JDK1.8新特性（一）Optional类
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认识Optional
类注解源码：
/**
* A container object which may or may not contain a non-null value.
* If a value is present, {@code isPresent()} will return {@code true} and
* {@code get()} will return the value.
*
* Additional methods that depend on the presence or absence of a contained
* value are provided, such as {@link #orElse(java.lang.Object) orElse()}
* (return a default value if value not present) and
* {@link #ifPresent(java.util.function.Consumer) 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
* ({@code ==}), identity hash code, or synchronization) on instances of
* {@code Optional} may have unpredictable results and should be avoided.
*
* @since 1.8
*/
public final class Optional {
/**
* Common instance for {@code empty()}.
*/
private static final Optional EMPTY = new Optional<>();
/**
* If non-null, the value; if null, indicates no value is present
*/
private final T value;
}
从类注解中我们可以看出，Optional是一个可以容纳空对象的object容器，可通过isPresent()方法来判断值是否存在，通过get()方法获取值。附加方法的使用取决于容器中的值是否存在。看来，Optional类的主要特性是区分对象是否为空，并分别作出处理的容器类。下面，我们看看这个容器在代码中是怎么使用的。
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API讲解
1.empty() 获取一个空容器。返回一个指向new Optional<>()的私有静态常量；
/**
* Returns an empty {@code Optional} instance. No value is present for this
* Optional.
*
* @apiNote Though it may be tempting to do so, avoid testing if an object
* is empty by comparing with {@code ==} against instances returned by
* {@code Option.empty()}. There is no guarantee that it is a singleton.
* Instead, use {@link #isPresent()}.
*
* @param Type of the non-existent value
* @return an empty {@code Optional}
*/
public static Optional empty() {
@SuppressWarnings("unchecked")
Optional t = (Optional) EMPTY;
return t;
} 2.of(T value) 获取一个容纳非空对象的容器。
/**
* Returns an {@code Optional} with the specified present non-null value.
*
* @param the class of the value
* @param value the value to be present, which must be non-null
* @return an {@code Optional} with the value present
* @throws NullPointerException if value is null
*/
public static Optional of(T value) {
return new Optional<>(value);
} 3.ofNullable(T value) 获取一个可容纳空对象的容器。
/**
* Returns an {@code Optional} describing the specified value, if non-null,
* otherwise returns an empty {@code Optional}.
*
* @param the class of the value
* @param value the possibly-null value to describe
* @return an {@code Optional} with a present value if the specified value
* is non-null, otherwise an empty {@code Optional}
*/
public static Optional ofNullable(T value) {
return value == null ? empty() : of(value);
} 4.get() 获取容器容纳的对象，当对象为空时抛出NoSuchElementException。
/**
* If a value is present in this {@code Optional}, returns the value,
* otherwise throws {@code NoSuchElementException}.
*
* @return the non-null value held by this {@code Optional}
* @throws NoSuchElementException if there is no value present
*
* @see Optional#isPresent()
*/
public T get() {
if (value == null) {
throw new NoSuchElementException("No value present");
}
return value;
}通过代码实践一下：
package optional;
import java.util.Optional;
public class User {
private String name, id;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
@Override
public String toString() {
return "User{" +
"name='" + name + '\'' +
", id='" + id + '\'' +
'}';
}
public static void main(String[] args) {
User user = new User();
User nullUser = null;
Optional userOpt = Optional.ofNullable(user);
Optional nullUserOpt = Optional.ofNullable(nullUser);
System.out.println(userOpt.get());
System.out.println(nullUserOpt.get());
}
}
控制台输出内容：
User{name='null', id='null'}
Exception in thread "main" java.util.NoSuchElementException: No value present
at java.util.Optional.get(Optional.java:135)
at optional.User.main(User.java:39)5.isPresent() 判断容纳的对象是否为空。
/**
* Return {@code true} if there is a value present, otherwise {@code false}.
*
* @return {@code true} if there is a value present, otherwise {@code false}
*/
public boolean isPresent() {
return value != null;
}代码实践：
System.out.println(userOpt.isPresent());
System.out.println(nullUserOpt.isPresent());
控制台输出：
true
false6.ifPresent(Consumer consumer)当容纳的对象存在时，执行参数中的方法。
/**
* If a value is present, invoke the specified consumer with the value,
* otherwise do nothing.
*
* @param consumer block to be executed if a value is present
* @throws NullPointerException if value is present and {@code consumer} is
* null
*/
public void ifPresent(Consumer consumer) {
if (value != null)
consumer.accept(value);
}
代码实践：
userOpt.ifPresent(item -> System.out.println(item));
nullUserOpt.ifPresent(item -> System.out.println(item));
控制台：
User{name='null', id='null'}
7.filter(Predicate predicate)通过一定条件过滤容器，返回null，this，或者空容器。
/**
* If a value is present, and the value matches the given predicate,
* return an {@code Optional} describing the value, otherwise return an
* empty {@code Optional}.
*
* @param predicate a predicate to apply to the value, if present
* @return an {@code Optional} describing the value of this {@code Optional}
* if a value is present and the value matches the given predicate,
* otherwise an empty {@code Optional}
* @throws NullPointerException if the predicate is null
*/
public Optional filter(Predicate predicate) {
Objects.requireNonNull(predicate);
if (!isPresent())
return this;
else
return predicate.test(value) ? this : empty();
} 代码实践：
Optional optional = nullUserOpt.filter((User item) -> item == null);
System.out.println(optional.isPresent());
Optional opt1 = userOpt.filter((User item) -> item == null);
System.out.println(opt1.isPresent());
Optional opt2 = userOpt.filter((User item) -> item != null);
System.out.println(opt2.get());
控制台输出：
false
false
User{name='null', id='null'} 8.map(Function mapper)获取容纳参数方法的返回值的容器。
/**
* If a value is present, apply the provided mapping function to it,
* and if the result is non-null, return an {@code Optional} describing the
* result. Otherwise return an empty {@code Optional}.
*
* @apiNote This method supports post-processing on optional values, without
* the need to explicitly check for a return status. For example, the
* following code traverses a stream of file names, selects one that has
* not yet been processed, and then opens that file, returning an
* {@code Optional}:
*
* {@code
* Optional fis =
* names.stream().filter(name -> !isProcessedYet(name))
* .findFirst()
* .map(name -> new FileInputStream(name));
* }
*
* Here, {@code findFirst} returns an {@code Optional}, and then
* {@code map} returns an {@code Optional} for the desired
* file if one exists.
*
* @param The type of the result of the mapping function
* @param mapper a mapping function to apply to the value, if present
* @return an {@code Optional} describing the result of applying a mapping
* function to the value of this {@code Optional}, if a value is present,
* otherwise an empty {@code Optional}
* @throws NullPointerException if the mapping function is null
*/
public Optional map(Function mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Optional.ofNullable(mapper.apply(value));
}
} 代码实践：
user.setId("123");
Optional idOpt = userOpt.map(User::getId);
System.out.println(idOpt.get());
控制台：
123 9.flatMap(Function> mapper)目的同上，不同的是参数方法的返回值必须由Optional包装。
代码实践：
user.setId("123");
Optional idOpt = userOpt.flatMap(item -> Optional.ofNullable(item.getId()));
System.out.println(idOpt.get());
控制台：123 10.orElse(T other) 当容纳的对象为空时，返回参数中的对象。
/**
* Return the value if present, otherwise return {@code other}.
*
* @param other the value to be returned if there is no value present, may
* be null
* @return the value, if present, otherwise {@code other}
*/
public T orElse(T other) {
return value != null ? value : other;
}代码实践：
User user1 = nullUserOpt.orElse(user);
System.out.println(user1);
控制台：
User{name='null', id='null'}11.orElseGet(Supplier other)当容纳的对象为空时，返回参数中方法的返回值，注意泛型一致。
/**
* Return the value if present, otherwise invoke {@code other} and return
* the result of that invocation.
*
* @param other a {@code Supplier} whose result is returned if no value
* is present
* @return the value if present otherwise the result of {@code other.get()}
* @throws NullPointerException if value is not present and {@code other} is
* null
*/
public T orElseGet(Supplier other) {
return value != null ? value : other.get();
}代码实践（本次直接使用userOpt）：
User user1 = nullUserOpt.orElseGet(userOpt::get);
System.out.println(user1);
控制台输出：
User{name='null', id='null'}
12.orElseThrow(Supplier exceptionSupplier) throws X 当容器中对象为空时，抛出参数中的异常。
/**
* Return the contained value, if present, otherwise throw an exception
* to be created by the provided supplier.
*
* @apiNote A method reference to the exception constructor with an empty
* argument list can be used as the supplier. For example,
* {@code IllegalStateException::new}
*
* @param Type of the exception to be thrown
* @param exceptionSupplier The supplier which will return the exception to
* be thrown
* @return the present value
* @throws X if there is no value present
* @throws NullPointerException if no value is present and
* {@code exceptionSupplier} is null
*/
public T orElseThrow(Supplier exceptionSupplier) throws X {
if (value != null) {
return value;
} else {
throw exceptionSupplier.get();
}
} 代码实践：
try {
nullUserOpt.orElseThrow(Exception::new);
}catch (Exception e){
System.err.println(e);
}
控制台输出：
java.lang.Exception13.equals(Object obj)对比容器中的对象
/**
* Indicates whether some other object is "equal to" this Optional. The
* other object is considered equal if:
*
* - it is also an {@code Optional} and;
*
- both instances have no value present or;
*
- the present values are "equal to" each other via {@code equals()}.
*
*
* @param obj an object to be tested for equality
* @return {code true} if the other object is "equal to" this object
* otherwise {@code false}
*/
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Optional)) {
return false;
}
Optional other = (Optional) obj;
return Objects.equals(value, other.value);
}代码实践：
boolean equals = userOpt.equals(Optional.ofNullable(user));
System.out.println(equals);
控制台输出：
true14.hashCode() 获取容器中对象的哈希值
/**
* Returns the hash code value of the present value, if any, or 0 (zero) if
* no value is present.
*
* @return hash code value of the present value or 0 if no value is present
*/
@Override
public int hashCode() {
return Objects.hashCode(value);
}