Guava小功能收集

自定义过滤条件的集合

List<Integer> numbers = Lists.newArrayList(1, 2, 3, 6, 10, 34, 57, 89);
Predicate<Integer> acceptEven = new Predicate<Integer>() {
    @Override
    public boolean apply(Integer number) {
        return (number % 2) == 0;
    }
};
List<Integer> evenNumbers = Lists.newArrayList(Collections2.filter(numbers, acceptEven));
Integer found = Collections.binarySearch(evenNumbers, 57);
assertThat(found, lessThan(0));

从集合中剔除掉空值

List<String> withNulls = Lists.newArrayList("a", "bc", null, "def");
Iterable<String> withoutNuls = Iterables.filter(withNulls, Predicates.notNull());
assertTrue(Iterables.all(withoutNuls, Predicates.notNull()));

从集合中检查某一个条件

List<Integer> evenNumbers = Lists.newArrayList(2, 6, 8, 10, 34, 90);
Predicate<Integer> acceptEven = new Predicate<Integer>() {
    @Override
    public boolean apply(Integer number) {
        return (number % 2) == 0;
    }
};
assertTrue(Iterables.all(evenNumbers, acceptEven));

集合的反向过滤

List<Integer> evenNumbers = Lists.newArrayList(2, 6, 8, 10, 34, 90);
Predicate<Integer> acceptOdd = new Predicate<Integer>() {
    @Override
    public boolean apply(Integer number) {
        return (number % 2) != 0;
    }
};
assertTrue(Iterables.all(evenNumbers, Predicates.not(acceptOdd)));

集合元素类型转换

List<Integer> numbers = Lists.newArrayList(1, 2, 3);
List<String> asStrings = Lists.transform(numbers, Functions.toStringFunction());
assertThat(asStrings, contains("1", "2", "3"));

集合的排序

List<Integer> numbers = Arrays.asList(2, 1, 11, 100, 8, 14);
Ordering<Object> ordering = Ordering.natural().onResultOf(Functions.toStringFunction());
List<Integer> inAlphabeticalOrder = ordering.sortedCopy(numbers);
List<Integer> correctAlphabeticalOrder = Lists.newArrayList(1, 100, 11, 14, 2, 8);
assertThat(correctAlphabeticalOrder, equalTo(inAlphabeticalOrder));

集合的过滤与功能化

List<Integer> numbers = Arrays.asList(2, 1, 11, 100, 8, 14);
Predicate<Integer> acceptEvenNumber = new Predicate<Integer>() {
    @Override
    public boolean apply(Integer number) {
        return (number % 2) == 0;
    }
};
Function<Integer, Integer> powerOfTwo = new Function<Integer, Integer>() {
    @Override
    public Integer apply(Integer input) {
        return (int) Math.pow(input, 2);
    }
};
 
FluentIterable<Integer> powerOfTwoOnlyForEvenNumbers = 
FluentIterable.from(numbers).filter(acceptEvenNumber).transform(powerOfTwo);
assertThat(powerOfTwoOnlyForEvenNumbers, contains(4, 10000, 64, 196));

集合功能块的结合

List<Integer> numbers = Arrays.asList(2, 3);
Function<Integer, Integer> powerOfTwo = new Function<Integer, Integer>() {
    @Override
    public Integer apply(Integer input) {
        return (int) Math.pow(input, 2);
    }
};
List<Integer> result = Lists.transform(numbers, 
    Functions.compose(powerOfTwo, powerOfTwo));
assertThat(result, contains(16, 81));

集合通过过滤器创建功能块

List<Integer> numbers = Lists.newArrayList(1, 2, 3, 6);
Predicate<Integer> acceptEvenNumber = new Predicate<Integer>() {
    @Override
    public boolean apply(Integer number) {
        return (number % 2) == 0;
    }
};
Function<Integer, Boolean> isEventNumberFunction = Functions.forPredicate(acceptEvenNumber);
List<Boolean> areNumbersEven = Lists.transform(numbers, isEventNumberFunction);
 
assertThat(areNumbersEven, contains(false, true, false, true));

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