@GwtCompatible public final class Functions extends Object
com.google.common.base.Function
instances; see that
class for information about migrating to java.util.function
.
All methods return serializable functions as long as they're given serializable parameters.
See the Guava User Guide article on the use of Function
.
Modifier and Type | Method and Description |
---|---|
static <A,B,C> Function<A,C> |
compose(Function<B,C> g,
Function<A,? extends B> f)
Returns the composition of two functions.
|
static <E> Function<Object,E> |
constant(E value)
Returns a function that ignores its input and always returns
value . |
static <K,V> Function<K,V> |
forMap(Map<K,? extends V> map,
V defaultValue)
Returns a function which performs a map lookup with a default value.
|
static <K,V> Function<K,V> |
forMap(Map<K,V> map)
Returns a function which performs a map lookup.
|
static <T> Function<T,Boolean> |
forPredicate(Predicate<T> predicate)
Creates a function that returns the same boolean output as the given predicate for all inputs.
|
static <T> Function<Object,T> |
forSupplier(Supplier<T> supplier)
Returns a function that ignores its input and returns the result of
supplier.get() . |
static <E> Function<E,E> |
identity()
Returns the identity function.
|
static Function<Object,String> |
toStringFunction()
A function equivalent to the method reference
Object::toString , for users not yet using
Java 8. |
public static Function<Object,String> toStringFunction()
Object::toString
, for users not yet using
Java 8. The function simply invokes toString
on its argument and returns the result. It
throws a NullPointerException
on null input.
Warning: The returned function may not be consistent with equals (as
documented at Function.apply(F)
). For example, this function yields different results for
the two equal instances ImmutableSet.of(1, 2)
and ImmutableSet.of(2, 1)
.
Warning: as with all function types in this package, avoid depending on the specific
equals
, hashCode
or toString
behavior of the returned function. A
future migration to java.util.function
will not preserve this behavior.
For Java 8 users: use the method reference Object::toString
instead. In the
future, when this class requires Java 8, this method will be deprecated. See Function
for more important information about the Java 8 transition.
public static <K,V> Function<K,V> forMap(Map<K,V> map)
IllegalArgumentException
if given a key that does not exist in the map. See also forMap(Map, Object)
, which returns a default value in this case.
Note: if map
is a BiMap
(or can be one), you
can use Maps.asConverter
instead to get a
function that also supports reverse conversion.
Java 8 users: if you are okay with null
being returned for an unrecognized
key (instead of an exception being thrown), you can use the method reference map::get
instead.
public static <K,V> Function<K,V> forMap(Map<K,? extends V> map, V defaultValue)
defaultValue
for all inputs that do not belong to the map's key
set. See also forMap(Map)
, which throws an exception in this case.
Java 8 users: you can just write the lambda expression k ->
map.getOrDefault(k, defaultValue)
instead.
map
- source map that determines the function behaviordefaultValue
- the value to return for inputs that aren't map keysmap.get(a)
when a
is a key, or defaultValue
otherwisepublic static <A,B,C> Function<A,C> compose(Function<B,C> g, Function<A,? extends B> f)
f: A->B
and g: B->C
, composition
is defined as the function h such that h(a) == g(f(a))
for each a
.
Java 8 users: use g.compose(f)
or (probably clearer) f.andThen(g)
instead.
g
- the second function to applyf
- the first function to applyf
and g
public static <T> Function<T,Boolean> forPredicate(Predicate<T> predicate)
The returned function is consistent with equals (as documented at Function.apply(F)
) if and only if predicate
is itself consistent with equals.
Java 8 users: use the method reference predicate::test
instead.
public static <E> Function<Object,E> constant(E value)
value
.
Java 8 users: use the lambda expression o -> value
instead.
value
- the constant value for the function to returnvalue
public static <T> Function<Object,T> forSupplier(Supplier<T> supplier)
supplier.get()
.
Java 8 users: use the lambda expression o -> supplier.get()
instead.
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