Spring中提供的util CollectionUtils

CollectionUtils类

/*
 * Copyright 2002-2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.util;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Properties;
import java.util.Set;

/**
 * Miscellaneous collection utility methods.
 * Mainly for internal use within the framework.
 *
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @author Arjen Poutsma
 * @since 1.1.3
 */
public abstract class CollectionUtils {

   /**
    * Return {@code true} if the supplied Collection is {@code null} or empty.
    * Otherwise, return {@code false}.
    * @param collection the Collection to check
    * @return whether the given Collection is empty
    */
   public static boolean isEmpty(Collection collection) {
      return (collection == null || collection.isEmpty());
   }

   /**
    * Return {@code true} if the supplied Map is {@code null} or empty.
    * Otherwise, return {@code false}.
    * @param map the Map to check
    * @return whether the given Map is empty
    */
   public static boolean isEmpty(Map, ?> map) {
      return (map == null || map.isEmpty());
   }

   /**
    * Convert the supplied array into a List. A primitive array gets converted
    * into a List of the appropriate wrapper type.
    * 
NOTE: Generally prefer the standard {@link Arrays#asList} method.
    * This {@code arrayToList} method is just meant to deal with an incoming Object
    * value that might be an {@code Object[]} or a primitive array at runtime.
    * 
A {@code null} source value will be converted to an empty List.
    * @param source the (potentially primitive) array
    * @return the converted List result
    * @see ObjectUtils#toObjectArray(Object)
    * @see Arrays#asList(Object[])
    */
   @SuppressWarnings("rawtypes")
   public static List arrayToList(Object source) {
      return Arrays.asList(ObjectUtils.toObjectArray(source));
   }

   /**
    * Merge the given array into the given Collection.
    * @param array the array to merge (may be {@code null})
    * @param collection the target Collection to merge the array into
    */
   @SuppressWarnings("unchecked")
   public static <E> void mergeArrayIntoCollection(Object array, Collection<E> collection) {
      if (collection == null) {
         throw new IllegalArgumentException("Collection must not be null");
      }
      Object[] arr = ObjectUtils.toObjectArray(array);
      for (Object elem : arr) {
         collection.add((E) elem);
      }
   }

   /**
    * Merge the given Properties instance into the given Map,
    * copying all properties (key-value pairs) over.
    * 
Uses {@code Properties.propertyNames()} to even catch
    * default properties linked into the original Properties instance.
    * @param props the Properties instance to merge (may be {@code null})
    * @param map the target Map to merge the properties into
    */
   @SuppressWarnings("unchecked")
   public static <K, V> void mergePropertiesIntoMap(Properties props, Map<K, V> map) {
      if (map == null) {
         throw new IllegalArgumentException("Map must not be null");
      }
      if (props != null) {
         for (Enumeration en = props.propertyNames(); en.hasMoreElements();) {
            String key = (String) en.nextElement();
            Object value = props.getProperty(key);
            if (value == null) {
               // Potentially a non-String value...
               value = props.get(key);
            }
            map.put((K) key, (V) value);
         }
      }
   }


   /**
    * Check whether the given Iterator contains the given element.
    * @param iterator the Iterator to check
    * @param element the element to look for
    * @return {@code true} if found, {@code false} else
    */
   public static boolean contains(Iterator iterator, Object element) {
      if (iterator != null) {
         while (iterator.hasNext()) {
            Object candidate = iterator.next();
            if (ObjectUtils.nullSafeEquals(candidate, element)) {
               return true;
            }
         }
      }
      return false;
   }

   /**
    * Check whether the given Enumeration contains the given element.
    * @param enumeration the Enumeration to check
    * @param element the element to look for
    * @return {@code true} if found, {@code false} else
    */
   public static boolean contains(Enumeration enumeration, Object element) {
      if (enumeration != null) {
         while (enumeration.hasMoreElements()) {
            Object candidate = enumeration.nextElement();
            if (ObjectUtils.nullSafeEquals(candidate, element)) {
               return true;
            }
         }
      }
      return false;
   }

   /**
    * Check whether the given Collection contains the given element instance.
    * 
Enforces the given instance to be present, rather than returning
    * {@code true} for an equal element as well.
    * @param collection the Collection to check
    * @param element the element to look for
    * @return {@code true} if found, {@code false} else
    */
   public static boolean containsInstance(Collection collection, Object element) {
      if (collection != null) {
         for (Object candidate : collection) {
            if (candidate == element) {
               return true;
            }
         }
      }
      return false;
   }

   /**
    * Return {@code true} if any element in '{@code candidates}' is
    * contained in '{@code source}'; otherwise returns {@code false}.
    * @param source the source Collection
    * @param candidates the candidates to search for
    * @return whether any of the candidates has been found
    */
   public static boolean containsAny(Collection source, Collection candidates) {
      if (isEmpty(source) || isEmpty(candidates)) {
         return false;
      }
      for (Object candidate : candidates) {
         if (source.contains(candidate)) {
            return true;
         }
      }
      return false;
   }

   /**
    * Return the first element in '{@code candidates}' that is contained in
    * '{@code source}'. If no element in '{@code candidates}' is present in
    * '{@code source}' returns {@code null}. Iteration order is
    * {@link Collection} implementation specific.
    * @param source the source Collection
    * @param candidates the candidates to search for
    * @return the first present object, or {@code null} if not found
    */
   @SuppressWarnings("unchecked")
   public static <E> E findFirstMatch(Collection source, Collection<E> candidates) {
      if (isEmpty(source) || isEmpty(candidates)) {
         return null;
      }
      for (Object candidate : candidates) {
         if (source.contains(candidate)) {
            return (E) candidate;
         }
      }
      return null;
   }

   /**
    * Find a single value of the given type in the given Collection.
    * @param collection the Collection to search
    * @param type the type to look for
    * @return a value of the given type found if there is a clear match,
    * or {@code null} if none or more than one such value found
    */
   @SuppressWarnings("unchecked")
   public static <T> T findValueOfType(Collection collection, Class<T> type) {
      if (isEmpty(collection)) {
         return null;
      }
      T value = null;
      for (Object element : collection) {
         if (type == null || type.isInstance(element)) {
            if (value != null) {
               // More than one value found... no clear single value.
               return null;
            }
            value = (T) element;
         }
      }
      return value;
   }

   /**
    * Find a single value of one of the given types in the given Collection:
    * searching the Collection for a value of the first type, then
    * searching for a value of the second type, etc.
    * @param collection the collection to search
    * @param types the types to look for, in prioritized order
    * @return a value of one of the given types found if there is a clear match,
    * or {@code null} if none or more than one such value found
    */
   public static Object findValueOfType(Collection collection, Class[] types) {
      if (isEmpty(collection) || ObjectUtils.isEmpty(types)) {
         return null;
      }
      for (Class type : types) {
         Object value = findValueOfType(collection, type);
         if (value != null) {
            return value;
         }
      }
      return null;
   }

   /**
    * Determine whether the given Collection only contains a single unique object.
    * @param collection the Collection to check
    * @return {@code true} if the collection contains a single reference or
    * multiple references to the same instance, {@code false} else
    */
   public static boolean hasUniqueObject(Collection collection) {
      if (isEmpty(collection)) {
         return false;
      }
      boolean hasCandidate = false;
      Object candidate = null;
      for (Object elem : collection) {
         if (!hasCandidate) {
            hasCandidate = true;
            candidate = elem;
         }
         else if (candidate != elem) {
            return false;
         }
      }
      return true;
   }

   /**
    * Find the common element type of the given Collection, if any.
    * @param collection the Collection to check
    * @return the common element type, or {@code null} if no clear
    * common type has been found (or the collection was empty)
    */
   public static Class findCommonElementType(Collection collection) {
      if (isEmpty(collection)) {
         return null;
      }
      Class candidate = null;
      for (Object val : collection) {
         if (val != null) {
            if (candidate == null) {
               candidate = val.getClass();
            }
            else if (candidate != val.getClass()) {
               return null;
            }
         }
      }
      return candidate;
   }

   /**
    * Marshal the elements from the given enumeration into an array of the given type.
    * Enumeration elements must be assignable to the type of the given array. The array
    * returned will be a different instance than the array given.
    */
   public static <A, E extends A> A[] toArray(Enumeration<E> enumeration, A[] array) {
      ArrayList<A> elements = new ArrayList<A>();
      while (enumeration.hasMoreElements()) {
         elements.add(enumeration.nextElement());
      }
      return elements.toArray(array);
   }

   /**
    * Adapt an enumeration to an iterator.
    * @param enumeration the enumeration
    * @return the iterator
    */
   public static <E> Iterator<E> toIterator(Enumeration<E> enumeration) {
      return new EnumerationIterator<E>(enumeration);
   }

   /**
    * Adapt a {@code Map, List>} to an {@code MultiValueMap,V>}.
    * @param map the map
    * @return the multi-value map
    */
   public static <K, V> MultiValueMap<K, V> toMultiValueMap(Map<K, List<V>> map) {
      return new MultiValueMapAdapter<K, V>(map);

   }

   /**
    * Return an unmodifiable view of the specified multi-value map.
    * @param  map the map for which an unmodifiable view is to be returned.
    * @return an unmodifiable view of the specified multi-value map.
    */
   public static <K,V> MultiValueMap<K,V> unmodifiableMultiValueMap(MultiValueMapextends K, ? extends V> map) {
      Assert.notNull(map, "'map' must not be null");
      Map<K, List<V>> result = new LinkedHashMap<K, List<V>>(map.size());
      for (Map.Entryextends K, ? extends Listextends V>> entry : map.entrySet()) {
         List<V> values = Collections.unmodifiableList(entry.getValue());
         result.put(entry.getKey(), values);
      }
      Map<K, List<V>> unmodifiableMap = Collections.unmodifiableMap(result);
      return toMultiValueMap(unmodifiableMap);
   }


   /**
    * Iterator wrapping an Enumeration.
    */
   private static class EnumerationIterator<E> implements Iterator<E> {

      private Enumeration<E> enumeration;

      public EnumerationIterator(Enumeration<E> enumeration) {
         this.enumeration = enumeration;
      }

      @Override
      public boolean hasNext() {
         return this.enumeration.hasMoreElements();
      }

      @Override
      public E next() {
         return this.enumeration.nextElement();
      }

      @Override
      public void remove() throws UnsupportedOperationException {
         throw new UnsupportedOperationException("Not supported");
      }
   }

   /**
    * Adapts a Map to the MultiValueMap contract.
    */
   @SuppressWarnings("serial")
   private static class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {

      private final Map<K, List<V>> map;

      public MultiValueMapAdapter(Map<K, List<V>> map) {
         Assert.notNull(map, "'map' must not be null");
         this.map = map;
      }

      @Override
      public void add(K key, V value) {
         List<V> values = this.map.get(key);
         if (values == null) {
            values = new LinkedList<V>();
            this.map.put(key, values);
         }
         values.add(value);
      }

      @Override
      public V getFirst(K key) {
         List<V> values = this.map.get(key);
         return (values != null ? values.get(0) : null);
      }

      @Override
      public void set(K key, V value) {
         List<V> values = new LinkedList<V>();
         values.add(value);
         this.map.put(key, values);
      }

      @Override
      public void setAll(Map<K, V> values) {
         for (Entry<K, V> entry : values.entrySet()) {
            set(entry.getKey(), entry.getValue());
         }
      }

      @Override
      public Map<K, V> toSingleValueMap() {
         LinkedHashMap<K, V> singleValueMap = new LinkedHashMap<K,V>(this.map.size());
         for (Entry<K, List<V>> entry : map.entrySet()) {
            singleValueMap.put(entry.getKey(), entry.getValue().get(0));
         }
         return singleValueMap;
      }

      @Override
      public int size() {
         return this.map.size();
      }

      @Override
      public boolean isEmpty() {
         return this.map.isEmpty();
      }

      @Override
      public boolean containsKey(Object key) {
         return this.map.containsKey(key);
      }

      @Override
      public boolean containsValue(Object value) {
         return this.map.containsValue(value);
      }

      @Override
      public List<V> get(Object key) {
         return this.map.get(key);
      }

      @Override
      public List<V> put(K key, List<V> value) {
         return this.map.put(key, value);
      }

      @Override
      public List<V> remove(Object key) {
         return this.map.remove(key);
      }

      @Override
      public void putAll(Mapextends K, ? extends List<V>> m) {
         this.map.putAll(m);
      }

      @Override
      public void clear() {
         this.map.clear();
      }

      @Override
      public Set<K> keySet() {
         return this.map.keySet();
      }

      @Override
      public CollectionV>> values() {
         return this.map.values();
      }

      @Override
      public SetK, List<V>>> entrySet() {
         return this.map.entrySet();
      }

      @Override
      public boolean equals(Object other) {
         if (this == other) {
            return true;
         }
         return map.equals(other);
      }

      @Override
      public int hashCode() {
         return this.map.hashCode();
      }

      @Override
      public String toString() {
         return this.map.toString();
      }
   }

}