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<Integer> optional1 = Optional.of(1);
- // 参数可以是null
- Optional<Integer> optional2 = Optional.ofNullable(null);
- // 参数可以是非null
- Optional<Integer> optional3 = Optional.ofNullable(2);
Optional.empty():所有null包装成的Optional对象:
- Optional<Integer> optional1 = Optional.ofNullable(null);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- System.out.println(optional1 == optional2);// true
- System.out.println(optional1 == Optional.<Integer>empty());// true
- Object o1 = Optional.<Integer>empty();
- Object o2 = Optional.<String>empty();
- System.out.println(o1 == o2);// true
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- // isPresent判断值是否存在
- System.out.println(optional1.isPresent() == true);
- System.out.println(optional2.isPresent() == false);
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- // 如果不是null,调用Consumer
- optional1.ifPresent(new Consumer<Integer>() {
- @Override
- public void accept(Integer t) {
- System.out.println("value is " + t);
- }
- });
- // null,不调用Consumer
- optional2.ifPresent(new Consumer<Integer>() {
- @Override
- public void accept(Integer t) {
- System.out.println("value is " + t);
- }
- });
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- // orElse
- System.out.println(optional1.orElse(1000) == 1);// true
- System.out.println(optional2.orElse(1000) == 1000);// true
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- System.out.println(optional1.orElseGet(() -> {
- return 1000;
- }) == 1);//true
- System.out.println(optional2.orElseGet(() -> {
- return 1000;
- }) == 1000);//true
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- optional1.orElseThrow(()->{throw new IllegalStateException();});
- try
- {
- // 抛出异常
- optional2.orElseThrow(()->{throw new IllegalStateException();});
- }
- catch(IllegalStateException e )
- {
- e.printStackTrace();
- }
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- Optional<Integer> filter1 = optional1.filter((a) -> a == null);
- Optional<Integer> filter2 = optional1.filter((a) -> a == 1);
- Optional<Integer> filter3 = optional2.filter((a) -> a == null);
- System.out.println(filter1.isPresent());// false
- System.out.println(filter2.isPresent());// true
- System.out.println(filter2.get().intValue() == 1);// true
- System.out.println(filter3.isPresent());// false
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Integer> optional2 = Optional.ofNullable(null);
- Optional<String> str1Optional = optional1.map((a) -> "key" + a);
- Optional<String> str2Optional = optional2.map((a) -> "key" + a);
- System.out.println(str1Optional.get());// key1
- System.out.println(str2Optional.isPresent());// false
- Optional<Integer> optional1 = Optional.ofNullable(1);
- Optional<Optional<String>> str1Optional = optional1.map((a) -> {
- return Optional.<String>of("key" + a);
- });
- Optional<String> str2Optional = optional1.flatMap((a) -> {
- return Optional.<String>of("key" + a);
- });
- System.out.println(str1Optional.get().get());// key1
- System.out.println(str2Optional.get());// key1