本文主要介绍Java 8的Optional的简单使用
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publicclassAddress {
private String province;
private String city;
}
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相关代码托管在java8_demo
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publicvoidemptyTest() {
// 声明一个空的Optional对象
Optional
nameOptional = Optional.empty();
// java.util.NoSuchElementException: No value present
nameOptional.get();
}
publicvoidofNullTest() {
// 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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Stream.map的定义：
Optional.map的定义： Optional map(Function 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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publicvoidmapTest() {
String province =
"Guangdong";
String city =
"Zhongshan";
Optional
addressOptional = Optional.of(new Address(province, city));
// mapper : Address -> String
// Optional-> Optional
Optional
assertTrue(stringOptional.isPresent());
assertEquals(city, stringOptional.get());
}
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Stream.flatMap的定义：
Optional.flatMap的定义： Optional flatMap(Function> mapper)
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publicvoidflatmapTest() {
String province =
"Guangdong";
String city =
"Zhongshan";
Optional
addressOptional = Optional.of(new Address(province, city));
// 对于 mapper 为 T -> Optional时，调用Optional.map，生成的是Optional
Optional
// 对于 mapper 为 T -> Optional时，调用Optional.map，生成的是Optional,被扁平化
Optional
assertTrue(stringOptional.isPresent());
assertEquals(city, stringOptional.get());
}
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publicvoiddereferenceTest() {
// get：最简单 + 最不安全
Address address = addressOptional.get();
assertNotNull(address);
address =
null;
Optional
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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publicvoidfilterTest() {
assertTrue(addressOptional.filter(address -> address.getCity().contains(
"Z")).isPresent());
}
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OptionalInt、OptionalLong、OptionalDouble
最多只有一个元素，并没有像StreamInt那样相对于Stream有性能优势
不支持map、flatmap、filter
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