JDK8新特性：使用Optional避免null导致的NullPointerException

NullPointerException

空指针异常是导致Java应⽤程序失败的最常见原因。以前，为了解决空指针异常，Google公司著名的Guava项⽬引⼊了Optional类，Guava通过使⽤检查空值的⽅式来防⽌代码污染，它⿎励程序员写更⼲净的代码。受到Google Guava的启发，Optional类已经成为Java 8类库的⼀部分。Optional实际上是个容器：它可以保存类型T的值，或者仅仅保存null。Optional提供很多有⽤的⽅法，这样我们就不⽤显式进⾏空值检测。

Optional.of()或者Optional.ofNullable()：创建Optional对象，差别在于of不允许参数是null，⽽ofNullable则⽆限制。// 参数不能是null

Optional optional1 = Optional.of(1);

// 参数可以是null

Optional optional2 = Optional.ofNullable(null);// 参数可以是⾮null

Optional optional3 = Optional.ofNullable(2);

Optional.empty()：所有null包装成的Optional对象：Optional optional1 = Optional.ofNullable(null);Optional optional2 = Optional.ofNullable(null);System.out.println(optional1 == optional2);// true

System.out.println(optional1 == Optional.empty());// trueObject o1 = Optional.empty();Object o2 = Optional.empty();System.out.println(o1 == o2);// true

isPresent()：判断值是否存在

Optional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);// isPresent判断值是否存在

System.out.println(optional1.isPresent() == true);System.out.println(optional2.isPresent() == false);

ifPresent(Consumer consumer)：如果option对象保存的值不是null，则调⽤consumer对象，否则不调⽤Optional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);// 如果不是null,调⽤Consumer

optional1.ifPresent(new Consumer() {@Override

public void accept(Integer t) {System.out.println(\"value is \" + t);}});

// null,不调⽤Consumer

optional2.ifPresent(new Consumer() {@Override

public void accept(Integer t) {System.out.println(\"value is \" + t);}});

orElse(value)：如果optional对象保存的值不是null，则返回原来的值，否则返回valueOptional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);// orElse

System.out.println(optional1.orElse(1000) == 1);// trueSystem.out.println(optional2.orElse(1000) == 1000);// true

orElseGet(Supplier supplier)：功能与orElse⼀样，只不过orElseGet参数是⼀个对象Optional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);System.out.println(optional1.orElseGet(() -> {return 1000;}) == 1);//true

System.out.println(optional2.orElseGet(() -> {return 1000;}) == 1000);//true

orElseThrow()：值不存在则抛出异常，存在则什么不做，有点类似Guava的PrecoditionsOptional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);optional1.orElseThrow(()->{throw new IllegalStateException();});try{

// 抛出异常

optional2.orElseThrow(()->{throw new IllegalStateException();});}

catch(IllegalStateException e ){

e.printStackTrace();}

filter(Predicate)：判断Optional对象中保存的值是否满⾜Predicate，并返回新的Optional。Optional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);Optional filter1 = optional1.filter((a) -> a == null);Optional filter2 = optional1.filter((a) -> a == 1);Optional filter3 = optional2.filter((a) -> a == null);System.out.println(filter1.isPresent());// falseSystem.out.println(filter2.isPresent());// true

System.out.println(filter2.get().intValue() == 1);// trueSystem.out.println(filter3.isPresent());// false

map(Function)：对Optional中保存的值进⾏函数运算，并返回新的Optional(可以是任何类型)Optional optional1 = Optional.ofNullable(1);Optional optional2 = Optional.ofNullable(null);Optional str1Optional = optional1.map((a) -> \"key\" + a);Optional str2Optional = optional2.map((a) -> \"key\" + a);System.out.println(str1Optional.get());// key1

System.out.println(str2Optional.isPresent());// false

flatMap()：功能与map()相似，差别请看如下代码。flatMap⽅法与map⽅法类似，区别在于mapping函数的返回值不同。map⽅法的mapping函数返回值可以是任何类型T，⽽flatMap⽅法的mapping函数必须是Optional。

Optional optional1 = Optional.ofNullable(1);

Optional> str1Optional = optional1.map((a) -> {return Optional.of(\"key\" + a);});

Optional str2Optional = optional1.flatMap((a) -> {return Optional.of(\"key\" + a);});

System.out.println(str1Optional.get().get());// key1System.out.println(str2Optional.get());// key1

--------------------- 作者：aitangyong

原⽂：https://blog.csdn.net/aitangyong/article/details/54564100 版权声明：本⽂为博主原创⽂章，转载请附上博⽂链接！