增加map.headMap测试样例 (#4)

Reviewed-on: #4

src/main/java/com/zeekling/cn/map/sortedmap/HeadMap.java

@@ -0,0 +1,19 @@
+package com.zeekling.cn.map.sortedmap;
+
+import java.util.SortedMap;
+import java.util.TreeMap;
+
+public class HeadMap {
+
+    public static void main(String[] args) {
+        SortedMap<String, String> mp = new TreeMap<>();
+        mp.put("One", "Geeks");
+        mp.put("Two", "For");
+        mp.put("Three", "Geeks");
+        mp.put("Four", "Code");
+        mp.put("Five", "It");
+        // 返回键小于H的映射
+        System.out.print(mp.headMap("H"));
+    }
+
+}