如何在
Java中执行以下
JavaScript代码？
var result = getA() || getB() || getC() || 'all of them were undefined!';
我想要做的是继续评估语句或方法,直到它得到一些而不是null.
解决方法
你可以为它创建一个方法.
public static <T> T coalesce(Supplier<T>... ts) {
return asList(ts)
.stream()
.map(t -> t.get())
.filter(t -> t != null)
.findFirst()
.orElse(null);
}
代码取自：http://benjiweber.co.uk/blog/2013/12/08/null-coalescing-in-java-8/
编辑如评论中所述.查找下面的小代码片如何使用它.使用Stream API比使用vargs作为方法参数有优势.如果方法返回的值是昂贵的,而不是由简单的getter返回,那么vargs解决方案将首先评估所有这些方法.
import static java.util.Arrays.asList;
import java.util.function.Supplier;
...
static class Person {
String name;
Person(String name) {
this.name = name;
}
public String name() {
System.out.println("name() called for = " + name);
return name;
}
}
public static <T> T coalesce(Supplier<T>... ts) {
System.out.println("called coalesce(Supplier<T>... ts)");
return asList(ts)
.stream()
.map(t -> t.get())
.filter(t -> t != null)
.findFirst()
.orElse(null);
}
public static <T> T coalesce(T... ts) {
System.out.println("called coalesce(T... ts)");
for (T t : ts) {
if (t != null) {
return t;
}
}
return null;
}
public static void main(String[] args) {
Person nobody = new Person(null);
Person john = new Person("John");
Person jane = new Person("Jane");
Person eve = new Person("Eve");
System.out.println("result Stream API: "
+ coalesce(nobody::name,john::name,jane::name,eve::name));
System.out.println();
System.out.println("result vargas : "
+ coalesce(nobody.name(),john.name(),jane.name(),eve.name()));
}
产量
called coalesce(Supplier<T>... ts) name() called for = null name() called for = John result Stream API: John name() called for = null name() called for = John name() called for = Jane name() called for = Eve called coalesce(T... ts) result vargas : John
如输出中所示.在Stream解决方案中,返回值的方法将在coalesce方法内进行求值.只有两个执行,因为第二个调用返回预期的非空值.在vargs解决方案中,在调用合并方法之前,将对所有返回值的方法进行求值.
