Question

java.util.Collections有几个unmodifiable方法，它们通过在装饰器中包装禁止变异操作的集合来提供不可修改的集合视图。

Java 6增加了对java.util.NavigableSet和java.util.NavigableMap的支持。

我希望能够拥有不可修改的NavigableSet和NavigableMap，但java.util.Collections#unmodifiableSortedSet(SortedSet)和java.util.Collections#unmodifiableSortedMap(SortedMap)是不够的，因为它们不支持特别适用于NavigableSet和NavigableMap。

是否存在unmodifiableNavigableSet和unmodifiableNavigableMap的事实上的实施？

Answer 1

Guava现在提供（自12.0版本，2012年4月30日发布）Maps.unmodifiableNavigableMap和Sets.unmodifiableNavigableSet。

Answer 2

Java SE 8包含了这两种方法。

请参阅Javadoc。

Answer 3

这只是猜测，但是未提供不可修改的实现的部分原因可能是由于NavigableSet / Map接口暴露了未标记为可选的变异方法：

NavigableSet.pollFirst
NavigableSet.pollLast
NavigableMap.pollFirstEntry
NavigableMap.pollLastEntry

也就是说，提供一个不可修改的实现来抛出UnsupportedOperationException似乎是合理的。这就是在这些实现中所做的（假设您正在使用GoogleGuava）：

<强> NavigableSet：

import static java.util.Collections.unmodifiableSortedSet;

import java.util.Collections;
import java.util.Iterator;
import java.util.NavigableSet;
import java.util.SortedSet;

import com.google.common.collect.ForwardingSortedSet;

/**
 * {@link NavigableSet} equivalent of {@link Collections#unmodifiableSortedSet(SortedSet)}.
 * This is unfortunately needed because {@link Collections} lacks "UnmodifiableNavigableSet"
 * (see http://stackoverflow.com/questions/2577706/unmodifiable-navigableset-navigablemap-in-java
 * and http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6907251).
 * 
 * This is just a guess, but part of the reason why an unmodifiable implementation was not provided may be due to the fact that 
 * {@link NavigableSet} exposes mutating methods that aren't marked as optional:
 * - {@link NavigableSet#pollFirst()}
 * - {@link NavigableSet#pollLast()}
 *  
 * In addition, one can't go the immutable route, as Google Guava doesn't provide an Immutable variant 
 * (see http://code.google.com/p/guava-libraries/issues/detail?id=664).
 *
 * @param <E> See {@link NavigableSet}
 */
public final class UnmodifiableNavigableSet<E> extends ForwardingSortedSet<E> implements NavigableSet<E> {

    private final NavigableSet<E> delegate;
    private final SortedSet<E> unmodifiableDelegate;

    /**
     * @param delegate See {@link ForwardingSortedSet#delegate()}.
     */
    public UnmodifiableNavigableSet(NavigableSet<E> delegate) {
        this.delegate = delegate;
        unmodifiableDelegate = unmodifiableSortedSet(delegate);
    }

    /**
     * @param delegate
     * @return {@link #UnmodifiableNavigableSet(NavigableSet)}
     * @see EffectiveJava#Static_factories_instead_of_constructors
     */
    public static <E> UnmodifiableNavigableSet<E> newUnmodifiableNavigableSet(NavigableSet<E> delegate) {
        return new UnmodifiableNavigableSet<E>(delegate);
    }

    @Override
    protected SortedSet<E> delegate() {
        return unmodifiableDelegate;
    }

    @Override
    public E ceiling(E e) {
        return delegate.ceiling(e);
    }

    @Override
    public Iterator<E> descendingIterator() {
        // NavigableSet.descendingIterator explicitly states this behavior.
        // By doing this, we don't need to do anything extra to ensure the iterator is unmodifiable.
        return descendingSet().iterator();
    }

    @Override
    public NavigableSet<E> descendingSet() {
        return newUnmodifiableNavigableSet(delegate.descendingSet());
    }

    @Override
    public E floor(E e) {
        return delegate.floor(e);
    }

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
        return newUnmodifiableNavigableSet(delegate.headSet(toElement, inclusive));
    }

    @Override
    public E higher(E e) {
        return delegate.higher(e);
    }

    @Override
    public E lower(E e) {
        return delegate.lower(e);
    }

    @Override
    public E pollFirst() {
        throw new UnsupportedOperationException();
    }

    @Override
    public E pollLast() {
        throw new UnsupportedOperationException();
    }

    @Override
    public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
        return newUnmodifiableNavigableSet(delegate.subSet(fromElement, fromInclusive, toElement, toInclusive));
    }

    @Override
    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
        return newUnmodifiableNavigableSet(delegate.tailSet(fromElement, inclusive));
    }
}

<强>的NavigableMap：

import static UnmodifiableNavigableSet.newUnmodifiableNavigableSet;
import static java.util.Collections.unmodifiableSortedMap;

import java.util.AbstractMap;
import java.util.Map;
import java.util.NavigableMap;
import java.util.NavigableSet;
import java.util.SortedMap;

import com.google.common.collect.ForwardingSortedMap;

/**
 * This class has the same rational as {@link UnmodifiableNavigableSet}.
 * @param <K> See {@link NavigableMap}
 * @param <V> See {@link NavigableMap}
 */
public final class UnmodifiableNavigableMap<K,V> extends ForwardingSortedMap<K,V> implements NavigableMap<K,V> {

    private final NavigableMap<K,V> delegate;
    private final SortedMap<K,V> unmodifiableDelegate;

    /**
     * @param delegate See {@link ForwardingSortedMap#delegate()}.
     */
    public UnmodifiableNavigableMap(NavigableMap<K,V> delegate) {
        this.delegate = delegate;
        unmodifiableDelegate = unmodifiableSortedMap(delegate);
    }

    /**
     * @param delegate
     * @return {@link #UnmodifiableNavigableMap(NavigableMap)}
     * @see EffectiveJava#Static_factories_instead_of_constructors
     */
    public static <K,V> UnmodifiableNavigableMap<K,V> newUnmodifiableNavigableMap(NavigableMap<K,V> delegate) {
        return new UnmodifiableNavigableMap<K,V>(delegate);
    }

    @Override
    protected SortedMap<K,V> delegate() {
        return unmodifiableDelegate;
    }

    private Map.Entry<K,V> newImmutableEntry(Map.Entry<K,V> entry) {
        return entry == null ? null : new AbstractMap.SimpleImmutableEntry<K,V>(entry);
    }

    @Override
    public Map.Entry<K,V> ceilingEntry(K key) {
        return newImmutableEntry(delegate.ceilingEntry(key));
    }

    @Override
    public K ceilingKey(K key) {
        return delegate.ceilingKey(key);
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
        return newUnmodifiableNavigableSet(delegate.descendingKeySet());
    }

    @Override
    public NavigableMap<K,V> descendingMap() {
        return newUnmodifiableNavigableMap(delegate.descendingMap());
    }

    @Override
    public Map.Entry<K,V> firstEntry() {
        return newImmutableEntry(delegate.firstEntry());
    }

    @Override
    public Map.Entry<K,V> floorEntry(K key) {
        return newImmutableEntry(delegate.floorEntry(key));
    }

    @Override
    public K floorKey(K key) {
        return delegate.floorKey(key);
    }

    @Override
    public NavigableMap<K,V> headMap(K toKey, boolean inclusive) {
        return newUnmodifiableNavigableMap(delegate.headMap(toKey, inclusive));
    }

    @Override
    public Map.Entry<K,V> higherEntry(K key) {
        return newImmutableEntry(delegate.higherEntry(key));
    }

    @Override
    public K higherKey(K key) {
        return delegate.higherKey(key);
    }

    @Override
    public Map.Entry<K,V> lastEntry() {
        return newImmutableEntry(delegate.lastEntry());
    }

    @Override
    public Map.Entry<K,V> lowerEntry(K key) {
        return newImmutableEntry(delegate.lowerEntry(key));
    }

    @Override
    public K lowerKey(K key) {
        return delegate.lowerKey(key);
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
        return newUnmodifiableNavigableSet(delegate.navigableKeySet());
    }

    @Override
    public Map.Entry<K,V> pollFirstEntry() {
        throw new UnsupportedOperationException();
    }

    @Override
    public Map.Entry<K,V> pollLastEntry() {
        throw new UnsupportedOperationException();
    }

    @Override
    public NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
        return newUnmodifiableNavigableMap(delegate.subMap(fromKey, fromInclusive, toKey, toInclusive));
    }

    @Override
    public NavigableMap<K,V> tailMap(K fromKey, boolean inclusive) {
        return newUnmodifiableNavigableMap(delegate.tailMap(fromKey, inclusive));
    }
}

Answer 4

来自Christian Semrau（在问题评论中）：

JDK中没有这样的方法，如this bug report中所述。

Java中不可修改的NavigableSet / NavigableMap？

4 个答案: