英文:
Return the index of the first item in generics collections
问题
/*** 返回满足 aPredicate.test(o) 条件的第一个元素在 someCollection 中的索引，如果没有则返回 -1。*/public static <T> int find(Collection<T> someCollection, Predicate<T> aPredicate) {List<T> list = new ArrayList<>();for (Iterator<T> iterator = someCollection.iterator(); iterator.hasNext(); ) {T value = iterator.next();if (aPredicate.test(value)) {list.add(value);}}return list.indexOf(list.get(0)); // 或者直接返回 list.indexOf(list.get(0))}// 对于只适用于整数集合的情况，如何返回第一个元素的索引呢？
英文:
/*** Return the index of the first item in someCollection for which * aPredicate.test(o) is true, or -1.*/public static <T> int find(Collection<T> someCollection, Predicate<T> aPredicate) {List<T> list = new ArrayList<>();for (Iterator<T> iterator = someCollection.iterator(); iterator.hasNext(); ) {T value = iterator.next();if (aPredicate.test(value)) {list.add(value);}}return list[0]; // or return list.get(0)}
With the code above, I cannot use list[0] since it needs to be replaced with list.get(0), but this method is only applicable to the collection of Integers.
How can I return the index of the first element in such case?
答案1
得分: 1
你的函数目的是返回与给定的“Predicate”匹配的“Collection”中第一个元素的索引。因此，你应该存储一个List<int>而不是一个List<T>，如果要返回找到的第一个元素，根本没有存储列表的必要。因此，完全移除缓冲列表，在找到匹配的元素后立即返回。
public static int find(Collection<?> someCollection, Predicate<?> aPredicate) {int index = 0;for (Iterator<?> iterator = someCollection.iterator(); iterator.hasNext(); ) {Object value = iterator.next();if (aPredicate.test(value)) {return index;}index++;}return -1;}
英文:
The purpose of your function is to return the first index of the first element in a Collection that matches the given Predicate. As such, not only should you be storing a List<int> rather than a List<T>, there's no reason to be storing a list at all if the point is to return the first thing found. As such, remove the buffer list entirely and return as soon as you find a matching element.
public static <T> int find(Collection<T> someCollection, Predicate<T> aPredicate) {// Collections don't necessarily natively support indices, so you must// manually track the current indexint index = 0;for (Iterator<T> iterator = someCollection.iterator(); iterator.hasNext(); ) {T value = iterator.next();if (aPredicate.test(value)) {// A matching element was found, so there's no point continuing to loopreturn index;}index++;}// No element was found, so return the conventional -1return -1;}
答案2
得分: 0
你想要的不是存储 List<T>，而是一个 List<Integer>，基本上，它包含了你找到的元素的索引。你将不得不自己跟踪该索引，以及追踪调用了多少次 iterator.next()。
英文:
What you want isn't to store a List<T>, but a List<Integer>, essentially, containing the index of the elements you found. You will have to track that index yourself, but tracking how many calls to iterator.next() you have made.
答案3
得分: 0
让我们首先确保您的方法执行正确的操作：
public static <T> int find(Collection<T> someCollection, Predicate<T> aPredicate) {int i = -1;for (Iterator<T> iterator = someCollection.iterator(); iterator.hasNext();) {++i;T value = iterator.next();if (aPredicate.test(value)) {return i;}}return -1;}
让我们看看它是否有效：
public static void main(String[] args) {List<Integer> list = List.of(1, 2, 3);System.out.println(find(list, x -> x == 4)); // -1System.out.println(find(list, x -> x == 2)); // 1}
虽然它能正常工作，但它是否正确呢？...
public static void main(String[] args) {Set<String> set = Set.of("abc", "ade");System.out.println(find(set, "abc"::equals));}
多次运行此代码（使用 Java 9），您会惊讶地发现索引会发生变化。Set 没有任何顺序，而在 Java 9 中添加的不可变集合在创建时会进行一些内部随机化，因此结果可能会完全不正确。想要更广泛的了解，可以查看这里。
您可能会认为 Java 有一个通用的接口，用于确定特定集合是否有序，但实际上并没有。因此，除了重新考虑您想要做的事情之外，您真的无法做太多事情，也许会有所帮助。
英文:
Let's first make sure your method does the correct thing:
public static <T> int find(Collection<T> someCollection, Predicate<T> aPredicate) {int i = -1;for (Iterator<T> iterator = someCollection.iterator(); iterator.hasNext();) {++i;T value = iterator.next();if (aPredicate.test(value)) {return i;}}return -1;}
Let's see if it works:
public static void main(String[] args) {List<Integer> list = List.of(1, 2, 3);System.out.println(find(list, x -> x == 4)); // -1System.out.println(find(list, x -> x == 2)); // 1}
It does, but is it correct?...
public static void main(String[] args) {Set<String> set = Set.of("abc", "ade");System.out.println(find(set, "abc"::equals));}
Run this a few times (with java-9) and be surprised how the index will change. A Set does not have any order and immutable collections added in java-9 do some internal randomization when they are first created, so the results might get entirely incorrect. For a broader read, look here.
You could think that java has a common interface for when a certain Collection would be ordered or not, but it does not. So you can't really do much, other then rethink what you want to do, may be.
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论