type: B, subTypes [ { W, X, Z } ] }
myObject { type: C, subTypes [ { W, X, Z } ] }

I would pass in matchType B and subType Z, so I would expect one result -> myObject type B, subtypes: W, X, Z

the following currently returns 2 items in a list.

myObjects.stream()
    .filter(t -> t.getType().equals("B")
    .collect(Collectors.toList());

but I would like to add an additional filter over the each of the subtypes and only matching where 'Z' is present.

回答1:

You can do:

myObjects.stream()
         .filter(t -> t.getType().equals(someotherType) && 
                      t.getSubTypes().stream().anyMatch(<predicate>))
         .collect(Collectors.toList());

This will fetch all the MyObject objects which

meet a criteria regarding the type member.
contain objects in the nested List<String> that meet some other criteria, represented with <predicate>

回答2:

I saw the accepted answer from @kocko which is both a good answer and totally correct. However there is a slightly alternative approach where you simply chain the filters.

final List<MyObject> withBZ = myObjects.stream()
        .filter(myObj -> myObj.getType().equals("B"))
        .filter(myObj -> myObj.getSubTypes().stream().anyMatch("Z"::equals))
        .collect(Collectors.toList());

This is basically doing the same thing but the && operand is removed in favour of another filter. Chaining works really well for the Java 8 Stream API:s and IMO it is easier to read and follow the code.

来源：https://stackoverflow.com/questions/27822703/java-8-stream-filtering-value-of-list-in-a-list

标签

java

java-8