Java可选，如果object不为null-返回方法结果，如果为null-返回默认值 [英] Java Optional if object is not null - returns the method result, if null - returns default value

问题描述

是否可以将此代码转换为Java 8可选单行表达式?

Is it possible to transform this code to a Java 8 Optional one-line expression?

long lastPollTime;
if (object != null) {
    lastPollTime = object.getTime();
} else {
    lastPollTime = 0;
}

即如果某个对象不为null，则需要调用一个对象方法并返回其结果，否则返回0. Optional.ofNullable().orElse()不适合，因为它返回相同类型的对象，但是我需要方法调用的结果或某些默认值.

i.e. if some object is not null, I need to call an object method and return its result, or else return 0. Optional.ofNullable().orElse() is not suitable, as it returns the object of the same type, but i need the result of the method call or some default value.

推荐答案

几种形式:

long lastPollTime = Optional.ofNullable(object).map(o -> o.getTime()).orElse(0L);

long lastPollTime = Optional.ofNullable(object).map(YouObjectClass::getTime).orElse(0L);

long lastPollTime = Optional.ofNullable(object).isPresent() ? object.getTime() : 0;

long lastPollTime = object != null ? object.getTime() : 0;

其中，最后一个不使用Optional(因此不能严格回答您的问题！)更易于阅读，运行时开销也较小，因此应优先使用.

Of these, the last one, which doesn't use Optional (and therefore doesn't strictly answer your question!) is simpler to read and has fewer runtime overheads, and so should be preferred.

可以说，如果您反转选项，它甚至更简单:

Arguably, it's even simpler if you reverse the options:

long lastPollTime = object == null ? 0 : object.getTime();

...尽管您可能更喜欢默认的默认设置-这是个人喜好的问题.

... although you might prefer to have the default last -- it's a matter of personal taste.

如果您确实不能使用三元运算符，并且您正在执行很多操作，则可以编写自己的实用程序方法:

If you really can't use ternary operators, and you're doing this a lot, you could write your own utility method:

public <T,U> U mapWithFallback(T obj, Function<T,U> function, U fallback) {
    if(obj == null) {
        return fallback;
    } else {
        return function.apply(obj);
    }
}

...可调用为:

long lastPollTime = mapWithFallback(object, o -> o.getTime(), 0);

---

...或使用以下方法完全嘲笑您的无三元检查:

---

... or make a complete mockery of your no-ternaries check using:

public <T,U> U ifElse( Supplier<Boolean> a, Supplier<U> ifTrue, Supplier<U> ifFalse) {
     if(a.get()) {
          return ifTrue.get();
     } else {
          return ifFalse.get();
     }
}

long lastPollTime = ifElse( () -> object == null, () -> object.getTime(), () -> 0);

---

完全避免使用空引用是一种更好的选择，因此不需要这种检查-例如，使用空对象模式.

...或编写返回 Optional 而不是可能为null的方法. Optional 是一个很好的类；用它.只是不要仅将某些内容转换为 Optional ，这样您就可以立即检查其是否为空.

... or by writing methods that return Optional rather than potential nulls. Optional is a great class; use it. Just don't convert something to Optional purely so you can immediately check whether it's empty.

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