Introduction

The title of the post is really “holivny”, but in my opinion both Java and the popular Guava library have a number of architectural problems that in the long run lead to ambiguities and disagreements in the team.

Java Collections Framework and immutability on trust

Collections Framework , . java.util.Collection add, clear, remove ... : Collections.emptyList(), Collections.singletonList<T>(T t), Arrays.asList<T>(T.. values) . . :

List<Integer> list = Arrays.asList(21, 22, 23);
list.add(24)        //  ! UnsupportedOperationException

? java.util.List java.util.Collection, add. , Arrays.asList<T>(T... t) , add remove . : , . , - .

List<Integer> list = Arrays.asList(21, 22, 23);
list.set(0, 121);
System.out.println(list);        // [121, 22, 23]

. . , . , , , . .

List<Integer> filtered = filter(List<Integer> list);

filter List<Integer> . ? , ? try {} catch(Exception e) {} add remove — . ? ? list, filtered? . , . , — . , , list.clear(), , . SOLID, L — , , . , add , .

- , Stream API. , Java , , , . , , .

, java.util.Collection, java.util.Iterable, Collection. Iterable , Iterator, for-each . .

public interface Iterator<E> {
    boolean hasNext();

    E next();

    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

remove. , for-each, , , API , , . - , , , , Java , .

Guava

Guava? — ImmutableList.

ImmutableList<String> people = ImmutableList.of("Michael", "Simon", "John")
...

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Kotlin

? , JetBrains Kotlin. , , , . :

, , , .

val list: List<Int> = listOf(1, 2, 3)
val mutableList: MutableList<Int> = list as MutableList<Int>
mutableList.clear()      //  !  UnsupportedOperationException

- , MutableList List MutableList, , . , Java Collections Framework, Guava: , . , , . , Java, - , , , .

? ? , : , , , . . Java Collections Framework Immutable. :

, , «Immutable». , . , ImmutableQueue, , . . LinkedList , , ( , ). ImmutableLinkedList .

- , , . List ImmutableList. , toMutableList toImmutableList ImmutableList List . , , . , ArrayList ImmutableArrayList . . ArrayList , ImmutableArrayList, ImmutableArrayList ArrayList. API , .

, — java.util.Iterable. , for-each , . java.util.Iterator, , remove, . , .

1. ImmutableIterable, ImmutableIterator. , , Java for-each Iterable, ImmutableIterable.
2. ImmutableIterator, java.util.Iterator remove, final, . , .
3. remove Iterator, , Java, , .

, Java JP . , — , Java-. , , . !

P.S.

I'm currently working on an open-source library that provides fully immutable Java collections. Those interested can find the link on Github .