基本方法:
-
ofNullable()
为可能 null 的值创建一个 Optional 实例, 然后可以对该实例遍历/过滤, 判断是否存在,或者为空时执行.. -
ifPresent(...)
如果值存在则执行里面的方法
应用场景:
1> 默认值
传统方式
public static String getName(User u) {
if (u == null)
return "Unknown";
return u.name;
}
杜绝使用这种方式(不简洁)
public static String getName(User u) {
Optional<User> user = Optional.ofNullable(u);
if (!user.isPresent())
return "Unknown";
return user.get().name;
}
正确方式(链式调用):
public static String getName(User u) {
return Optional.ofNullable(u)
.map(user->user.name)
.orElse("Unknown");
//.orElseGet(() -> "john");
}
2>多重非空条件判断
传统方式
public static String getChampionName(Competition comp) throws IllegalArgumentException {
if (comp != null) {
CompResult result = comp.getResult();
if (result != null) {
User champion = result.getChampion();
if (champion != null) {
return champion.getName();
}
}
}
throw new IllegalArgumentException("The value of param comp isn't available.");
}
链式调用(map 遍历属性)
public static String getChampionName(Competition comp) throws IllegalArgumentException {
return Optional.ofNullable(comp)
.map(c->c.getResult())
.map(r->r.getChampion())
.map(u->u.getName())
.orElseThrow(()->new IllegalArgumentException("The value of param comp isn't available."));
}
3> 不为空才操作(单边判断)
string.ifPresent(System.out::println);
4> 指定条件过滤
public boolean priceIsInRange2(Modem modem2) {
return Optional.ofNullable(modem2)
.map(Modem::getPrice)
.filter(p -> p >= 10)
.isPresent();
}
5. filter 与 findFirst 结合
Optional<String> found = Stream.of(getEmpty(), getHello(), getBye())
.filter(Optional::isPresent)
.map(Optional::get)
.findFirst();
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