java - Iterators.filter 与 for-each

Question

也许这是一个显而易见或愚蠢的问题，所以我提前表示歉意。

假设我有以下两种使用简单条件迭代集合的方法:

1

Predicate<SomeObject> pred1 = new Predicate<SomeObject> () {... some condition_1 ...};
Predicate<SomeObject> pred2 = new Predicate<SomeObject> () {... some condition_2 ...};
Iterator<SomeObject> newIterator = Iterators.filter(iterator, pred1);
... do something like Sets.newHashSet(newIterator) ...
newIterator = Iterators.filter(iterator, pred2);
... do something like Sets.newHashSet(newIterator) ...

2

while(iterator.hasNext()) {
 SomeObject someObject = iterator.next();
   if (condition_1) {
    ...do something like set.add(someObject)...
   } else if (condition_2) {
    ...do something like set.add(someObject)...
   }
}

我说得对吗，#2 比 #1 更快？

感谢您的回答!

Answer 1

命令式版本会更快，但是快多少很难确定。在我的测试中，一旦热了，两个版本都会在 1.1 微秒内返回。命令式版本方法在 95% 的情况下运行速度超过 5.15 微秒(随机执行运行 1,000,000 次)，而 FluentIterable 版本在 95% 的情况下运行速度超过 6.6 微秒。

Guava 可以非常快，特别是如果可以静态定义谓词和函数。

在下面的示例中，请注意谓词组合和 FluentIterable 的用法。

FluentIterable 特别有吸引力，因为只需进行最少的修改即可用 Java 8 流替换用法。

public static class SomeObject {

    private final String someString;

    public SomeObject(final String someString) {
        this.someString = someString;
    }

    public String getSomeString() {
        return this.someString;
    }

    @Override
    public String toString() {
        return this.someString;
    }
}

public static final Predicate<SomeObject> CONTAINS_A_FILTER = new Predicate<GuavaExample.SomeObject>() {

    @Override
    public boolean apply(final SomeObject someObject) {

        return someObject.getSomeString()
            .contains("A");
    }
};

public static final Predicate<SomeObject> CONTAINS_B_FILTER = new Predicate<GuavaExample.SomeObject>() {

    @Override
    public boolean apply(final SomeObject someObject) {

        return someObject.getSomeString()
            .contains("B");
    }
};

public static final Predicate<SomeObject> CONTAINS_A_AND_B_FILTER = Predicates.and(CONTAINS_A_FILTER,
        CONTAINS_B_FILTER);

public static final Predicate<SomeObject> CONTAINS_A_OR_B_FILTER = Predicates.or(CONTAINS_A_FILTER,
        CONTAINS_B_FILTER);

public static void main(final String... args) {

   final List<SomeObject> someObjects = ImmutableList.of(new SomeObject("A"), new SomeObject("AB"),
            new SomeObject("B"), new SomeObject("C"));

    final Set<SomeObject> containsAandB = FluentIterable.from(someObjects)
            .filter(CONTAINS_A_AND_B_FILTER)
            .toSet();

    // prints [AB]
    System.out.println(containsAandB);

    final Set<SomeObject> containsAOrB = FluentIterable.from(someObjects)
            .filter(CONTAINS_A_OR_B_FILTER)
            .toSet();

    // prints [A, AB, B]
    System.out.println(containsAOrB);
}