本文共 2161 字，大约阅读时间需要 7 分钟。

Java Optional?????

Java 8???Optional???????????null???????Optional?????????????????????????null????NullPointerException????????Optional??????????

1. Optional?????

Optional?????of()?ofNullable()???????of()?????????null???ofNullable()???????null?????????????

Optional.of("?null?");Optional.ofNullable(null);

2. Optional????????

Optional?????????Present?????Absent????????isPresent()??????true??????false?????????empty()???????????Optional???

Optional
   
     optional = Optional.ofNullable(null);if (optional.isPresent()) {    System.out.println("???");} else {    System.out.println("???");}
   

3. ????????

????????Optional???????????????orElse()????????????

String defaultValue = "???";String result = optional.orElse(defaultValue);

???????????????orElseThrow()????????Supplier???????????

try {    String value = optional.orElseThrow(() -> new IllegalArgumentException("???"));    System.out.println(value);} catch (IllegalArgumentException e) {    System.out.println(e.getMessage());}

4. ??ally??

ifPresent()????????????????Consumer?

optional.ifPresent(System.out::println);

???????????Consumer???????????

5. ??????

???????Optional????map()?flatMap()???????????Optional?????????????

optional.map(String::toUpperCase);optional.flatMap(String::isEmpty);

6. ????

?????????????

public class OptionalExample {    public static void main(String[] args) {        Optional
   
     optional = Optional.ofNullable(null);                // ???????        if (optional.isPresent()) {            System.out.println("???");        } else {            System.out.println("???");        }                // ?????        String value = optional.orElse("???");        System.out.println(value);                // ????        try {            String valueOrThrow = optional.orElseThrow(() -> new IllegalArgumentException("???"));            System.out.println(valueOrThrow);        } catch (IllegalArgumentException e) {            System.out.println(e.getMessage());        }                // ???????        optional.ifPresent(System.out::println);    }}
   

??

Optional?????????????null???????????????orElse?orElseThrow?ifPresent??????????????????????Optional??????????????????