Unmodifiable NavigableSet/NavigableMap in Java?

Question

java.util.Collections has several unmodifiable methods that provide unmodifiable collection views by wrapping collections in decorators that prohibit mutation operations.

Java 6 added support for java.util.NavigableSet and java.util.NavigableMap .

I'd like to be able to have unmodifiable NavigableSet s and NavigableMap s, but java.util.Collections#unmodifiableSortedSet(SortedSet) and java.util.Collections#unmodifiableSortedMap(SortedMap) are not sufficient because they do not support the operations that are particular to NavigableSet and NavigableMap .

Are there de-facto implementations for unmodifiableNavigableSet and unmodifiableNavigableMap ?

Answer 1

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

Answer 2

Java SE 8 included these two methods.

See the Javadoc .

Answer 3

This is just a guess, but part of the reason why an unmodifiable implementation was not provided may be due to the fact that the NavigableSet/Map interfaces expose mutating methods that aren't marked as optional:

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

That said, it seems reasonable when providing an unmodifiable implementation to just throw an UnsupportedOperationException. That is what is done in these implementations (which assume you're using 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:

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

From Christian Semrau (in the question comments):

There are no such methods within the JDK, as mentioned in this bug report .