https://github.com/spring-projects/spring-framework: Spring Core (Spring IO)
  • Juergen Hoeller 
/*
 * Copyright 2002-2020 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
 *
 *      https://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.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Properties;
import java.util.Set;
import java.util.SortedSet;

import org.springframework.lang.Nullable;


Miscellaneous collection utility methods.
Mainly for internal use within the framework.

Author:Juergen Hoeller, Rob Harrop, Arjen Poutsma
Since:1.1.3
/**
 * 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 {

	
Default load factor for HashMap/LinkedHashMap variants. 
See Also:
/**
	 * Default load factor for {@link HashMap}/{@link LinkedHashMap} variants.
	 * @see #newHashMap(int)
	 * @see #newLinkedHashMap(int)
	 */
	static final float DEFAULT_LOAD_FACTOR = 0.75f;


	
Return true if the supplied Collection is null or empty. Otherwise, return false. 
Params:
  • collection – the Collection to check
Returns:whether the given Collection is empty
/**
	 * 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(@Nullable Collection<?> collection) {
		return (collection == null || collection.isEmpty());
	}

	
Return true if the supplied Map is null or empty. Otherwise, return false. 
Params:
  • map – the Map to check
Returns:whether the given Map is empty
/**
	 * 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(@Nullable Map<?, ?> map) {
		return (map == null || map.isEmpty());
	}

	
Instantiate a new HashMap with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected. 
This differs from the regular HashMap constructor which takes an initial capacity relative to a load factor but is effectively aligned with the JDK's ConcurrentHashMap(int). 
Params:
  • expectedSize – the expected number of elements (with a corresponding
capacity to be derived so that no resize/rehash operations are needed)
See Also:
Since:5.3
/**
	 * Instantiate a new {@link HashMap} with an initial capacity
	 * that can accommodate the specified number of elements without
	 * any immediate resize/rehash operations to be expected.
	 * <p>This differs from the regular {@link HashMap} constructor
	 * which takes an initial capacity relative to a load factor
	 * but is effectively aligned with the JDK's
	 * {@link java.util.concurrent.ConcurrentHashMap#ConcurrentHashMap(int)}.
	 * @param expectedSize the expected number of elements (with a corresponding
	 * capacity to be derived so that no resize/rehash operations are needed)
	 * @since 5.3
	 * @see #newLinkedHashMap(int)
	 */
	public static <K, V> HashMap<K, V> newHashMap(int expectedSize) {
		return new HashMap<>((int) (expectedSize / DEFAULT_LOAD_FACTOR), DEFAULT_LOAD_FACTOR);
	}

	
Instantiate a new LinkedHashMap with an initial capacity that can accommodate the specified number of elements without any immediate resize/rehash operations to be expected. 
This differs from the regular LinkedHashMap constructor which takes an initial capacity relative to a load factor but is aligned with Spring's own LinkedCaseInsensitiveMap and LinkedMultiValueMap constructor semantics as of 5.3. 
Params:
  • expectedSize – the expected number of elements (with a corresponding
capacity to be derived so that no resize/rehash operations are needed)
See Also:
Since:5.3
/**
	 * Instantiate a new {@link LinkedHashMap} with an initial capacity
	 * that can accommodate the specified number of elements without
	 * any immediate resize/rehash operations to be expected.
	 * <p>This differs from the regular {@link LinkedHashMap} constructor
	 * which takes an initial capacity relative to a load factor but is
	 * aligned with Spring's own {@link LinkedCaseInsensitiveMap} and
	 * {@link LinkedMultiValueMap} constructor semantics as of 5.3.
	 * @param expectedSize the expected number of elements (with a corresponding
	 * capacity to be derived so that no resize/rehash operations are needed)
	 * @since 5.3
	 * @see #newHashMap(int)
	 */
	public static <K, V> LinkedHashMap<K, V> newLinkedHashMap(int expectedSize) {
		return new LinkedHashMap<>((int) (expectedSize / DEFAULT_LOAD_FACTOR), DEFAULT_LOAD_FACTOR);
	}

	
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 Arrays.asList method. This arrayToList method is just meant to deal with an incoming Object value that might be an Object[] or a primitive array at runtime. 
A null source value will be converted to an empty List. 
Params:
  • source – the (potentially primitive) array
See Also:
Returns:the converted List result
/**
	 * 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[])
	 */
	public static List<?> arrayToList(@Nullable Object source) {
		return Arrays.asList(ObjectUtils.toObjectArray(source));
	}

	
Merge the given array into the given Collection.

Params:
  • array – the array to merge (may be null)
  • collection – the target Collection to merge the array into
/**
	 * 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(@Nullable Object array, Collection<E> collection) {
		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 Properties.propertyNames() to even catch default properties linked into the original Properties instance. 
Params:
  • props – the Properties instance to merge (may be null)
  • map – the target Map to merge the properties into
/**
	 * 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(@Nullable Properties props, Map<K, V> map) {
		if (props != null) {
			for (Enumeration<?> en = props.propertyNames(); en.hasMoreElements();) {
				String key = (String) en.nextElement();
				Object value = props.get(key);
				if (value == null) {
					// Allow for defaults fallback or potentially overridden accessor...
					value = props.getProperty(key);
				}
				map.put((K) key, (V) value);
			}
		}
	}


	
Check whether the given Iterator contains the given element.

Params:
  • iterator – the Iterator to check
  • element – the element to look for
Returns:true if found, false otherwise
/**
	 * 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} otherwise
	 */
	public static boolean contains(@Nullable 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.

Params:
  • enumeration – the Enumeration to check
  • element – the element to look for
Returns:true if found, false otherwise
/**
	 * 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} otherwise
	 */
	public static boolean contains(@Nullable 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 true for an equal element as well. 
Params:
  • collection – the Collection to check
  • element – the element to look for
Returns:true if found, false otherwise
/**
	 * 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} otherwise
	 */
	public static boolean containsInstance(@Nullable Collection<?> collection, Object element) {
		if (collection != null) {
			for (Object candidate : collection) {
				if (candidate == element) {
					return true;
				}
			}
		}
		return false;
	}

	
Return true if any element in 'candidates' is contained in 'source'; otherwise returns false. 
Params:
  • source – the source Collection
  • candidates – the candidates to search for
Returns:whether any of the candidates has been found
/**
	 * 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) {
		return findFirstMatch(source, candidates) != null;
	}

	
Return the first element in 'candidates' that is contained in 'source'. If no element in 'candidates' is present in 'source' returns null. Iteration order is Collection implementation specific. 
Params:
  • source – the source Collection
  • candidates – the candidates to search for
Returns:the first present object, or null if not found
/**
	 * 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")
	@Nullable
	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.

Params:
  • collection – the Collection to search
  • type – the type to look for
Returns:a value of the given type found if there is a clear match, or null if none or more than one such value found
/**
	 * 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")
	@Nullable
	public static <T> T findValueOfType(Collection<?> collection, @Nullable 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.

Params:
  • collection – the collection to search
  • types – the types to look for, in prioritized order
Returns:a value of one of the given types found if there is a clear match, or null if none or more than one such value found
/**
	 * 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
	 */
	@Nullable
	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.

Params:
  • collection – the Collection to check
Returns:true if the collection contains a single reference or multiple references to the same instance, false otherwise
/**
	 * 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} otherwise
	 */
	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.

Params:
  • collection – the Collection to check
Returns:the common element type, or null if no clear common type has been found (or the collection was empty)
/**
	 * 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)
	 */
	@Nullable
	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;
	}

	
Retrieve the first element of the given Set, using SortedSet.first() or otherwise using the iterator. 
Params:
  • set – the Set to check (may be null or empty)
See Also:
Returns:the first element, or null if none
Since:5.2.3
/**
	 * Retrieve the first element of the given Set, using {@link SortedSet#first()}
	 * or otherwise using the iterator.
	 * @param set the Set to check (may be {@code null} or empty)
	 * @return the first element, or {@code null} if none
	 * @since 5.2.3
	 * @see SortedSet
	 * @see LinkedHashMap#keySet()
	 * @see java.util.LinkedHashSet
	 */
	@Nullable
	public static <T> T firstElement(@Nullable Set<T> set) {
		if (isEmpty(set)) {
			return null;
		}
		if (set instanceof SortedSet) {
			return ((SortedSet<T>) set).first();
		}

		Iterator<T> it = set.iterator();
		T first = null;
		if (it.hasNext()) {
			first = it.next();
		}
		return first;
	}

	
Retrieve the first element of the given List, accessing the zero index.

Params:
  • list – the List to check (may be null or empty)
Returns:the first element, or null if none
Since:5.2.3
/**
	 * Retrieve the first element of the given List, accessing the zero index.
	 * @param list the List to check (may be {@code null} or empty)
	 * @return the first element, or {@code null} if none
	 * @since 5.2.3
	 */
	@Nullable
	public static <T> T firstElement(@Nullable List<T> list) {
		if (isEmpty(list)) {
			return null;
		}
		return list.get(0);
	}

	
Retrieve the last element of the given Set, using SortedSet.last() or otherwise iterating over all elements (assuming a linked set). 
Params:
  • set – the Set to check (may be null or empty)
See Also:
Returns:the last element, or null if none
Since:5.0.3
/**
	 * Retrieve the last element of the given Set, using {@link SortedSet#last()}
	 * or otherwise iterating over all elements (assuming a linked set).
	 * @param set the Set to check (may be {@code null} or empty)
	 * @return the last element, or {@code null} if none
	 * @since 5.0.3
	 * @see SortedSet
	 * @see LinkedHashMap#keySet()
	 * @see java.util.LinkedHashSet
	 */
	@Nullable
	public static <T> T lastElement(@Nullable Set<T> set) {
		if (isEmpty(set)) {
			return null;
		}
		if (set instanceof SortedSet) {
			return ((SortedSet<T>) set).last();
		}

		// Full iteration necessary...
		Iterator<T> it = set.iterator();
		T last = null;
		while (it.hasNext()) {
			last = it.next();
		}
		return last;
	}

	
Retrieve the last element of the given List, accessing the highest index.

Params:
  • list – the List to check (may be null or empty)
Returns:the last element, or null if none
Since:5.0.3
/**
	 * Retrieve the last element of the given List, accessing the highest index.
	 * @param list the List to check (may be {@code null} or empty)
	 * @return the last element, or {@code null} if none
	 * @since 5.0.3
	 */
	@Nullable
	public static <T> T lastElement(@Nullable List<T> list) {
		if (isEmpty(list)) {
			return null;
		}
		return list.get(list.size() - 1);
	}

	
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.

/**
	 * 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<>();
		while (enumeration.hasMoreElements()) {
			elements.add(enumeration.nextElement());
		}
		return elements.toArray(array);
	}

	
Adapt an Enumeration to an Iterator. 
Params:
  • enumeration – the original Enumeration
Returns:the adapted Iterator
/**
	 * Adapt an {@link Enumeration} to an {@link Iterator}.
	 * @param enumeration the original {@code Enumeration}
	 * @return the adapted {@code Iterator}
	 */
	public static <E> Iterator<E> toIterator(@Nullable Enumeration<E> enumeration) {
		return (enumeration != null ? new EnumerationIterator<>(enumeration) : Collections.emptyIterator());
	}

	
Adapt a Map<K, List<V>> to an MultiValueMap<K, V>. 
Params:
  • targetMap – the original map
Returns:the adapted multi-value map (wrapping the original map)
Since:3.1
/**
	 * Adapt a {@code Map<K, List<V>>} to an {@code MultiValueMap<K, V>}.
	 * @param targetMap the original map
	 * @return the adapted multi-value map (wrapping the original map)
	 * @since 3.1
	 */
	public static <K, V> MultiValueMap<K, V> toMultiValueMap(Map<K, List<V>> targetMap) {
		return new MultiValueMapAdapter<>(targetMap);
	}

	
Return an unmodifiable view of the specified multi-value map.

Params:
  • targetMap – the map for which an unmodifiable view is to be returned.
Returns:an unmodifiable view of the specified multi-value map
Since:3.1
/**
	 * Return an unmodifiable view of the specified multi-value map.
	 * @param targetMap the map for which an unmodifiable view is to be returned.
	 * @return an unmodifiable view of the specified multi-value map
	 * @since 3.1
	 */
	@SuppressWarnings("unchecked")
	public static <K, V> MultiValueMap<K, V> unmodifiableMultiValueMap(
			MultiValueMap<? extends K, ? extends V> targetMap) {

		Assert.notNull(targetMap, "'targetMap' must not be null");
		Map<K, List<V>> result = newLinkedHashMap(targetMap.size());
		targetMap.forEach((key, value) -> {
			List<? extends V> values = Collections.unmodifiableList(value);
			result.put(key, (List<V>) values);
		});
		Map<K, List<V>> unmodifiableMap = Collections.unmodifiableMap(result);
		return toMultiValueMap(unmodifiableMap);
	}


	
Iterator wrapping an Enumeration.

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

		private final 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");
		}
	}


}