java SortedMap 升序、降序操作
阿新 • • 發佈:2019-01-09
參考:
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降序操作:按key降序
import java.util.Comparator; import java.util.Iterator; import java.util.Map; import java.util.Set; import java.util.TreeMap; public class test { public static void main(String[] args) { Map<Double, String> map = new TreeMap<Double, String>( new Comparator<Double>() { public int compare(Double obj1, Double obj2) { // 降序排序 return obj2.compareTo(obj1); } }); map.put(2.33, "ccccc"); map.put(2.0, "aaaaa"); map.put(3.0, "bbbbb"); map.put(4.0, "ddddd"); Set<Double> keySet = map.keySet(); Iterator<Double> iter = keySet.iterator(); while (iter.hasNext()) { Double key = iter.next(); System.out.println(key + ":" + map.get(key)); } } }
TreeMap預設是升序的,如果我們需要改變排序方式,則需要使用比較器:Comparator。Comparator可以對集合物件或者陣列進行排序的比較器介面,實現該介面的public
compare(T o1,To2)
方法即可實現排序,
import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
public class TreeMapTest {
public static void main(String[] args) {
Map<String, String> map = new TreeMap<String, String>(
new Comparator<String>() {
public int compare(String obj1, String obj2) {
// 降序排序
return obj2.compareTo(obj1);
}
});
map.put("b", "ccccc");
map.put("d", "aaaaa");
map.put("c", "bbbbb");
map.put("a", "ddddd");
Set<String> keySet = map.keySet();
Iterator<String> iter = keySet.iterator();
while (iter.hasNext()) {
String key = iter.next();
System.out.println(key + ":" + map.get(key));
}
}
}
按value:
上面例子是對根據TreeMap的key值來進行排序的,但是有時我們需要根據TreeMap的value來進行排序。對value排序我們就需要藉助於Collections的sort(List<T> list, Comparator<? super T> c)方法,該方法根據指定比較器產生的順序對指定列表進行排序。但是有一個前提條件,那就是所有的元素都必須能夠根據所提供的比較器來進行比較
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;
public class TreeMapTest {
public static void main(String[] args) {
Map<String, String> map = new TreeMap<String, String>();
map.put("a", "ddddd");
map.put("c", "bbbbb");
map.put("d", "aaaaa");
map.put("b", "ccccc");
//這裡將map.entrySet()轉換成list
List<Map.Entry<String,String>> list = new ArrayList<Map.Entry<String,String>>(map.entrySet());
//然後通過比較器來實現排序
Collections.sort(list,new Comparator<Map.Entry<String,String>>() {
//升序排序
public int compare(Entry<String, String> o1,
Entry<String, String> o2) {
return o1.getValue().compareTo(o2.getValue());
}
});
for(Map.Entry<String,String> mapping:list){
System.out.println(mapping.getKey()+":"+mapping.getValue());
}
}
}
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升序排序:按key
import java.util.Map.Entry;
import java.util.*;
public class test {
public static void main(String[] args) {
HashMap<String,String> map=new HashMap<String, String>();
map.put("1","11");
map.put("2", "22");
map.put("3", "33");
for (Entry<String,String> entry: map.entrySet()) {
System.out.println("排序之前:"+entry.getKey()+" 值"+entry.getValue());
}
System.out.println("======================================================");
SortedMap<String,String> sort=new TreeMap<String,String>(map);
Set<Entry<String,String>> entry1=sort.entrySet();
Iterator<Entry<String,String>> it=entry1.iterator();
while(it.hasNext())
{
Entry<String,String> entry=it.next();
System.out.println("排序之後:"+entry.getKey()+" 值"+entry.getValue());
}
}
}