Optional解决判断为null问题

Optional 的本质，就是内部储存了一个真实的值，在构造的时候，就直接判断其值是否为空。

1、构造方法

        Optional 有两个构造方法，都是 private 权限，不能由外部调用。

    /**
     * Constructs an empty instance.
     *
     * @implNote Generally only one empty instance, {@link Optional#EMPTY},
     * should exist per VM.
     */
    private Optional() {
        this.value = null;
    }

    /**
     * Constructs an instance with the value present.
     *
     * @param value the non-null value to be present
     * @throws NullPointerException if value is null
     */
    private Optional(T value) {
        this.value = Objects.requireNonNull(value);
    }

        这里调用了 Objects 的方法，该方法源码如下：

    /**
     * Checks that the specified object reference is not {@code null}. This
     * method is designed primarily for doing parameter validation in methods
     * and constructors, as demonstrated below:
     * <blockquote><pre>
     * public Foo(Bar bar) {
     *     this.bar = Objects.requireNonNull(bar);
     * }
     * </pre></blockquote>
     *
     * @param obj the object reference to check for nullity
     * @param <T> the type of the reference
     * @return {@code obj} if not {@code null}
     * @throws NullPointerException if {@code obj} is {@code null}
     */
    public static <T> T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }

2、empty()

        Optional类内部维护了一个 value 为 null 的对象：

    /**
     * Common instance for {@code empty()}.
     */
    private static final Optional<?> EMPTY = new Optional<>();

        而 empty() 的作用就是返回 EMPTY 对象：

    /**
     * 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 <T> Type of the non-existent value
     * @return an empty {@code Optional}
     */
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }

3、of(T value)

    /**
     * Returns an {@code Optional} with the specified present non-null value.
     *
     * @param <T> 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 <T> Optional<T> of(T value) {
        return new Optional<>(value);
    }

   也就是说 of(T value) 函数内部调用了构造函数。根据构造函数的源码我们可以得出两个结论：

1. 通过 of(T value) 函数所构造出的 Optional 对象，当 Value 值为空时，依然会报 NullPointerException。
2. 通过 of(T value) 函数所构造出的 Optional 对象，当 Value 值不为空时，能正常构造 Optional 对象。

ofNullable(T value)

    /**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     * @param <T> 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 <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

相比较 of(T value) 的区别就是，当 value 值为 null 时，of(T value) 会报 NullPointerException 异常；ofNullable(T value) 不会 throw Exception，ofNullable(T value) 直接返回一个 EMPTY 对象。

在实际应用过程中不想隐藏 NullPointerException，这种情况下就用 of(T value) 方法，但是实际上不想隐藏 NPE 的情况少之又少。

get()

    /**
     * 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;
    }

获取 value。

isPresent()

    /**
     * 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;
    }

判断value值是否为空。

ifPresent(Consumer<? super T> consumer)

    /**
     * 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<? super T> consumer) {
        if (value != null)
            consumer.accept(value);
    }

相比较于 isPresent()，ifPresent(Consumer<? super T> consumer) 在 value 值不为空时，做一些操作。

filter(Predicate<? super T> predicate)

    /**
     * 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<T> filter(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate);
        if (!isPresent())
            return this;
        else
            return predicate.test(value) ? this : empty();
    }

filter 方法接受一个 Predicate 来对 Optional 中包含的值进行过滤，如果包含的值满足条件，那么还是返回这个 Optional；否则返回 Optional.empty。

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 {@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<FileInputStream>}:
     *
     * <pre>{@code
     *     Optional<FileInputStream> fis =
     *         names.stream().filter(name -> !isProcessedYet(name))
     *                       .findFirst()
     *                       .map(name -> new FileInputStream(name));
     * }</pre>
     *
     * Here, {@code findFirst} returns an {@code Optional<String>}, and then
     * {@code map} returns an {@code Optional<FileInputStream>} for the desired
     * file if one exists.
     *
     * @param <U> 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<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent())
            return empty();
        else {
            return Optional.ofNullable(mapper.apply(value));
        }
    }

如果 User 类的结构是：

public class User {
    private String name;
    public String getName() {
        return name;
    }
}

那么获取 name 的写法如下：

String name = Optional.ofNullable(user).map(u-> u.getName()).get();

flatMap(Function<? super T, Optional<U>> mapper)

    /**
     * If a value is present, apply the provided {@code Optional}-bearing
     * mapping function to it, return that result, otherwise return an empty
     * {@code Optional}.  This method is similar to {@link #map(Function)},
     * but the provided mapper is one whose result is already an {@code Optional},
     * and if invoked, {@code flatMap} does not wrap it with an additional
     * {@code Optional}.
     *
     * @param <U> The type parameter to the {@code Optional} returned by
     * @param mapper a mapping function to apply to the value, if present
     *           the mapping function
     * @return the result of applying an {@code Optional}-bearing 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 or returns
     * a null result
     */
    public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent())
            return empty();
        else {
            return Objects.requireNonNull(mapper.apply(value));
        }
    }

相较于 map(Function<? super T, ? extends U> mapper) 而言，flatMap(Function<? super T, Optional<U>> mapper) 的唯一区别就是入参类型不同。

如果 User 类的结构是：

public class User {
    private String name;
    public Optional<String> getName() {
        return Optional.ofNullable(name);
    }
}

那么获取 name 的写法如下：

String name = Optional.ofNullable(user).flatMap(u-> u.getName()).get();

orElse(T other)

    /**
     * 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;
    }

 在构造函数传入的 value 值为 null 时，进行调用的。相当于 value 值为 null 时，给予一个默认值。用法如下：

public User createUser(){
    User user = new User();
    user.setName("tyy");
    return user;
}

Optional.ofNullable(user).orElse(createUser());

orElseGet(Supplier<? extends T> other)

    /**
     * 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<? extends T> other) {
        return value != null ? value : other.get();
    }

在构造函数传入的 value 值为 null 时，进行调用的。相当于 value 值为 null 时，给予一个默认值。用法如下：

Optional.ofNullable(user).orElseGet(() -> createUser());

orElseGet(Supplier<? extends T> other) 和 orElse(T other) 的区别在于当 user 值不为 null 时，orElse() 依然会执行 createUser() 方法，而 orElseGet() 并不会执行 createUser() 方法。

orElseThrow(Supplier<? extends X> exceptionSupplier)

    /**
     * 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 <X> 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 <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
        if (value != null) {
            return value;
        } else {
            throw exceptionSupplier.get();
        }
    }

当 value 值为 null 时，直接抛一个异常出去，用法如下：

Optional.ofNullable(user).orElseThrow(()->new Exception("用户不存在"));

使用示例

public class User {
   private String name;
   private City address;

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public City getAddress() {
      return address;
   }

   public void setAddress(City address) {
      this.address = address;
   }
}

public class City {
   private String city;

   public String getCity() {
      return city;
   }

   public void setCity(String city) {
      this.city = city;
   }
}

例一： 

以前的写法：

    public String getCity(User user)  throws Exception{
        if(user!=null){
            if(user.getAddress()!=null){
                Address address = user.getAddress();
                if(address.getCity()!=null){
                    return address.getCity();
                }
            }
        }
        throw new Excpetion("取值错误"); 
    }

 使用 Optional 的写法：

public String getCity(User user) throws Exception{
    return Optional.ofNullable(user)
                   .map(u-> u.getAddress())
                   .map(a->a.getCity())
                   .orElseThrow(()->new Exception("取值错误"));
}

例二：

以前的写法：

if(user!=null){
    dosomething(user);
}

使用 Optional 的写法：

Optional.ofNullable(user)
    .ifPresent(u->{
        dosomething(u);
});

例三：

以前的写法：

public User getUser(User user) throws Exception{
    if(user!=null){
        String name = user.getName();
        if("tyy".equals(name)){
            return user;
        }
    }else{
        user = new User();
        user.setName("tyy");
        return user;
    }
}

使用 Optional 的写法：

public User getUser(User user) {
    return Optional.ofNullable(user)
                   .filter(u->"tyy".equals(u.getName()))
                   .orElseGet(()-> {
                        User user1 = new User();
                        user1.setName("tyy");
                        return user1;
                   });
}