Package io.vavr

Class Lazy<T>

java.lang.Object

io.vavr.Lazy<T>

All Implemented Interfaces:

Value<T>, java.io.Serializable, java.lang.Iterable<T>, java.util.function.Supplier<T>

public final class Lazy<T>
extends java.lang.Object
implements Value<T>, java.util.function.Supplier<T>, java.io.Serializable

Represents a lazy evaluated value. Compared to a Supplier, Lazy is memoizing, i.e. it evaluates only once and therefore is referential transparent.

 
 final Lazy<Double> l = Lazy.of(Math::random);
 l.isEvaluated(); // = false
 l.get();         // = 0.123 (random generated)
 l.isEvaluated(); // = true
 l.get();         // = 0.123 (memoized)
 
 

Example of creating a real lazy value (works only with interfaces):

final CharSequence chars = Lazy.val(() -> "Yay!", CharSequence.class);

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field	Description
private java.util.concurrent.locks.ReentrantLock	lock
private static long	serialVersionUID
private java.util.function.Supplier<? extends T>	supplier
private T	value

Constructor Summary

Constructors

Modifier	Constructor	Description
private	Lazy(java.util.function.Supplier<? extends T> supplier)

Method Summary

Modifier and Type	Method	Description
private T	computeValue()
boolean	equals(java.lang.Object o)	Clarifies that values have a proper equals() method implemented.
Option<T>	filter(java.util.function.Predicate<? super T> predicate)
T	get()	Evaluates this lazy value and caches it, when called the first time.
int	hashCode()	Clarifies that values have a proper hashCode() method implemented.
boolean	isAsync()	A Lazy's value is computed synchronously.
boolean	isEmpty()	Checks, this Value is empty, i.e.
boolean	isEvaluated()	Checks, if this lazy value is evaluated.
boolean	isLazy()	A Lazy's value is computed lazily.
boolean	isSingleValued()	States whether this is a single-valued type.
Iterator<T>	iterator()	Returns a rich io.vavr.collection.Iterator.
<U> Lazy<U>	map(java.util.function.Function<? super T,? extends U> mapper)	Maps the underlying value to a different component type.
static <T> Lazy<T>	narrow(Lazy<? extends T> lazy)	Narrows a widened Lazy<? extends T> to Lazy<T> by performing a type-safe cast.
static <T> Lazy<T>	of(java.util.function.Supplier<? extends T> supplier)	Creates a Lazy that requests its value from a given Supplier.
Lazy<T>	peek(java.util.function.Consumer<? super T> action)	Performs the given action on the first element if this is an eager implementation.
static <T> Lazy<Seq<T>>	sequence(java.lang.Iterable<? extends Lazy<? extends T>> values)	Reduces many Lazy values into a single Lazy by transforming an Iterable<Lazy<? extends T>> into a Lazy<Seq<T>>.
java.lang.String	stringPrefix()	Returns the name of this Value type, which is used by toString().
java.lang.String	toString()	Clarifies that values have a proper toString() method implemented.
<U> U	transform(java.util.function.Function<? super Lazy<T>,? extends U> f)	Transforms this Lazy.
static <T> T	val(java.util.function.Supplier<? extends T> supplier, java.lang.Class<T> type)	Creates a real _lazy value_ of type T, backed by a Proxy which delegates to a Lazy instance.
private void	writeObject(java.io.ObjectOutputStream s)	Ensures that the value is evaluated before serialization.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface io.vavr.Value

collect, collect, contains, corresponds, eq, exists, forAll, forEach, getOrElse, getOrElse, getOrElseThrow, getOrElseTry, getOrNull, out, out, spliterator, stderr, stdout, toArray, toCharSeq, toCompletableFuture, toEither, toEither, toInvalid, toInvalid, toJavaArray, toJavaArray, toJavaArray, toJavaCollection, toJavaList, toJavaList, toJavaMap, toJavaMap, toJavaMap, toJavaOptional, toJavaParallelStream, toJavaSet, toJavaSet, toJavaStream, toLeft, toLeft, toLinkedMap, toLinkedMap, toLinkedSet, toList, toMap, toMap, toOption, toPriorityQueue, toPriorityQueue, toQueue, toRight, toRight, toSet, toSortedMap, toSortedMap, toSortedMap, toSortedMap, toSortedSet, toSortedSet, toStream, toTree, toTree, toTry, toTry, toValid, toValid, toValidation, toValidation, toVector

Field Detail

serialVersionUID

private static final long serialVersionUID

See Also:

Constant Field Values

lock

private final java.util.concurrent.locks.ReentrantLock lock

supplier

private transient volatile java.util.function.Supplier<? extends T> supplier

value

private T value

Constructor Detail

Lazy

private Lazy(java.util.function.Supplier<? extends T> supplier)

Method Detail

narrow

public static <T> Lazy<T> narrow(Lazy<? extends T> lazy)

Narrows a widened Lazy<? extends T> to Lazy<T> by performing a type-safe cast. This is eligible because immutable/read-only collections are covariant.

Type Parameters:

T - Component type of the Lazy.

Parameters:

lazy - A Lazy.

Returns:

the given lazy instance as narrowed type Lazy<T>.

of

public static <T> Lazy<T> of(java.util.function.Supplier<? extends T> supplier)

Creates a Lazy that requests its value from a given Supplier. The supplier is asked only once, the value is memoized.

Type Parameters:

T - type of the lazy value

Parameters:

supplier - A supplier

Returns:

A new instance of Lazy

sequence

public static <T> Lazy<Seq<T>> sequence(java.lang.Iterable<? extends Lazy<? extends T>> values)

Reduces many Lazy values into a single Lazy by transforming an Iterable<Lazy<? extends T>> into a Lazy<Seq<T>>.

Type Parameters:

T - Type of the lazy values.

Parameters:

values - An iterable of lazy values.

Returns:

A lazy sequence of values.

Throws:

java.lang.NullPointerException - if values is null

val

@GwtIncompatible("reflection is not supported")
public static <T> T val(java.util.function.Supplier<? extends T> supplier,
                        java.lang.Class<T> type)

Creates a real _lazy value_ of type T, backed by a Proxy which delegates to a Lazy instance.

Type Parameters:

T - type of the lazy value

Parameters:

supplier - A supplier

type - An interface

Returns:

A new instance of T

filter

public Option<T> filter(java.util.function.Predicate<? super T> predicate)

get

public T get()

Evaluates this lazy value and caches it, when called the first time. On subsequent calls, returns the cached value.

Specified by:

get in interface java.util.function.Supplier<T>

Specified by:

get in interface Value<T>

Returns:

the lazy evaluated value

computeValue

private T computeValue()

isAsync

public boolean isAsync()

A Lazy's value is computed synchronously.

Specified by:

isAsync in interface Value<T>

Returns:

false

isEmpty

public boolean isEmpty()

Description copied from interface: Value

Checks, this Value is empty, i.e. if the underlying value is absent.

Specified by:

isEmpty in interface Value<T>

Returns:

false, if no underlying value is present, true otherwise.

isEvaluated

public boolean isEvaluated()

Checks, if this lazy value is evaluated.

Note: A value is internally evaluated (once) by calling get().

Returns:

true, if the value is evaluated, false otherwise.

Throws:

java.lang.UnsupportedOperationException - if this value is undefined

isLazy

public boolean isLazy()

A Lazy's value is computed lazily.

Specified by:

isLazy in interface Value<T>

Returns:

true

isSingleValued

public boolean isSingleValued()

Description copied from interface: Value

States whether this is a single-valued type.

Specified by:

isSingleValued in interface Value<T>

Returns:

true if this is single-valued, false otherwise.

iterator

public Iterator<T> iterator()

Description copied from interface: Value

Returns a rich io.vavr.collection.Iterator.

Specified by:

iterator in interface java.lang.Iterable<T>

Specified by:

iterator in interface Value<T>

Returns:

A new Iterator

map

public <U> Lazy<U> map(java.util.function.Function<? super T,? extends U> mapper)

Description copied from interface: Value

Maps the underlying value to a different component type.

Specified by:

map in interface Value<T>

Type Parameters:

U - The new component type

Parameters:

mapper - A mapper

Returns:

A new value

peek

public Lazy<T> peek(java.util.function.Consumer<? super T> action)

Description copied from interface: Value

Performs the given action on the first element if this is an eager implementation. Performs the given action on all elements (the first immediately, successive deferred), if this is a lazy implementation.

Specified by:

peek in interface Value<T>

Parameters:

action - The action that will be performed on the element(s).

Returns:

this instance

transform

public <U> U transform(java.util.function.Function<? super Lazy<T>,? extends U> f)

Transforms this Lazy.

Type Parameters:

U - Type of transformation result

Parameters:

f - A transformation

Returns:

An instance of type U

Throws:

java.lang.NullPointerException - if f is null

stringPrefix

public java.lang.String stringPrefix()

Description copied from interface: Value

Returns the name of this Value type, which is used by toString().

Specified by:

stringPrefix in interface Value<T>

Returns:

This type name.

equals

public boolean equals(java.lang.Object o)

Description copied from interface: Value

Clarifies that values have a proper equals() method implemented.

See Object.equals(Object).

Specified by:

equals in interface Value<T>

Overrides:

equals in class java.lang.Object

Parameters:

o - An object

Returns:

true, if this equals o, false otherwise

hashCode

public int hashCode()

Description copied from interface: Value

Clarifies that values have a proper hashCode() method implemented.

See Object.hashCode().

Specified by:

hashCode in interface Value<T>

Overrides:

hashCode in class java.lang.Object

Returns:

The hashcode of this object

toString

public java.lang.String toString()

Description copied from interface: Value

Clarifies that values have a proper toString() method implemented.

See Object.toString().

Specified by:

toString in interface Value<T>

Overrides:

toString in class java.lang.Object

Returns:

A String representation of this object

writeObject

@GwtIncompatible("The Java serialization protocol is explicitly not supported")
private void writeObject(java.io.ObjectOutputStream s)
                  throws java.io.IOException

Ensures that the value is evaluated before serialization.

Parameters:

s - An object serialization stream.

Throws:

java.io.IOException - If an error occurs writing to the stream.