public class

SetUtils

extends Object
java.lang.Object
   ↳ org.apache.commons.collections.SetUtils

Class Overview

Provides utility methods and decorators for Set and SortedSet instances.

Summary

Fields
public static final Set EMPTY_SET An empty unmodifiable set.
public static final SortedSet EMPTY_SORTED_SET An empty unmodifiable sorted set.
Public Constructors
SetUtils()
SetUtils should not normally be instantiated.
Public Methods
static int hashCodeForSet(Collection set)
Generates a hash code using the algorithm specified in hashCode().
static boolean isEqualSet(Collection set1, Collection set2)
Tests two sets for equality as per the equals() contract in equals(java.lang.Object).
static Set orderedSet(Set set)
Returns a set that maintains the order of elements that are added backed by the given set.
static Set predicatedSet(Set set, Predicate predicate)
Returns a predicated (validating) set backed by the given set.
static SortedSet predicatedSortedSet(SortedSet set, Predicate predicate)
Returns a predicated (validating) sorted set backed by the given sorted set.
static Set synchronizedSet(Set set)
Returns a synchronized set backed by the given set.
static SortedSet synchronizedSortedSet(SortedSet set)
Returns a synchronized sorted set backed by the given sorted set.
static Set transformedSet(Set set, Transformer transformer)
Returns a transformed set backed by the given set.
static SortedSet transformedSortedSet(SortedSet set, Transformer transformer)
Returns a transformed sorted set backed by the given set.
static Set typedSet(Set set, Class type)
Returns a typed set backed by the given set.
static SortedSet typedSortedSet(SortedSet set, Class type)
Returns a typed sorted set backed by the given set.
static Set unmodifiableSet(Set set)
Returns an unmodifiable set backed by the given set.
static SortedSet unmodifiableSortedSet(SortedSet set)
Returns an unmodifiable sorted set backed by the given sorted set.
[Expand]
Inherited Methods
From class java.lang.Object

Fields

public static final Set EMPTY_SET

An empty unmodifiable set. This uses the Collections implementation and is provided for completeness.

public static final SortedSet EMPTY_SORTED_SET

An empty unmodifiable sorted set. This is not provided in the JDK.

Public Constructors

public SetUtils ()

SetUtils should not normally be instantiated.

Public Methods

public static int hashCodeForSet (Collection set)

Generates a hash code using the algorithm specified in hashCode().

This method is useful for implementing Set when you cannot extend AbstractSet. The method takes Collection instances to enable other collection types to use the Set implementation algorithm.

Parameters
set the set to calculate the hash code for, may be null
Returns
  • the hash code
See Also

public static boolean isEqualSet (Collection set1, Collection set2)

Tests two sets for equality as per the equals() contract in equals(java.lang.Object).

This method is useful for implementing Set when you cannot extend AbstractSet. The method takes Collection instances to enable other collection types to use the Set implementation algorithm.

The relevant text (slightly paraphrased as this is a static method) is:

Two sets are considered equal if they have the same size, and every member of the first set is contained in the second. This ensures that the equals method works properly across different implementations of the Set interface.

This implementation first checks if the two sets are the same object: if so it returns true. Then, it checks if the two sets are identical in size; if not, it returns false. If so, it returns a.containsAll((Collection) b).

Parameters
set1 the first set, may be null
set2 the second set, may be null
Returns
  • whether the sets are equal by value comparison
See Also

public static Set orderedSet (Set set)

Returns a set that maintains the order of elements that are added backed by the given set.

If an element is added twice, the order is determined by the first add. The order is observed through the iterator or toArray.

Parameters
set the set to order, must not be null
Returns
  • an ordered set backed by the given set
Throws
IllegalArgumentException if the Set is null

public static Set predicatedSet (Set set, Predicate predicate)

Returns a predicated (validating) set backed by the given set.

Only objects that pass the test in the given predicate can be added to the set. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original set after invoking this method, as it is a backdoor for adding invalid objects.

Parameters
set the set to predicate, must not be null
predicate the predicate for the set, must not be null
Returns
  • a predicated set backed by the given set
Throws
IllegalArgumentException if the Set or Predicate is null

public static SortedSet predicatedSortedSet (SortedSet set, Predicate predicate)

Returns a predicated (validating) sorted set backed by the given sorted set.

Only objects that pass the test in the given predicate can be added to the set. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original set after invoking this method, as it is a backdoor for adding invalid objects.

Parameters
set the sorted set to predicate, must not be null
predicate the predicate for the sorted set, must not be null
Returns
  • a predicated sorted set backed by the given sorted set
Throws
IllegalArgumentException if the Set or Predicate is null

public static Set synchronizedSet (Set set)

Returns a synchronized set backed by the given set.

You must manually synchronize on the returned buffer's iterator to avoid non-deterministic behavior:

 Set s = SetUtils.synchronizedSet(mySet);
 synchronized (s) {
     Iterator i = s.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 
This method uses the implementation in the decorators subpackage.

Parameters
set the set to synchronize, must not be null
Returns
  • a synchronized set backed by the given set
Throws
IllegalArgumentException if the set is null

public static SortedSet synchronizedSortedSet (SortedSet set)

Returns a synchronized sorted set backed by the given sorted set.

You must manually synchronize on the returned buffer's iterator to avoid non-deterministic behavior:

 Set s = SetUtils.synchronizedSet(mySet);
 synchronized (s) {
     Iterator i = s.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 
This method uses the implementation in the decorators subpackage.

Parameters
set the sorted set to synchronize, must not be null
Returns
  • a synchronized set backed by the given set
Throws
IllegalArgumentException if the set is null

public static Set transformedSet (Set set, Transformer transformer)

Returns a transformed set backed by the given set.

Each object is passed through the transformer as it is added to the Set. It is important not to use the original set after invoking this method, as it is a backdoor for adding untransformed objects.

Parameters
set the set to transform, must not be null
transformer the transformer for the set, must not be null
Returns
  • a transformed set backed by the given set
Throws
IllegalArgumentException if the Set or Transformer is null

public static SortedSet transformedSortedSet (SortedSet set, Transformer transformer)

Returns a transformed sorted set backed by the given set.

Each object is passed through the transformer as it is added to the Set. It is important not to use the original set after invoking this method, as it is a backdoor for adding untransformed objects.

Parameters
set the set to transform, must not be null
transformer the transformer for the set, must not be null
Returns
  • a transformed set backed by the given set
Throws
IllegalArgumentException if the Set or Transformer is null

public static Set typedSet (Set set, Class type)

Returns a typed set backed by the given set.

Only objects of the specified type can be added to the set.

Parameters
set the set to limit to a specific type, must not be null
type the type of objects which may be added to the set
Returns
  • a typed set backed by the specified set

public static SortedSet typedSortedSet (SortedSet set, Class type)

Returns a typed sorted set backed by the given set.

Only objects of the specified type can be added to the set.

Parameters
set the set to limit to a specific type, must not be null
type the type of objects which may be added to the set
Returns
  • a typed set backed by the specified set

public static Set unmodifiableSet (Set set)

Returns an unmodifiable set backed by the given set.

This method uses the implementation in the decorators subpackage.

Parameters
set the set to make unmodifiable, must not be null
Returns
  • an unmodifiable set backed by the given set
Throws
IllegalArgumentException if the set is null

public static SortedSet unmodifiableSortedSet (SortedSet set)

Returns an unmodifiable sorted set backed by the given sorted set.

This method uses the implementation in the decorators subpackage.

Parameters
set the sorted set to make unmodifiable, must not be null
Returns
  • an unmodifiable set backed by the given set
Throws
IllegalArgumentException if the set is null