还用 != null 判空？新姿势，值得学习！
来源：blog.csdn.net/zjhred/article/details/84976734
引言
在文章的开头，先说下NPE问题，NPE问题就是，我们在开发中经常碰到的NullPointerException.假设我们有两个类，他们的UML类图如下图所示
在这种情况下，有如下代码：
user.getAddress().getProvince();
这种写法，在user为null时，是有可能报NullPointerException异常的。为了解决这个问题，于是采用下面的写法：
if(user!=null){Address address = user.getAddress();if(address!=null){String province = address.getProvince();}}
这种写法是比较丑陋的，为了避免上述丑陋的写法，让丑陋的设计变得优雅。JAVA8提供了Optional类来优化这种写法，接下来的正文部分进行详细说明
API介绍
1、Optional(T value),empty(),of(T value),ofNullable(T value)
public static <T> Optional<T> of(T value) {return new Optional<>(value);}
public final class Optional<T> {//省略....private static final Optional<?> EMPTY = new Optional<>();private Optional() {this.value = null;}//省略...public static<T> Optional<T> empty() {("unchecked")Optional<T> t = (Optional<T>) EMPTY;return t;}}
public static <T> Optional<T> ofNullable(T value) {return value == null ? empty() : of(value);}
2、orElse(T other)，orElseGet(Supplier<? extends T> other)和orElseThrow(Supplier<? extends X> exceptionSupplier)
public void test() {User user = null;user = Optional.ofNullable(user).orElse(createUser());user = Optional.ofNullable(user).orElseGet(() -> createUser());}public User createUser(){User user = new User();user.setName("zhangsan");return user;}
User user = null;Optional.ofNullable(user).orElseThrow(()->new Exception("用户不存在"));
3、map(Function<? super T, ? extends U> mapper)和flatMap(Function<? super T, Optional> mapper)
public final class Optional<T> {//省略....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));}}//省略...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));}}}
public class User {private String name;public String getName() {return name;}}
String city = Optional.ofNullable(user).map(u-> u.getName()).get();
public class User {private String name;public Optional<String> getName() {return Optional.ofNullable(name);}}
String city = Optional.ofNullable(user).flatMap(u-> u.getName()).get();
4、isPresent()和ifPresent(Consumer<? super T> consumer)
public final class Optional<T> {//省略....public boolean isPresent() {return value != null;}//省略...public void ifPresent(Consumer<? super T> consumer) {if (value != null)consumer.accept(value);}}
if (user != null){// TODO: do something}
User user = Optional.ofNullable(user);if (Optional.isPresent()){// TODO: do something}
Optional.ofNullable(user).ifPresent(u->{// TODO: do something});
5、filter(Predicate<? super T> predicate)
public final class Optional<T> {//省略....Objects.requireNonNull(predicate);if (!isPresent())return this;elsereturn predicate.test(value) ? this : empty();}
Optional<User> user1 = Optional.ofNullable(user).filter(u -> u.getName().length()<6);
实战使用
例一
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");}
public String getCity(User user) throws Exception{return Optional.ofNullable(user).map(u-> u.getAddress()).map(a->a.getCity()).orElseThrow(()->new Exception("取指错误"));}
例二
if(user!=null){dosomething(user);}
Optional.ofNullable(user).ifPresent(u->{dosomething(u);});
例三
public User getUser(User user) throws Exception{if(user!=null){String name = user.getName();if("zhangsan".equals(name)){return user;}}else{user = new User();user.setName("zhangsan");return user;}}
public User getUser(User user) {return Optional.ofNullable(user).filter(u->"zhangsan".equals(u.getName())).orElseGet(()-> {User user1 = new User();user1.setName("zhangsan");return user1;});}
（完）
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