这只是一个猜测，但未提供不可修改实现的部分原因可能是 NavigableSet/Map 接口公开了未标记为可选的变异方法：
- NavigableSet.pollFirst
- NavigableSet.pollLast
- NavigableMap.pollFirstEntry
- NavigableMap.pollLastEntry
也就是说，在提供不可修改的实现时仅抛出 UnsupportedOperationException 似乎是合理的。这就是在这些实现中所做的（假设您使用的是 GoogleGuava）：
可导航集：
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));
}
}
可导航地图：
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));
}
}