Java 8——Optional
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2022-03-10 20:09:50
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本文主要介绍Java 8的 Optional 的简单使用
Address
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public class Address {
private String province;
private String city;
}
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of + ofNullable
相关代码托管在java8_demo
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public void emptyTest() {
// 声明一个空的Optional对象
Optional<Address> nameOptional = Optional.empty();
// java.util.NoSuchElementException: No value present
nameOptional.get();
}
public void ofNullTest() {
// 依据实例创建Optional对象
Optional.of(new Address("Guangdong", "Zhongshan"));
// Optional.ofNullable(null)返回Optional.empty()
assertEquals(Optional.empty(), Optional.ofNullable(null));
// java.lang.NullPointerException
Optional.of(null);
}
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map
Stream.map的定义：<R> Stream<R> map(Function<? super T, ? extends R> mapper)Optional.map的定义：<U> Optional<U> map(Function<? super T, ? extends U> mapper)上面两者定义非常类似，
Optional可看成最多包含一个元素的Stream
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public void mapTest() {
String province = "Guangdong";
String city = "Zhongshan";
Optional<Address> addressOptional = Optional.of(new Address(province, city));
// mapper : Address -> String
// Optional<Address> -> Optional<String>
Optional<String> stringOptional = addressOptional.map(Address::getCity);
assertTrue(stringOptional.isPresent());
assertEquals(city, stringOptional.get());
}
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flatMap
Stream.flatMap的定义：<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)Optional.flatMap的定义：<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper)
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public void flatmapTest() {
String province = "Guangdong";
String city = "Zhongshan";
Optional<Address> addressOptional = Optional.of(new Address(province, city));
// 对于 mapper 为 T -> Optional<U>时，调用Optional.map，生成的是Optional<Optional<U>>
Optional<Optional<String>> optionalOptional = addressOptional.map(address -> Optional.ofNullable(address.getCity()));
// 对于 mapper 为 T -> Optional<U>时，调用Optional.map，生成的是Optional<U>,被扁平化
Optional<String> stringOptional = addressOptional.flatMap(address -> Optional.ofNullable(address.getCity()));
assertTrue(stringOptional.isPresent());
assertEquals(city, stringOptional.get());
}
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解引用
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public void dereferenceTest() {
// get：最简单 + 最不安全
Address address = addressOptional.get();
assertNotNull(address);
address = null;
Optional<Address> emptyAddressOptional = Optional.ofNullable(address);
String defaultValue = "Unknown";
// orElse：设置默认值
Address elseAddress = emptyAddressOptional.orElse(new Address(defaultValue, defaultValue));
assertEquals(defaultValue, elseAddress.getProvince());
assertEquals(defaultValue, elseAddress.getCity());
// orElseGet：orElse的延迟调用版本
Address elseGetAddress = emptyAddressOptional.orElseGet(Address::new);
assertNull(elseGetAddress.getProvince());
assertNull(elseGetAddress.getCity());
// ifPresent：存在值则运行consumer，否则不进行任何操作
emptyAddressOptional.ifPresent(System.out::println);
// orElseThrow：不存在时，抛出异常
emptyAddressOptional.orElseThrow(UnsupportedOperationException::new);
}
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filter
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public void filterTest() {
assertTrue(addressOptional.filter(address -> address.getCity().contains("Z")).isPresent());
}
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基础类型 + Optional
OptionalInt、OptionalLong、OptionalDouble最多
只有一个元素，并没有像StreamInt那样相对于Stream<Integer>有性能优势不支持
map、flatmap、filter
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