Java中不可修改的NavigableSet / NavigableMap？

Question

java.util.Collections有幾個unmodifiable方法，通過在裝飾器中包裝禁止變異操作的集合來提供不可修改的集合視圖。

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

我希望能夠擁有不可修改的NavigableSet和NavigableMap ，但java.util.Collections#unmodifiableSortedSet(SortedSet)和java.util.Collections#unmodifiableSortedMap(SortedMap)是不夠的，因為它們不支持特別是NavigableSet和NavigableMap 。

是否存在unmodifiableNavigableSet和unmodifiableNavigableMap的事實上的實現？

Answer 1

Guava現在提供（自2012年4月30日發布的12.0版本以來） Maps.unmodifiableNavigableMap和Sets.unmodifiableNavigableSet 。

Answer 2

Java SE 8包含這兩種方法。

見Javadoc 。

Answer 3

這只是一個猜測，但是未提供不可修改的實現的部分原因可能是由於NavigableSet / Map接口暴露了未標記為可選的變異方法：

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

也就是說，提供不可修改的實現只是拋出UnsupportedOperationException似乎是合理的。 這就是在這些實現中所做的（假設您正在使用GoogleGuava）：

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));
    }
}

NavigableMap的：

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

來自Christian Semrau（在問題評論中）：

JDK中沒有這樣的方法，如本錯誤報告中所述 。