假设您有一个map<String, Object> myMap
。
给定表达式"some.string.*"
,我必须从myMap
中检索其键以该表达式开头的所有值。
我尽量避免使用for loop
s,因为myMap
会被赋予一组表达式,而不是只有一个,而且在性能方面,为每个表达式使用for loop
会变得很麻烦。
完成此操作的最快方法是什么?
发布于 2012-11-23 23:11:25
我最近写了一个MapFilter
就是为了满足这样的需要。你也可以过滤过滤后的地图,这让它变得非常有用。
如果您的表达式有像"some.byte“和"some.string”这样的公共根,那么首先按公共根过滤(“Some.在这种情况下)将为您节省大量时间。有关一些简单的示例,请参阅main
。
请注意,对过滤后的地图进行更改会更改底层地图。
public class MapFilter<T> implements Map<String, T> {
// The enclosed map -- could also be a MapFilter.
final private Map<String, T> map;
// Use a TreeMap for predictable iteration order.
// Store Map.Entry to reflect changes down into the underlying map.
// The Key is the shortened string. The entry.key is the full string.
final private Map<String, Map.Entry<String, T>> entries = new TreeMap<>();
// The prefix they are looking for in this map.
final private String prefix;
public MapFilter(Map<String, T> map, String prefix) {
// Store my backing map.
this.map = map;
// Record my prefix.
this.prefix = prefix;
// Build my entries.
rebuildEntries();
}
public MapFilter(Map<String, T> map) {
this(map, "");
}
private synchronized void rebuildEntries() {
// Start empty.
entries.clear();
// Build my entry set.
for (Map.Entry<String, T> e : map.entrySet()) {
String key = e.getKey();
// Retain each one that starts with the specified prefix.
if (key.startsWith(prefix)) {
// Key it on the remainder.
String k = key.substring(prefix.length());
// Entries k always contains the LAST occurrence if there are multiples.
entries.put(k, e);
}
}
}
@Override
public String toString() {
return "MapFilter(" + prefix + ") of " + map + " containing " + entrySet();
}
// Constructor from a properties file.
public MapFilter(Properties p, String prefix) {
// Properties extends HashTable<Object,Object> so it implements Map.
// I need Map<String,T> so I wrap it in a HashMap for simplicity.
// Java-8 breaks if we use diamond inference.
this(new HashMap<>((Map) p), prefix);
}
// Helper to fast filter the map.
public MapFilter<T> filter(String prefix) {
// Wrap me in a new filter.
return new MapFilter<>(this, prefix);
}
// Count my entries.
@Override
public int size() {
return entries.size();
}
// Are we empty.
@Override
public boolean isEmpty() {
return entries.isEmpty();
}
// Is this key in me?
@Override
public boolean containsKey(Object key) {
return entries.containsKey(key);
}
// Is this value in me.
@Override
public boolean containsValue(Object value) {
// Walk the values.
for (Map.Entry<String, T> e : entries.values()) {
if (value.equals(e.getValue())) {
// Its there!
return true;
}
}
return false;
}
// Get the referenced value - if present.
@Override
public T get(Object key) {
return get(key, null);
}
// Get the referenced value - if present.
public T get(Object key, T dflt) {
Map.Entry<String, T> e = entries.get((String) key);
return e != null ? e.getValue() : dflt;
}
// Add to the underlying map.
@Override
public T put(String key, T value) {
T old = null;
// Do I have an entry for it already?
Map.Entry<String, T> entry = entries.get(key);
// Was it already there?
if (entry != null) {
// Yes. Just update it.
old = entry.setValue(value);
} else {
// Add it to the map.
map.put(prefix + key, value);
// Rebuild.
rebuildEntries();
}
return old;
}
// Get rid of that one.
@Override
public T remove(Object key) {
// Do I have an entry for it?
Map.Entry<String, T> entry = entries.get((String) key);
if (entry != null) {
entries.remove(key);
// Change the underlying map.
return map.remove(prefix + key);
}
return null;
}
// Add all of them.
@Override
public void putAll(Map<? extends String, ? extends T> m) {
for (Map.Entry<? extends String, ? extends T> e : m.entrySet()) {
put(e.getKey(), e.getValue());
}
}
// Clear everything out.
@Override
public void clear() {
// Just remove mine.
// This does not clear the underlying map - perhaps it should remove the filtered entries.
for (String key : entries.keySet()) {
map.remove(prefix + key);
}
entries.clear();
}
@Override
public Set<String> keySet() {
return entries.keySet();
}
@Override
public Collection<T> values() {
// Roll them all out into a new ArrayList.
List<T> values = new ArrayList<>();
for (Map.Entry<String, T> v : entries.values()) {
values.add(v.getValue());
}
return values;
}
@Override
public Set<Map.Entry<String, T>> entrySet() {
// Roll them all out into a new TreeSet.
Set<Map.Entry<String, T>> entrySet = new TreeSet<>();
for (Map.Entry<String, Map.Entry<String, T>> v : entries.entrySet()) {
entrySet.add(new Entry<>(v));
}
return entrySet;
}
/**
* An entry.
*
* @param <T> The type of the value.
*/
private static class Entry<T> implements Map.Entry<String, T>, Comparable<Entry<T>> {
// Note that entry in the entry is an entry in the underlying map.
private final Map.Entry<String, Map.Entry<String, T>> entry;
Entry(Map.Entry<String, Map.Entry<String, T>> entry) {
this.entry = entry;
}
@Override
public String getKey() {
return entry.getKey();
}
@Override
public T getValue() {
// Remember that the value is the entry in the underlying map.
return entry.getValue().getValue();
}
@Override
public T setValue(T newValue) {
// Remember that the value is the entry in the underlying map.
return entry.getValue().setValue(newValue);
}
@Override
public boolean equals(Object o) {
if (!(o instanceof Entry)) {
return false;
}
Entry e = (Entry) o;
return getKey().equals(e.getKey()) && getValue().equals(e.getValue());
}
@Override
public int hashCode() {
return getKey().hashCode() ^ getValue().hashCode();
}
@Override
public String toString() {
return getKey() + "=" + getValue();
}
@Override
public int compareTo(Entry<T> o) {
return getKey().compareTo(o.getKey());
}
}
// Simple tests.
public static void main(String[] args) {
String[] samples = {
"Some.For.Me",
"Some.For.You",
"Some.More",
"Yet.More"};
Map map = new HashMap();
for (String s : samples) {
map.put(s, s);
}
Map all = new MapFilter(map);
Map some = new MapFilter(map, "Some.");
Map someFor = new MapFilter(some, "For.");
System.out.println("All: " + all);
System.out.println("Some: " + some);
System.out.println("Some.For: " + someFor);
Properties props = new Properties();
props.setProperty("namespace.prop1", "value1");
props.setProperty("namespace.prop2", "value2");
props.setProperty("namespace.iDontKnowThisNameAtCompileTime", "anothervalue");
props.setProperty("someStuff.morestuff", "stuff");
Map<String, String> filtered = new MapFilter(props, "namespace.");
System.out.println("namespace props " + filtered);
}
}
发布于 2019-05-30 03:24:34
删除所有不以所需前缀开头的键:
yourMap.keySet().removeIf(key -> !key.startsWith(keyPrefix));
发布于 2021-01-21 19:01:06
公认的答案在99%的情况下都有效,但关键在于细节。
具体地说,当映射的关键字以前缀开头,然后是Character.MAX_VALUE
,然后是其他任何内容时,接受的答案将不起作用。发表在被接受的答案上的评论会带来一些小的改进,但仍然不能涵盖所有情况。
下面的解决方案还使用NavigableMap来挑选一个给定关键字前缀的子映射。解决方案是subMapFrom()
方法,技巧是不撞/递增前缀的最后一个字符,而是不是MAX_VALUE
的最后一个字符,同时去掉所有尾随的MAX_VALUE
s。但是如果前缀是"ab“+ MAX_VALUE
,我们丢弃最后一个字符,而不是前面的字符,结果是"ac”。
import static java.lang.Character.MAX_VALUE;
public class App
{
public static void main(String[] args) {
NavigableMap<String, String> map = new TreeMap<>();
String[] keys = {
"a",
"b",
"b" + MAX_VALUE,
"b" + MAX_VALUE + "any",
"c"
};
// Populate map
Stream.of(keys).forEach(k -> map.put(k, ""));
// For each key that starts with 'b', find the sub map
Stream.of(keys).filter(s -> s.startsWith("b")).forEach(p -> {
System.out.println("Looking for sub map using prefix \"" + p + "\".");
// Always returns expected sub maps with no misses
// [b, b, bany], [b, bany] and [bany]
System.out.println("My solution: " +
subMapFrom(map, p).keySet());
// WRONG! Prefix "b" misses "bany"
System.out.println("SO answer: " +
map.subMap(p, true, p + MAX_VALUE, true).keySet());
// WRONG! Prefix "b" misses "b" and "bany"
System.out.println("SO comment: " +
map.subMap(p, true, tryIncrementLastChar(p), false).keySet());
System.out.println();
});
}
private static <V> NavigableMap<String, V> subMapFrom(
NavigableMap<String, V> map, String keyPrefix)
{
final String fromKey = keyPrefix, toKey; // undefined
// Alias
String p = keyPrefix;
if (p.isEmpty()) {
// No need for a sub map
return map;
}
// ("ab" + MAX_VALUE + MAX_VALUE + ...) returns index 1
final int i = lastIndexOfNonMaxChar(p);
if (i == -1) {
// Prefix is all MAX_VALUE through and through, so grab rest of map
return map.tailMap(p, true);
}
if (i < p.length() - 1) {
// Target char for bumping is not last char; cut out the residue
// ("ab" + MAX_VALUE + MAX_VALUE + ...) becomes "ab"
p = p.substring(0, i + 1);
}
toKey = bumpChar(p, i);
return map.subMap(fromKey, true, toKey, false);
}
private static int lastIndexOfNonMaxChar(String str) {
int i = str.length();
// Walk backwards, while we have a valid index
while (--i >= 0) {
if (str.charAt(i) < MAX_VALUE) {
return i;
}
}
return -1;
}
private static String bumpChar(String str, int pos) {
assert !str.isEmpty();
assert pos >= 0 && pos < str.length();
final char c = str.charAt(pos);
assert c < MAX_VALUE;
StringBuilder b = new StringBuilder(str);
b.setCharAt(pos, (char) (c + 1));
return b.toString();
}
private static String tryIncrementLastChar(String p) {
char l = p.charAt(p.length() - 1);
return l == MAX_VALUE ?
// Last character already max, do nothing
p :
// Bump last character
p.substring(0, p.length() - 1) + ++l;
}
}
输出:
Looking for sub map using prefix "b".
My solution: [b, b, bany]
SO answer: [b, b]
SO comment: [b, b, bany]
Looking for sub map using prefix "b".
My solution: [b, bany]
SO answer: [b, bany]
SO comment: []
Looking for sub map using prefix "bany".
My solution: [bany]
SO answer: [bany]
SO comment: [bany]
也许应该补充的是,我还尝试了其他各种方法,包括我在互联网上其他地方找到的代码。它们都因产生不正确的结果而失败,或者由于各种异常而崩溃。
https://stackoverflow.com/questions/13530999
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