转自:https://blog.csdn.net/fangwenzheng88/article/details/78457850
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.
* <p><b>NOTE:</b> 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.
* <p>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.
* <p>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.
* <p>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<K, List<V>>} to an {@code MultiValueMap<K,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(MultiValueMap<? extends 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.Entry<? extends K, ? extends List<? extends 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(Map<? extends 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 Collection<List<V>> values() {
return this.map.values();
}
@Override
public Set<Entry<K, 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();
}
}
}