Optional (abacus API Documentation)

java.lang.Object
- com.landawn.abacus.util.Optional<T>

```
public final class Optional<T>
extends java.lang.Object
```
Note: It's copied from OpenJDK at: http://hg.openjdk.java.net/jdk8u/hs-dev/jdk
A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.
Additional methods that depend on the presence or absence of a contained value are provided, such as orElse() (return a default value if value not present) and ifPresent() (execute a block of code if the value is present).
This is a value-based class; use of identity-sensitive operations (including reference equality (==), identity hash code, or synchronization) on instances of Optional may have unpredictable results and should be avoided.

Since:

1.8

Method Summary

All Methods Static Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`java.util.Optional<T>`	`__()`
`static <T> Optional<T>`	`empty()` Returns an empty `Optional` instance.
`boolean`	`equals(java.lang.Object obj)` Indicates whether some other object is "equal to" this Optional.
`Optional<T>`	`filter(Predicate<? super T> predicate)` If a value is present, and the value matches the given predicate, return an `Optional` describing the value, otherwise return an empty `Optional`.
`<U> Optional<U>`	`flatMap(Function<? super T,Optional<U>> mapper)` If a value is present, apply the provided `Optional`-bearing mapping function to it, return that result, otherwise return an empty `Optional`.
`T`	`get()` If a value is present in this `Optional`, returns the value, otherwise throws `NoSuchElementException`.
`int`	`hashCode()` Returns the hash code value of the present value, if any, or 0 (zero) if no value is present.
`void`	`ifPresent(Consumer<? super T> action)` If a value is present, invoke the specified consumer with the value, otherwise do nothing.
`void`	`ifPresentOrElse(Consumer<? super T> action, java.lang.Runnable emptyAction)` If a value is present, performs the given action with the value, otherwise performs the given empty-based action.
`boolean`	`isPresent()` Return `true` if there is a value present, otherwise `false`.
`<U> Optional<U>`	`map(Function<? super T,? extends U> mapper)` If a value is present, apply the provided mapping function to it, and if the result is non-null, return an `Optional` describing the result.
`OptionalBoolean`	`mapToBoolean(ToBooleanFunction<? super T> mapper)`
`OptionalByte`	`mapToByte(ToByteFunction<? super T> mapper)`
`OptionalChar`	`mapToChar(ToCharFunction<? super T> mapper)`
`OptionalDouble`	`mapToDouble(ToDoubleFunction<? super T> mapper)`
`OptionalFloat`	`mapToFloat(ToFloatFunction<? super T> mapper)`
`OptionalInt`	`mapToInt(ToIntFunction<? super T> mapper)`
`OptionalLong`	`mapToLong(ToLongFunction<? super T> mapper)`
`OptionalShort`	`mapToShort(ToShortFunction<? super T> mapper)`
`static <T> Optional<T>`	`of(T value)` Returns an `Optional` with the specified present non-null value.
`static <T> Optional<T>`	`ofNullable(T value)` Returns an `Optional` describing the specified value, if non-null, otherwise returns an empty `Optional`.
`T`	`orElse(T other)` Return the value if present, otherwise return `other`.
`T`	`orElseGet(Supplier<? extends T> other)` Return the value if present, otherwise invoke `other` and return the result of that invocation.
`<X extends java.lang.Throwable> T`	`orElseThrow(Supplier<? extends X> exceptionSupplier)` Return the contained value, if present, otherwise throw an exception to be created by the provided supplier.
`Stream<T>`	`stream()`
`java.util.List<T>`	`toList()` Returns a `List` with the value if it presents, otherwise an empty `List`.
`java.util.Set<T>`	`toSet()` Returns a `Set` with the value if it presents, otherwise an empty `Set`.
`java.lang.String`	`toString()` Returns a non-empty string representation of this Optional suitable for debugging.

Methods inherited from class java.lang.Object
getClass, notify, notifyAll, wait, wait, wait

- Method Detail
 - empty
```
public static <T> Optional<T> empty()
```
 Returns an empty Optional instance. No value is present for this Optional.
 
 Type Parameters:
 
 T - Type of the non-existent value
 
 Returns:
 
 an empty Optional
 - of
```
public static <T> Optional<T> of(T value)
```
 Returns an Optional with the specified present non-null value.
 
 Type Parameters:
 
 T - the class of the value
 
 Parameters:
 
 value - the value to be present, which must be non-null
 
 Returns:
 
 an Optional with the value present
 
 Throws:
 
 java.lang.NullPointerException - if value is null
 - ofNullable
```
public static <T> Optional<T> ofNullable(T value)
```
 Returns an Optional describing the specified value, if non-null, otherwise returns an empty Optional.
 
 Type Parameters:
 
 T - the class of the value
 
 Parameters:
 
 value - the possibly-null value to describe
 
 Returns:
 
 an Optional with a present value if the specified value is non-null, otherwise an empty Optional
 - get
```
public T get()
```
 If a value is present in this Optional, returns the value, otherwise throws NoSuchElementException.
 
 Returns:
 
 the non-null value held by this Optional
 
 Throws:
 
 java.util.NoSuchElementException - if there is no value present
 
 See Also:
 
 isPresent()
 - isPresent
```
public boolean isPresent()
```
 Return true if there is a value present, otherwise false.
 
 Returns:
 
 true if there is a value present, otherwise false
 - ifPresent
```
public void ifPresent(Consumer<? super T> action)
```
 If a value is present, invoke the specified consumer with the value, otherwise do nothing.
 
 Parameters:
 
 action - block to be executed if a value is present
 
 Throws:
 
 java.lang.NullPointerException - if value is present and consumer is null
 - ifPresentOrElse
```
public void ifPresentOrElse(Consumer<? super T> action,
 java.lang.Runnable emptyAction)
```
 If a value is present, performs the given action with the value, otherwise performs the given empty-based action.
 
 Parameters:
 
 action -
 
 emptyAction -
 - filter
```
public Optional<T> filter(Predicate<? super T> predicate)
```
 If a value is present, and the value matches the given predicate, return an Optional describing the value, otherwise return an empty Optional.
 
 Parameters:
 
 predicate - a predicate to apply to the value, if present
 
 Returns:
 
 an Optional describing the value of this Optional if a value is present and the value matches the given predicate, otherwise an empty Optional
 
 Throws:
 
 java.lang.NullPointerException - if the predicate is null
 - map
```
public  Optional map(Function<? super T,? extends U> mapper)
```
 If a value is present, apply the provided mapping function to it, and if the result is non-null, return an Optional describing the result. Otherwise return an empty Optional.
 
 Type Parameters:
 
 U - The type of the result of the mapping function
 
 Parameters:
 
 mapper - a mapping function to apply to the value, if present
 
 Returns:
 
 an Optional describing the result of applying a mapping function to the value of this Optional, if a value is present, otherwise an empty Optional
 
 Throws:
 
 java.lang.NullPointerException - if the mapping function is null
 - mapToBoolean
```
public OptionalBoolean mapToBoolean(ToBooleanFunction<? super T> mapper)
```
 - mapToChar
```
public OptionalChar mapToChar(ToCharFunction<? super T> mapper)
```
 - mapToByte
```
public OptionalByte mapToByte(ToByteFunction<? super T> mapper)
```
 - mapToShort
```
public OptionalShort mapToShort(ToShortFunction<? super T> mapper)
```
 - mapToInt
```
public OptionalInt mapToInt(ToIntFunction<? super T> mapper)
```
 - mapToLong
```
public OptionalLong mapToLong(ToLongFunction<? super T> mapper)
```
 - mapToFloat
```
public OptionalFloat mapToFloat(ToFloatFunction<? super T> mapper)
```
 - mapToDouble
```
public OptionalDouble mapToDouble(ToDoubleFunction<? super T> mapper)
```
 - flatMap
```
public  Optional flatMap(Function<? super T,Optional> mapper)
```
 If a value is present, apply the provided Optional-bearing mapping function to it, return that result, otherwise return an empty Optional. This method is similar to map(Function), but the provided mapper is one whose result is already an Optional, and if invoked, flatMap does not wrap it with an additional Optional.
 
 Type Parameters:
 
 U - The type parameter to the Optional returned by
 
 Parameters:
 
 mapper - a mapping function to apply to the value, if present the mapping function
 
 Returns:
 
 the result of applying an Optional-bearing mapping function to the value of this Optional, if a value is present, otherwise an empty Optional
 
 Throws:
 
 java.lang.NullPointerException - if the mapping function is null or returns a null result
 - orElse
```
public T orElse(T other)
```
 Return the value if present, otherwise return other.
 
 Parameters:
 
 other - the value to be returned if there is no value present, may be null
 
 Returns:
 
 the value, if present, otherwise other
 - orElseGet
```
public T orElseGet(Supplier<? extends T> other)
```
 Return the value if present, otherwise invoke other and return the result of that invocation.
 
 Parameters:
 
 other - a Supplier whose result is returned if no value is present
 
 Returns:
 
 the value if present otherwise the result of other.get()
 
 Throws:
 
 java.lang.NullPointerException - if value is not present and other is null
 - orElseThrow
```
public <X extends java.lang.Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier)
 throws X extends java.lang.Throwable
```
 Return the contained value, if present, otherwise throw an exception to be created by the provided supplier.
 
 Type Parameters:
 
 X - Type of the exception to be thrown
 
 Parameters:
 
 exceptionSupplier - The supplier which will return the exception to be thrown
 
 Returns:
 
 the present value
 
 Throws:
 
 X - if there is no value present
 
 java.lang.NullPointerException - if no value is present and exceptionSupplier is null
 
 X extends java.lang.Throwable
 - stream
```
public Stream<T> stream()
```
 - __
```
public java.util.Optional<T> __()
```
 - toList
```
public java.util.List<T> toList()
```
 Returns a List with the value if it presents, otherwise an empty List.
 
 Returns:
 - toSet
```
public java.util.Set<T> toSet()
```
 Returns a Set with the value if it presents, otherwise an empty Set.
 
 Returns:
 - equals
```
public boolean equals(java.lang.Object obj)
```
 Indicates whether some other object is "equal to" this Optional. The other object is considered equal if:
 - it is also an Optional and;
 - both instances have no value present or;
 - the present values are "equal to" each other via equals().
 Overrides:
 
 equals in class java.lang.Object
 
 Parameters:
 
 obj - an object to be tested for equality
 
 Returns:
 
 {code true} if the other object is "equal to" this object otherwise false
 - hashCode
```
public int hashCode()
```
 Returns the hash code value of the present value, if any, or 0 (zero) if no value is present.
 
 Overrides:
 
 hashCode in class java.lang.Object
 
 Returns:
 
 hash code value of the present value or 0 if no value is present
 - toString
```
public java.lang.String toString()
```
 Returns a non-empty string representation of this Optional suitable for debugging. The exact presentation format is unspecified and may vary between implementations and versions.
 
 Overrides:
 
 toString in class java.lang.Object
 
 Returns:
 
 the string representation of this instance

Class Optional<T>

Method Summary

Methods inherited from class java.lang.Object

Method Detail

empty

of

ofNullable

get

isPresent

ifPresent

ifPresentOrElse

filter

map

mapToBoolean

mapToChar

mapToByte

mapToShort

mapToInt

mapToLong

mapToFloat

mapToDouble

flatMap

orElse

orElseGet

orElseThrow

stream

__

toList

toSet

equals

hashCode

toString