Java 8 Stream filtering value of list in a list

Question

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.

Answer 1

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.

Answer 2

I found a bunch of examples at this site: https://zetcode.com/java/streamfilter/

Quoting the example for Java Stream multiple filter operations

It is possible to apply multiple filter operations on a stream.

package com.zetcode;

import java.util.Arrays;
import java.util.function.IntConsumer;

public class JavaStreamMultipleFilters {

public static void main(String[] args) {

    int[] inums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
    
    IntConsumer icons = i -> System.out.print(i + " ");
    
    Arrays.stream(inums).filter(e -> e < 6 || e > 10)
            .filter(e -> e % 2 == 0).forEach(icons);
}

}

In the example, we apply multiple filter operations on a stream of integers.

int[] inums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };

We have an array of integer values.

IntConsumer icons = i -> System.out.print(i + " ");

IntConsumer is an operation that accepts a single integer value argument and returns no result.

Arrays.stream(inums).filter(e -> e < 6 || e > 10)
        .filter(e -> e % 2 == 0).forEach(icons);

A stream is created from the array with the Arrays.stream method. Multiple filtering operations are performed.

2 4 12 14

These integers fulfill all the filtering conditions.