操作集合时，查找列表中的元素是常见操作，本文记录两个平常使用的List查找工具方法。
使用示例
@Test
public void testPositionAndFind() {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
//查找list中2的下标，不存在则为-1
Integer twoIndex = CollectionUtil.position(list, 2).orElse(-1);
//查找list中index为3（包含）到5（不包含）之间是否存在0
boolean zeroPresent = CollectionUtil.position(list, 0, 3, 5).isPresent();
//同上
zeroPresent = CollectionUtil.find(list, 0, 3,5 ).isPresent();
//查找list中第一个偶数，不存在就是-1
Integer evenNum = CollectionUtil.findIf(list, i -> i % 2 == 0).orElse(-1);
assertEquals((int) CollectionUtil.position(list, 1).get(), 0);
assertEquals((int) CollectionUtil.position(list, 2).get(), 1);
assertEquals((int) CollectionUtil.position(list, 20).orElse(-1), -1);
assertTrue(CollectionUtil.position(list, 2).isPresent());
assertFalse(CollectionUtil.position(list, 20).isPresent());
assertTrue(CollectionUtil.position(list, 2, 1).isPresent());
assertFalse(CollectionUtil.position(list, 2, 3).isPresent());
assertTrue(CollectionUtil.position(list, 6, 0, 6).isPresent());
assertFalse(CollectionUtil.position(list, 6, 0, 5).isPresent());
assertEquals((int) CollectionUtil.positionIf(list, i -> Objects.equals(i, 1)).get(), 0);
assertEquals((int) CollectionUtil.positionIf(list, i -> Objects.equals(i, 2)).get(), 1);
assertEquals((int) CollectionUtil.positionIf(list, i -> Objects.equals(i, 20)).orElse(-1), -1);
assertTrue(CollectionUtil.positionIf(list, i -> Objects.equals(i, 2)).isPresent());
assertFalse(CollectionUtil.positionIf(list, i -> Objects.equals(i, 20)).isPresent());
assertTrue(CollectionUtil.positionIf(list, i -> Objects.equals(i, 2), 1).isPresent());
assertFalse(CollectionUtil.positionIf(list, i -> Objects.equals(i, 2), 3).isPresent());
assertTrue(CollectionUtil.positionIf(list, i -> Objects.equals(i, 6), 0, 6).isPresent());
assertFalse(CollectionUtil.positionIf(list, i -> Objects.equals(i, 6), 0, 5).isPresent());
assertEquals((int) CollectionUtil.find(list, 1).get(), 1);
assertEquals((int) CollectionUtil.find(list, 2).get(), 2);
assertEquals((int) CollectionUtil.find(list, 20).orElse(-1), -1);
assertTrue(CollectionUtil.find(list, 2).isPresent());
assertFalse(CollectionUtil.find(list, 20).isPresent());
assertTrue(CollectionUtil.find(list, 2, 1).isPresent());
assertFalse(CollectionUtil.find(list, 2, 3).isPresent());
assertTrue(CollectionUtil.find(list, 6, 0, 6).isPresent());
assertFalse(CollectionUtil.find(list, 6, 0, 5).isPresent());
assertEquals((int) CollectionUtil.findIf(list, i -> Objects.equals(i, 1)).get(), 1);
assertEquals((int) CollectionUtil.findIf(list, i -> Objects.equals(i, 2)).get(), 2);
assertEquals((int) CollectionUtil.findIf(list, i -> Objects.equals(i, 20)).orElse(-1), -1);
assertTrue(CollectionUtil.findIf(list, i -> Objects.equals(i, 2)).isPresent());
assertFalse(CollectionUtil.findIf(list, i -> Objects.equals(i, 20)).isPresent());
assertTrue(CollectionUtil.findIf(list, i -> Objects.equals(i, 2), 1).isPresent());
assertFalse(CollectionUtil.findIf(list, i -> Objects.equals(i, 2), 3).isPresent());
assertTrue(CollectionUtil.findIf(list, i -> Objects.equals(i, 6), 0, 6).isPresent());
assertFalse(CollectionUtil.findIf(list, i -> Objects.equals(i, 6), 0, 5).isPresent());
}
API介绍
-
<T> Optional<T> find(List<T> list, T item):查找
list中和itemequals的元素，不存在就返回Optional.empty(); -
<T> Optional<T> find(List<T> list, T item, int start):同上，从下标为
start的位置开始（包含start）; -
<T> Optional<T> find(List<T> list, T item, int start, int end):同上，从下标为
start的位置开始（包含start），到下标为end的位置结束（不包含end）; -
<T> Optional<T> findIf(List<T> list, Predicate<? super T> predicate):查找
list中满足predicate的元素，不存在就返回Optional.empty(); -
<T> Optional<T> findIf(List<T> list, Predicate<? super T> predicate, int start):同上，从下标为
start的位置开始（包含start）; -
<T> Optional<T> findIf(List<T> list, Predicate<? super T> predicate, int start, int end):同上，从下标为
start的位置开始（包含start），到下标为end的位置结束（不包含end）; -
<T> Optional<Integer> position(List<T> list, T item):查找
list中和itemequals的元素的下标，不存在就返回Optional.empty(); -
<T> Optional<Integer> position(List<T> list, T item, int start):同上，从下标为
start的位置开始（包含start）; -
<T> Optional<Integer> position(List<T> list, T item, int start, int end):同上，从下标为
start的位置开始（包含start），到下标为end的位置结束（不包含end）; -
<T> Optional<Integer> positionIf(List<T> list, Predicate<? super T> predicate):查找
list中满足predicate的元素的下标，不存在就返回Optional.empty(); -
<T> Optional<Integer> positionIf(List<T> list, Predicate<? super T> predicate, int start):同上，从下标为
start的位置开始（包含start）; -
<T> Optional<Integer> positionIf(List<T> list, Predicate<? super T> predicate, int start, int end):同上，从下标为
start的位置开始（包含start），到下标为end的位置结束（不包含end）;
完整实现
public CollectionUtil{
public static <T> Optional<T> find(List<T> list, T item) {
return find(list, item, 0);
}
public static <T> Optional<T> find(List<T> list, T item, int start) {
return find(list, item, start, list.size());
}
public static <T> Optional<T> find(List<T> list, T item, int start, int end) {
return findIf(list, t -> Objects.equals(t, item), start, end);
}
public static <T> Optional<T> findIf(List<T> list, Predicate<? super T> predicate) {
return findIf(list, predicate, 0);
}
public static <T> Optional<T> findIf(List<T> list, Predicate<? super T> predicate, int start) {
return findIf(list, predicate, start, list.size());
}
public static <T> Optional<T> findIf(List<T> list, Predicate<? super T> predicate, int start, int end) {
Iterator<T> iterator = list.iterator();
int index = 0;
while (iterator.hasNext()) {
T item = iterator.next();
if (index >= start) {
if (predicate.test(item)) {
return Optional.of(item);
}
}
index++;
if (index >= end) {
return Optional.empty();
}
}
return Optional.empty();
}
public static <T> Optional<Integer> position(List<T> list, T item) {
return position(list, item, 0);
}
public static <T> Optional<Integer> position(List<T> list, T item, int start) {
return position(list, item, start, list.size());
}
public static <T> Optional<Integer> position(List<T> list, T item, int start, int end) {
return positionIf(list, t -> Objects.equals(t, item), start, end);
}
public static <T> Optional<Integer> positionIf(List<T> list, Predicate<? super T> predicate) {
return positionIf(list, predicate, 0);
}
public static <T> Optional<Integer> positionIf(List<T> list, Predicate<? super T> predicate, int start) {
return positionIf(list, predicate, start, list.size());
}
public static <T> Optional<Integer> positionIf(List<T> list, Predicate<? super T> predicate, int start, int end) {
Iterator<T> iterator = list.iterator();
int index = 0;
while (iterator.hasNext()) {
T item = iterator.next();
if (index >= start) {
if (predicate.test(item)) {
return Optional.of(index);
}
}
index++;
if (index >= end) {
return Optional.empty();
}
}
return Optional.empty();
}
}