[英]How to write a custom Comparator for TreeMap in Java?
我想將鍵值對存儲在TreeMap中,並根據以下邏輯基於Key的值對條目進行排序:
按鍵的長度排序。 如果兩個鍵的長度相同,則按字母順序對其進行排序。 例如,對於以下鍵/值對。
IBARAKI MitoCity
TOCHIGI UtunomiyaCity
GUNMA MaehashiCity
SAITAMA SaitamaCity
CHIBA ChibaCity
TOKYO Sinjyuku
KANAGAWA YokohamaCity
預期的輸出是這樣的。
CHIBA : ChibaCity
GUNMA : MaehashiCity
TOKYO : Sinjyuku
IBARAKI : MitoCity
SAITAMA : SaitamaCity
TOCHIGI : UtunomiyaCity
KANAGAWA : YokohamaCity
您可以將Comparator作為參數傳遞給Map的構造函數。 根據文檔,它僅用於密鑰:
/**
* Constructs a new, empty tree map, ordered according to the given
* comparator. All keys inserted into the map must be <em>mutually
* comparable</em> by the given comparator: {@code comparator.compare(k1,
* k2)} must not throw a {@code ClassCastException} for any keys
* {@code k1} and {@code k2} in the map. If the user attempts to put
* a key into the map that violates this constraint, the {@code put(Object
* key, Object value)} call will throw a
* {@code ClassCastException}.
*
* @param comparator the comparator that will be used to order this map.
* If {@code null}, the {@linkplain Comparable natural
* ordering} of the keys will be used.
*/
public TreeMap(Comparator<? super K> comparator) {
this.comparator = comparator;
}
這樣,您可以像這樣通過密鑰的長度傳遞比較器:
new TreeMap<>(Comparator.comparingInt(String::length).thenComparing(Comparator.naturalOrder()))
您可以按照以下步驟進行操作。
public static void main(String[] args) {
Map<String, String> map = new TreeMap<>(new CustomSortComparator());
map.put("IBARAKI", "MitoCity");
map.put("TOCHIGI", "UtunomiyaCity");
map.put("GUNMA", "MaehashiCity");
map.put("SAITAMA", "SaitamaCity");
map.put("CHIBA", "ChibaCity");
map.put("TOKYO", "Sinjyuku");
map.put("KANAGAWA", "YokohamaCity");
System.out.println(map);
}
CustomSortComparator的定義如下。
public class CustomSortComparator implements Comparator<String> {
@Override
public int compare(String o1, String o2) {
if (o1.length() > o2.length()) {
return 1;
}
if (o1.length() < o2.length()) {
return -1;
}
return returnCompareBytes(o1, o2);
}
private int returnCompareBytes(String key1, String key2) {
for (int i = 0; i < key1.length() - 1; i++) {
if (key1.charAt(i) > key2.charAt(i)) {
return 1;
}
if (key1.charAt(i) < key2.charAt(i)) {
return -1;
}
}
return 0;
}
}
您需要為此編寫自己的comparator
,並在TreeMap
使用它,例如:
public class StringComparator implements Comparator<String> {
@Override
public int compare(String s1, String s2) {
return s1.length() == s2.length() ? s1.compareTo(s2) : s1.length() - s2.length();
}
public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
Map<String, String> map = new TreeMap<>(new StringComparator());
map.put("IBARAKI", "MitoCity");
map.put("TOCHIGI", "UtunomiyaCity");
map.put("GUNMA", "MaehashiCity");
map.put("SAITAMA", "SaitamaCity");
map.put("CHIBA", "ChibaCity");
map.put("TOKYO", "Sinjyuku");
map.put("KANAGAWA", "YokohamaCity");
System.out.println(map);
}
}
這不會處理null
值,但是如果在用例中期望null
值,則可以添加處理。
您應該創建一個唯一的比較器來比較地圖的鍵。 但是因為您也想打印這些值,所以應該比較整個條目集:
Comparator<Map.Entry<String, String>> c = new Comparator<Map.Entry<String, String>>() {
@Override
public int compare(Map.Entry<String, String> o1, Map.Entry<String, String> o2) {
int q = Integer.compare(o1.getKey().length(), o2.getKey().length());
return q != 0 ? q : o1.getKey().compareTo(o2.getKey());
}
};
然后可以使用此比較器進行排序:
map.entrySet().stream().sorted(c).forEach(System.out::println);
您可以使用此方法來代替將Map直接轉換為TreeMap
public static Map toTreeMap(Map hashMap)
{
// Create a new TreeMap
Map treeMap = new TreeMap<>(new Comparator<Map.Entry<String, String>>(){
public int compare(Map.Entry<String, String> o1, Map.Entry<String, String> o2 )
{
if (o1.getKey().length() > o2.getKey().length()) {
return 1;
}
if (o1.getKey().length() > o2.getKey().length()) {
return -1;
}
return o1.getKey().compareTo(o2.getKey());
}
});
for(Map.entry e : hashmap){
treeMap.put(e.getKey(),e.getValue);
}
return treeMap;
}
您可以在對TreeMap
的構造函數調用中定義所需的Comparator<String>
:
import java.util.Comparator;
import java.util.Map;
import java.util.TreeMap;
public class Main {
static final Map<String, String> map =
new TreeMap<String, String> (new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
int diff_length = o1.length() - o2.length();
if (diff_length != 0) return diff_length;
return o1.compareTo(o2);
}
});
public static final void main(String[] args) {
map.put("IBARAKI", "MitoCity");
map.put("TOCHIGI", "UtunomiyaCity");
map.put("GUNMA", "MaehashiCity");
map.put("SAITAMA", "SaitamaCity");
map.put("CHIBA", "ChibaCity");
map.put("TOKYO", "Sinjyuku");
map.put("KANAGAWA", "YokohamaCity");
System.out.println(map);
}
}
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