Optional类如何解决空指针异常(2)

Optional类如何解决空指针异常

7. 过滤和映射操作

Optional类还提供了filter(Predicate)和map(Function)方法，用于对Optional对象中保存的值进行过滤和映射

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<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

8. flatMap操作

flatMap()方法与map()相似，不同之处在于flatMap()的mapping函数返回值必须是Optional类型

Optional<Integer> optional1 = Optional.ofNullable(1);
Optional<Optional<String>> str1Optional = optional1.map(a -> Optional.of("key" + a));
Optional<String> str2Optional = optional1.flatMap(a -> Optional.of("key" + a));
System.out.println(str1Optional.get().get()); // key1
System.out.println(str2Optional.get()); // key1

9. Optional类的方法总结

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