我们知道HashMap默认不保留任何顺序。如果有需要，我们需要根据需求明确排序。在本教程中，我们将学习如何对TreeMap使用Comparator按键和按值进行排序。
HashMap按键排序
在这个例子中，我们使用TreeMap集合类基于键对HashMap进行排序。
package beginnersbook.com;import java.util.HashMap;import java.util.Map;import java.util.TreeMap;import java.util.Set;import java.util.Iterator;public class Details { public static void main(String[] args) { HashMap<Integer, String> hmap = new HashMap<Integer, String>(); hmap.put(5, "A"); hmap.put(11, "C"); hmap.put(4, "Z"); hmap.put(77, "Y"); hmap.put(9, "P"); hmap.put(66, "Q"); hmap.put(0, "R"); System.out.println("Before Sorting:"); Set set = hmap.entrySet(); Iterator iterator = set.iterator(); while(iterator.hasNext()) { Map.Entry me = (Map.Entry)iterator.next(); System.out.print(me.getKey() + ": "); System.out.println(me.getValue()); } Map<Integer, String> map = new TreeMap<Integer, String>(hmap); System.out.println("After Sorting:"); Set set2 = map.entrySet(); Iterator iterator2 = set2.iterator(); while(iterator2.hasNext()) { Map.Entry me2 = (Map.Entry)iterator2.next(); System.out.print(me2.getKey() + ": "); System.out.println(me2.getValue()); } }}
输出：
Before Sorting:0: R4: Z5: A66: Q9: P77: Y11: CAfter Sorting:0: R4: Z5: A9: P11: C66: Q77: Y
HashMap按值排序
在这个例子中，我们使用Comparator按值对HashMap进行排序。
package beginnersbook.com;import java.util.Collections;import java.util.Comparator;import java.util.HashMap;import java.util.Iterator;import java.util.LinkedHashMap;import java.util.LinkedList;import java.util.List;import java.util.Map;import java.util.Set;public class HMapSortingByvalues { public static void main(String[] args) { HashMap<Integer, String> hmap = new HashMap<Integer, String>(); hmap.put(5, "A"); hmap.put(11, "C"); hmap.put(4, "Z"); hmap.put(77, "Y"); hmap.put(9, "P"); hmap.put(66, "Q"); hmap.put(0, "R"); System.out.println("Before Sorting:"); Set set = hmap.entrySet(); Iterator iterator = set.iterator(); while(iterator.hasNext()) { Map.Entry me = (Map.Entry)iterator.next(); System.out.print(me.getKey() + ": "); System.out.println(me.getValue()); } Map<Integer, String> map = sortByValues(hmap); System.out.println("After Sorting:"); Set set2 = map.entrySet(); Iterator iterator2 = set2.iterator(); while(iterator2.hasNext()) { Map.Entry me2 = (Map.Entry)iterator2.next(); System.out.print(me2.getKey() + ": "); System.out.println(me2.getValue()); } } private static HashMap sortByValues(HashMap map) { List list = new LinkedList(map.entrySet()); // Defined Custom Comparator here Collections.sort(list, new Comparator() { public int compare(Object o1, Object o2) { return ((Comparable) ((Map.Entry) (o1)).getValue()) .compareTo(((Map.Entry) (o2)).getValue()); } }); // Here I am copying the sorted list in HashMap // using LinkedHashMap to preserve the insertion order HashMap sortedHashMap = new LinkedHashMap(); for (Iterator it = list.iterator(); it.hasNext();) { Map.Entry entry = (Map.Entry) it.next(); sortedHashMap.put(entry.getKey(), entry.getValue()); } return sortedHashMap; }}输出：
Before Sorting:0: R4: Z5: A66: Q9: P77: Y11: CAfter Sorting:5: A11: C9: P66: Q0: R77: Y4: Z
