java8中Optional的使用

防止NPE

1. 返回类型为基本数据类型，return包装数据类型的对象时，自动拆箱有可能产生NPE。 反例：public int f() { return Integer对象}， 如果为null，自动解箱抛NPE
2. 数据库的查询结果可能为null
3. 集合里的元素即使isNotEmpty，取出的数据元素也可能为null
4. 远程调用返回对象时，一律要求进行空指针判断，防止NPE
5. 对于Session中获取的数据，建议进行NPE检查，避免空指针
6. 级联调用obj.getA().getB().getC()；一连串调用，易产生NPE

public final class Optional {

    private static final Optional EMPTY = new Optional<>();

    private final T value;

    private Optional() {
        this.value = null;
    }
    
    private Optional(T value) {
        this.value = Objects.requireNonNull(value);
    }

    public static  Optional ofNullable(T value) {
        return value == null ? empty() : of(value);
    }
    
    public static  Optional of(T value) {
        return new Optional<>(value);
    }
    
    public static Optional empty() {
        @SuppressWarnings("unchecked")
        Optional t = (Optional) EMPTY;
        return t;
    }
    
    ... ...
}

ofNullable方法和of方法唯一区别就是当 value 为 null 时，ofNullable返回的是EMPTY，of 会抛出NullPointerException` 异常

// 源码
public void ifPresent(Consumer consumer) {
        if (value != null)
            consumer.accept(value);
    }

 public static void main(String[] args) {
        Zoo zoo = new Zoo();
        Dog dog = new Dog();
        dog.setAge(1);
        zoo.setDog(dog);
        Optional.of(zoo).ifPresent(System.out::println);
    }

// 源码 
public boolean isPresent() {
        return value != null;
}

Optional.of(zoo).filter(z -> z.getDog().getAge() > 0).ifPresent(System.out::println);

// 源码
public Optional map(Function mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent())
            return empty();
        else {
            return Optional.ofNullable(mapper.apply(value));
        }
    }

public static void main(String[] args) {
        UserRequest u1 = new UserRequest();
        u1.setUserName("u1");
        u1.setId(1L);
        Optional.of(u1).map(new Function() {
            @Override
            public Object apply(UserRequest userRequest) {
                return userRequest.getId();
            }
        }).ifPresent(System.out::println);

        Optional aLong = Optional.of(u1).map(UserRequest::getId);
        aLong.ifPresent(System.out::println);

    }

// 源码
public Optional flatMap(Function> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent())
            return empty();
        else {
            return Objects.requireNonNull(mapper.apply(value));
        }
    }

Optional.ofNullable(student).flatMap(u -> Optional.ofNullable(u.getAge())); 

public T orElse(T other) {    return value != null ? value : other;}

public static String getGender(Student student) {    if (null == student) {        return "Unkown";    }    return student.getGender();}

public static String getGender(Student student) {	return Optional.ofNullable(student).map(u -> u.getGender()).orElse("Unkown");}

// 源码  public T get() {    if (value == null) {        throw new NoSuchElementException("No value present");    }    return value;}   

// 源码public T orElseGet(Supplier other) {        return value != null ? value : other.get();    }

Optional.ofNullable(u1).map(u -> u.getId()).orElseGet(() -> "Unkown");   

public  T orElseThrow(Supplier exceptionSupplier) throws X {        if (value != null) {            return value;        } else {            throw exceptionSupplier.get();        }    }

Optional.ofNullable(student).map(u -> u.getGender()).orElseThrow(() -> new RuntimeException("Unkown"));