[英]Sort Guava Multimap by number of values
如果我有一個 Guava Multimap,我將如何根據給定鍵的值數量對條目進行排序?
例如:
Multimap<String, String> multiMap = ArrayListMultimap.create();
multiMap.put("foo", "1");
multiMap.put("bar", "2");
multiMap.put("bar", "3");
multiMap.put("bar", "99");
鑒於此,當迭代 multiMap 時,我如何讓“bar”條目先出現(因為“bar”有 3 個值,而“foo”只有 1 個值)?
提取列表中的條目,然后對列表進行排序:
List<Map.Entry<String, String>> entries = new ArrayList<Map.Entry<String, String>>(map.entries());
Collections.sort(entries, new Comparator<Map.Entry<String, String>>() {
@Override
public int compare(Map.Entry<String, String> e1, Map.Entry<String, String> e2) {
return Ints.compare(map.get(e2.getKey()).size(), map.get(e1.getKey()).size());
}
});
然后遍歷條目。
編輯 :
如果您實際上想要迭代內部映射( Entry<String, Collection<String>>
)的Entry<String, Collection<String>>
,則執行以下操作:
List<Map.Entry<String, Collection<String>>> entries =
new ArrayList<Map.Entry<String, Collection<String>>>(map.asMap().entrySet());
Collections.sort(entries, new Comparator<Map.Entry<String, Collection<String>>>() {
@Override
public int compare(Map.Entry<String, Collection<String>> e1,
Map.Entry<String, Collection<String>> e2) {
return Ints.compare(e2.getValue().size(), e1.getValue().size());
}
});
// and now iterate
for (Map.Entry<String, Collection<String>> entry : entries) {
System.out.println("Key = " + entry.getKey());
for (String value : entry.getValue()) {
System.out.println(" Value = " + value);
}
}
我會使用 Multimap 的鍵 Multiset條目,按降序對它們進行排序(一旦將issue 356 中描述的功能添加到 Guava 中會更容易),並通過迭代排序的鍵來構建新的 Multimap,從原始 Multimap 獲取值:
/**
* @return a {@link Multimap} whose entries are sorted by descending frequency
*/
public Multimap<String, String> sortedByDescendingFrequency(Multimap<String, String> multimap) {
// ImmutableMultimap.Builder preserves key/value order
ImmutableMultimap.Builder<String, String> result = ImmutableMultimap.builder();
for (Multiset.Entry<String> entry : DESCENDING_COUNT_ORDERING.sortedCopy(multimap.keys().entrySet())) {
result.putAll(entry.getElement(), multimap.get(entry.getElement()));
}
return result.build();
}
/**
* An {@link Ordering} that orders {@link Multiset.Entry Multiset entries} by ascending count.
*/
private static final Ordering<Multiset.Entry<?>> ASCENDING_COUNT_ORDERING = new Ordering<Multiset.Entry<?>>() {
@Override
public int compare(Multiset.Entry<?> left, Multiset.Entry<?> right) {
return Ints.compare(left.getCount(), right.getCount());
}
};
/**
* An {@link Ordering} that orders {@link Multiset.Entry Multiset entries} by descending count.
*/
private static final Ordering<Multiset.Entry<?>> DESCENDING_COUNT_ORDERING = ASCENDING_COUNT_ORDERING.reverse();
編輯:如果某些條目具有相同的頻率,這將不起作用(請參閱我的評論)
另一種方法,使用基於 Multimaps 鍵 Multiset 和ImmutableMultimap.Builder.orderKeysBy()的排序:
/**
* @return a {@link Multimap} whose entries are sorted by descending frequency
*/
public Multimap<String, String> sortedByDescendingFrequency(Multimap<String, String> multimap) {
return ImmutableMultimap.<String, String>builder()
.orderKeysBy(descendingCountOrdering(multimap.keys()))
.putAll(multimap)
.build();
}
private static Ordering<String> descendingCountOrdering(final Multiset<String> multiset) {
return new Ordering<String>() {
@Override
public int compare(String left, String right) {
return Ints.compare(multiset.count(left), multiset.count(right));
}
};
}
第二種方法較短,但我不喜歡 Ordering 具有狀態的事實(它取決於 Multimap 的鍵 Multiset 來比較鍵)。
使用 Java8 流:
ListMultimap<String, String> multiMap = ArrayListMultimap.create();
multiMap.put("foo", "f1");
multiMap.put("baz", "z1");
multiMap.put("baz", "z2");
multiMap.put("bar", "b1");
multiMap.put("bar", "b2");
multiMap.put("bar", "b3");
Multimaps.asMap(multiMap).entrySet().stream()
.sorted(Comparator.comparing(e -> -e.getValue().size()))
.forEach(e -> System.out.println(e.getKey() + " -> "
+ Arrays.toString(e.getValue().toArray())));
輸出:
bar -> [b1, b2, b3]
baz -> [z1, z2]
foo -> [f1]
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