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## ix.util
Class Collect

    java.lang.Object
      ix.util.Collect

    public final class Collect

extends java.lang.Object

Collection and Map utilities

See Also:
  `Fn`, `Strings`, `Util`

Nested Class Summary
`static class` | `Collect.ShortestFirstComparator` A Comparator that orders shorter Collections before longer ones.

Method Summary
`static java.util.List` | `append(java.util.List a, java.util.List b)` Returns a new list of the same class as its first argument and that contains the elements of the first list followed by those of the second.
`static boolean` | `areConsistent(java.util.Map m1, java.util.Map m2)` Checks whether two maps are consistent.
`static boolean` | `containsEq(java.util.Collection col, java.lang.Object item)` Determines whether a collection contains an element == to an object.
`static java.util.Collection` | `difference(java.util.Collection a, java.util.Collection b)` Returns a new collection of the same class as its first argument and that contains the elements of the first collection that do not appear in the second.
`static java.util.Map` | `difference(java.util.Map a, java.util.Map b)` Returns a map of the same class as its first argument and that contains the entries from the first map that do not appear in the second.
`static java.lang.String` | `elementsToString(java.util.Collection c)` Returns the `toString()` representation of the elements of the collection, separated by commas.
`static java.lang.String` | `elementsToString(java.util.Collection c, Function1 f)` Returns the `toString()` representation of the elements of the collection as transformed by the function and separated by commas.
`static java.util.List` | `ensureList(java.util.List list)` Ensures that a value is a List object by returning an empty list if the value is `null`.
`static java.util.Map` | `ensureMap(java.util.Map map)` Ensures that a value is a Map object by returning an empty map if the value is `null`.
`static boolean` | `equalAsSets(java.util.Collection a, java.util.Collection b)` Determines whether two collections have the same elements regardless of order.
`static boolean` | `every(java.util.Collection c, Predicate1 p)` Tests whether a predicate is true of every element of a collection.
`static java.util.Map` | `extendMap(java.util.Map m, java.util.Collection keys, java.util.Collection values)` Adds entries to a map from a collection of keys and a parallel collection of values.
`static java.util.Collection` | `filter(java.util.Collection result, java.util.Collection c, Predicate1 p)` Returns the result collection after adding the elements of another collection for which the predicate returned true;
`static java.util.Collection` | `filter(java.util.Collection c, Predicate1 p)` Returns a new collection of the same class as the original and containing only the elements for which the predicate returned true;
`static java.util.Map` | `filter(java.util.Map result, java.util.Map m, Predicate2 p)` Returns the result map after adding a map's (key, value) entries for which a predicate returns true.
`static java.util.Map` | `filter(java.util.Map m, Predicate2 p)` Returns a new map of the same class as the original and containing only the (key, value) entries for which the predicate returned true;
`static java.lang.Object` | `findIf(java.util.Collection col, Predicate1 p)` Returns the first element of the collection such that the predicate is true.
`static boolean` | `haveCommonElements(java.util.Collection a, java.util.Collection b)` Checks whether two colletions have any elements in common.
`static java.util.List` | `insertBeforeFirst(java.lang.Object target, java.util.List list, java.util.List add)` Returns a same-class copy of a list in which a list of additional elements has been inserted before the first occurrence of a specified target element.
`static java.util.Collection` | `intersection(java.util.Collection a, java.util.Collection b)` Returns a new collection of the same class as its first argument and that contains the elements that appear in both collections.
`static boolean` | `isEmpty(java.util.Collection c)` Returns true iff the collection is either `null` or empty.
`static boolean` | `isEmpty(java.util.Map m)` Returns true iff the Map is either `null` or empty.
`static java.util.Iterator` | `iterator(java.util.Collection c)` Returns the collections's iterator, treating `null` as an empty list.
`static java.util.Collection` | `map(java.util.Collection result, java.util.Collection c, Function1 f)` Returns the result collection after adding the values returned by a function applied to each element of another collection.
`static java.util.Collection` | `map(java.util.Collection c, Function1 f)` Returns a new collection of the same class as the original and with corresponding elements obtained by calling the specified function.
`static java.util.List` | `powerset(java.util.List elements)` Returns the powerset of a List in a nice order.
`static
`<E> E`
` | `randomth(java.util.List<E> list, java.util.Random random)` Returns a random element of the list.
`static boolean` | `removeAll(java.util.Map a, java.util.Map b)` Removes all entries from a map that appear in a second map.
`static java.util.List` | `replaceFirst(java.lang.Object from, java.lang.Object to, java.util.List source)` Replaces the first occurrence of an item in a list, returning the modified list.
`static boolean` | `some(java.util.Collection c, Predicate1 p)` Tests whether a prediate is true of at least one element of a collection.
`static Function1` | `toFunction(java.util.Map m)` Returns a function based on the map.
`static java.util.Map` | `transposeMap(java.util.Map m)` Returns a map of the same class as the original but with keys and values exchanged.
`static java.util.Collection` | `union(java.util.Collection a, java.util.Collection b)` Returns a new collection of the same class as its first argument and that contains the elements that appear in the first collection plus all elements from the second collection that were not already in the first.
`static void` | `walkTriangle(java.util.List list, Proc walker)` Performs "triangle iteration" on a list.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Method Detail

### isEmpty

    public static final boolean isEmpty(java.util.Collection c)

  Returns true iff the collection is either `null` or empty.

### iterator

    public static final java.util.Iterator iterator(java.util.Collection c)

  Returns the collections's iterator, treating `null` as an empty list.

### ensureList

    public static java.util.List ensureList(java.util.List list)

  Ensures that a value is a List object by returning an empty list if the value is `null`.

### randomth

    public static <E> E randomth(java.util.List<E> list,
                                 java.util.Random random)

  Returns a random element of the list.

### containsEq

    public static boolean containsEq(java.util.Collection col,
                                     java.lang.Object item)

  Determines whether a collection contains an element == to an object.

  Returns:
    true if the collection contains an element == to the specified item, and false if it does not.

### findIf

    public static java.lang.Object findIf(java.util.Collection col,
                                          Predicate1 p)

  Returns the first element of the collection such that the predicate is true.

### replaceFirst

    public static java.util.List replaceFirst(java.lang.Object from,
                                              java.lang.Object to,
                                              java.util.List source)

  Replaces the first occurrence of an item in a list, returning the modified list. If the item does not occur, the list is returned unchanged.

### equalAsSets

    public static boolean equalAsSets(java.util.Collection a,
                                      java.util.Collection b)

  Determines whether two collections have the same elements regardless of order.

### haveCommonElements

    public static boolean haveCommonElements(java.util.Collection a,
                                             java.util.Collection b)

  Checks whether two colletions have any elements in common.

  For some reason, the Java collections have a containsAll method but no containsAny. This method fills that gap.

### intersection

    public static java.util.Collection intersection(java.util.Collection a,
                                                    java.util.Collection b)

  Returns a new collection of the same class as its first argument and that contains the elements that appear in both collections.

### union

    public static java.util.Collection union(java.util.Collection a,
                                             java.util.Collection b)

  Returns a new collection of the same class as its first argument and that contains the elements that appear in the first collection plus all elements from the second collection that were not already in the first.

### difference

    public static java.util.Collection difference(java.util.Collection a,
                                                  java.util.Collection b)

  Returns a new collection of the same class as its first argument and that contains the elements of the first collection that do not appear in the second.

### append

    public static java.util.List append(java.util.List a,
                                        java.util.List b)

  Returns a new list of the same class as its first argument and that contains the elements of the first list followed by those of the second.

### insertBeforeFirst

    public static java.util.List insertBeforeFirst(java.lang.Object target,
                                                   java.util.List list,
                                                   java.util.List add)

  Returns a same-class copy of a list in which a list of additional elements has been inserted before the first occurrence of a specified target element. If the target does not occur in the original list, an IllegalArgumentException is thrown.

  Parameters:
    `target` - the item to insert before
    `list` - the list to insert into
    `add` - the elements to insert

### map

    public static java.util.Collection map(java.util.Collection c,
                                           Function1 f)

  Returns a new collection of the same class as the original and with corresponding elements obtained by calling the specified function.

### map

    public static java.util.Collection map(java.util.Collection result,
                                           java.util.Collection c,
                                           Function1 f)

  Returns the result collection after adding the values returned by a function applied to each element of another collection.

### some

    public static boolean some(java.util.Collection c,
                               Predicate1 p)

  Tests whether a prediate is true of at least one element of a collection.

### every

    public static boolean every(java.util.Collection c,
                                Predicate1 p)

  Tests whether a predicate is true of every element of a collection.

### filter

    public static java.util.Collection filter(java.util.Collection c,
                                              Predicate1 p)

  Returns a new collection of the same class as the original and containing only the elements for which the predicate returned true;

### filter

    public static java.util.Collection filter(java.util.Collection result,
                                              java.util.Collection c,
                                              Predicate1 p)

  Returns the result collection after adding the elements of another collection for which the predicate returned true;

### elementsToString

    public static java.lang.String elementsToString(java.util.Collection c,
                                                    Function1 f)

  Returns the `toString()` representation of the elements of the collection as transformed by the function and separated by commas.

### elementsToString

    public static java.lang.String elementsToString(java.util.Collection c)

  Returns the `toString()` representation of the elements of the collection, separated by commas.

### walkTriangle

    public static void walkTriangle(java.util.List list,
                                    Proc walker)

  Performs "triangle iteration" on a list. For each element in the list, the specified 2-argument procedure is called on that element and each element after it in the list.

### powerset

    public static java.util.List powerset(java.util.List elements)

  Returns the powerset of a List in a nice order. For example, for list `[a, b, c]`, the result will be:

        [[], [a], [b], [c], [a, b], [a, c], [b, c], [a, b, c]]

### isEmpty

    public static final boolean isEmpty(java.util.Map m)

  Returns true iff the Map is either `null` or empty.

### ensureMap

    public static java.util.Map ensureMap(java.util.Map map)

  Ensures that a value is a Map object by returning an empty map if the value is `null`.

### areConsistent

    public static boolean areConsistent(java.util.Map m1,
                                        java.util.Map m2)

  Checks whether two maps are consistent.

  Returns:
    true iff agree on the values of every key they have in common.

### toFunction

    public static Function1 toFunction(java.util.Map m)

  Returns a function based on the map. This is useful in conjunction with `map(Collection, Function1)` and similar methods.

### extendMap

    public static java.util.Map extendMap(java.util.Map m,
                                          java.util.Collection keys,
                                          java.util.Collection values)

  Adds entries to a map from a collection of keys and a parallel collection of values.

  Throws:
    `java.lang.IllegalArgumentException` - if a key is already present in the Map or appears more than once in the collection of keys.
    `AssertionFailure` - if the key and value collections are of different lengths.

### transposeMap

    public static java.util.Map transposeMap(java.util.Map m)

  Returns a map of the same class as the original but with keys and values exchanged.

### difference

    public static java.util.Map difference(java.util.Map a,
                                           java.util.Map b)

  Returns a map of the same class as its first argument and that contains the entries from the first map that do not appear in the second. This method calls `removeAll(Map, Map)` on a copy of the first map.

### removeAll

    public static boolean removeAll(java.util.Map a,
                                    java.util.Map b)

  Removes all entries from a map that appear in a second map. Returns true if any change was made. Entries count as the same if the second map has an entry for a key and either both values are null or equals regards the values as the same.

### filter

    public static java.util.Map filter(java.util.Map m,
                                       Predicate2 p)

  Returns a new map of the same class as the original and containing only the (key, value) entries for which the predicate returned true;

### filter

    public static java.util.Map filter(java.util.Map result,
                                       java.util.Map m,
                                       Predicate2 p)

  Returns the result map after adding a map's (key, value) entries for which a predicate returns true.

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