Java-8 stream expression to 'OR' several enum values together

问题

I am aggregating a bunch of enum values (different from the ordinal values) in a foreach loop.

   int output = 0;
   for (TestEnum testEnum: setOfEnums) {
        output |= testEnum.getValue();
   }

Is there a way to do this in streams API?

If I use a lambda like this in a Stream<TestEnum> :

   setOfEnums.stream().forEach(testEnum -> (output |= testEnum.getValue());

I get a compile time error that says, 'variable used in lambda should be effectively final'.

回答1:

Predicate represents a boolean valued function, you need to use reduce method of stream to aggregate bunch of enum values.

if we consider that you have HashSet as named SetOfEnums :

    //int initialValue = 0; //this is effectively final for next stream pipeline if you wont modify this value in that stream
    final int initialValue = 0;//final
    int output = SetOfEnums.stream()
                     .map(TestEnum::getValue)
                     .reduce(initialValue, (e1,e2)-> e1|e2);

回答2:

You nedd to reduce stream of enums like this:

int output = Arrays.stream(TestEnum.values()).mapToInt(TestEnum::getValue).reduce(0, (acc, value) -> acc | value);

回答3:

I like the recommendations to use reduction, but perhaps a more complete answer would illustrate why it is a good idea.

In a lambda expression, you can reference variables like output that are in scope where the lambda expression is defined, but you cannot modify the values. The reason for that is that, internally, the compiler must be able to implement your lambda, if it chooses to do so, by creating a new function with your lambda as its body. The compiler may choose to add parameters as needed so that all of the values used in this generated function are available in the parameter list. In your case, such a function would definitely have the lambda's explicit parameter, testEnum, but because you also reference the local variable output in the lambda body, it could add that as a second parameter to the generated function. Effectively, the compiler might generate this function from your lambda:

private void generatedFunction1(TestEnum testEnum, int output) {
    output |= testEnum.getValue();
}

As you can see, the output parameter is a copy of the output variable used by the caller, and the OR operation would only be applied to the copy. Since the original output variable wouldn't be modified, the language designers decided to prohibit modification of values passed implicitly to lambdas.

To get around the problem in the most direct way, setting aside for the moment that the use of reduction is a far better approach, you could wrap the output variable in a wrapper (e.g. an int[] array of size 1 or an AtomicInteger. The wrapper's reference would be passed by value to the generated function, and since you would now update the contents of output, not the value of output, output remains effectively final, so the compiler won't complain. For example:

AtomicInteger output = new AtomicInteger();
setOfEnums.stream().forEach(testEnum -> (output.set(output.get() | testEnum.getValue()));

or, since we're using AtomicInteger, we may as well make it thread-safe in case you later choose to use a parallel Stream,

AtomicInteger output = new AtomicInteger();
setOfEnums.stream().forEach(testEnum -> (output.getAndUpdate(prev -> prev | testEnum.getValue())));

Now that we've gone over an answer that most resembles what you asked about, we can talk about the superior solution of using reduction, that other answers have already recommended.

There are two kinds of reduction offered by Stream, stateless reduction (reduce(), and stateful reduction (collect()). To visualize the difference, consider a conveyer belt delivering hamburgers, and your goal is to collect all of the hamburger patties into one big hamburger. With stateful reduction, you would start with a new hamburger bun, and then collect the patty out of each hamburger as it arrives, and you add it to the stack of patties in the hamburger bun you set up to collect them. In stateless reduction, you start out with an empty hamburger bun (called the "identity", since that empty hamburger bun is what you end up with if the conveyer belt is empty), and as each hamburger arrives on the belt, you make a copy of the previous accumulated burger and add the patty from the new one that just arrived, discarding the previous accumulated burger.

The stateless reduction may seem like a huge waste, but there are cases when copying the accumulated value is very cheap. One such case is when accumulating primitive types -- primitive types are very cheap to copy, so stateless reduction is ideal when crunching primitives in applications such as summing, ORing, etc.

So, using stateless reduction, your example might become:

setOfEnums.stream()
    .mapToInt(TestEnum::getValue)    // or .mapToInt(testEnum -> testEnum.getValue())
    .reduce(0, (resultSoFar, testEnum) -> resultSoFar | testEnum);

Some points to ponder:

• Your original for loop is probably faster than using streams, except perhaps if your set is very large and you use parallel streams. Don't use streams for the sake of using streams. Use them if they make sense.
• In my first example, I showed the use of Stream.forEach(). If you ever find yourself creating a Stream and just calling forEach(), it is more efficient just to call forEach() on the collection directly.
• You didn't mention what kind of Set you are using, but I hope you are using EnumSet<TestEnum>. Because it is implemented as a bit field, It performs much better (O(1)) than any other kind of Set for all operations, even copying. EnumSet.noneOf(TestEnum.class) creates an empty Set, EnumSet.allOf(TestEnum.class) gives you a set of all enum values, etc.

来源：https://stackoverflow.com/questions/42552126/java-8-stream-expression-to-or-several-enum-values-together