Add java doc. (#1681)

* add javadoc for PartialPath

* add javadoc for Path

* Update Path.java

* add java doc
Co-authored-by: Nqiaojialin <646274302@qq.com>

server/src/main/java/org/apache/iotdb/db/metadata/PartialPath.java

@@ -26,7 +26,8 @@ import org.apache.iotdb.tsfile.common.constant.TsFileConstant;
 import org.apache.iotdb.tsfile.read.common.Path;
 
 /**
- * A prefix path, suffix path or fullPath generated from SQL
+ * A prefix path, suffix path or fullPath generated from SQL.
+ * Usually used in the IoTDB server module
  */
 public class PartialPath extends Path implements Comparable<Path> {
 
@@ -36,15 +37,30 @@ public class PartialPath extends Path implements Comparable<Path> {
   // alias of time series used in SELECT AS
   private String tsAlias = null;
 
+  /**
+   * Construct the PartialPath using a String, will split the given String into String[]
+   * E.g., path = "root.sg.\"d.1\".\"s.1\""
+   * nodes = {"root", "sg", "\"d.1\"", "\"s.1\""}
+   *
+   * @param path a full String of a time series path
+   * @throws IllegalPathException
+   */
   public PartialPath(String path) throws IllegalPathException {
     this.nodes = MetaUtils.splitPathToDetachedPath(path);
     this.fullPath = path;
   }
 
+  /**
+   * @param partialNodes nodes of a time series path
+   */
   public PartialPath(String[] partialNodes) {
     nodes = partialNodes;
   }
 
+  /**
+   * @param path path
+   * @param needSplit needSplit is basically false, whether need to be split to device and measurement, doesn't support escape character yet.
+   */
   public PartialPath(String path, boolean needSplit) {
     super(path, needSplit);
   }

tsfile/src/main/java/org/apache/iotdb/tsfile/read/common/Path.java

@@ -22,10 +22,11 @@ import java.io.Serializable;
 import org.apache.iotdb.tsfile.common.constant.TsFileConstant;
 
 /**
- * This class define an Object named Path to represent a series in IoTDB.
- * AndExpression in batch read, this definition is also used in query
- * processing. Note that, Path is unmodified after a new object has been
- * created.
+ * This class represent a time series in TsFile,
+ * which is usually defined by a device and a measurement.
+ *
+ * If you want to use one String such as "device1.measurement1" to init Path in TsFile API,
+ * please use the new Path(string, true) to split it to device and measurement.
  */
 public class Path implements Serializable, Comparable<Path> {
 
@@ -56,8 +57,6 @@ public class Path implements Serializable, Comparable<Path> {
     }
     if(!needSplit) {
       fullPath = pathSc;
-      device = "";
-      measurement = pathSc;
     } else {
       if (pathSc.length() > 0) {
         if (pathSc.charAt(pathSc.length() - 1) == TsFileConstant.DOUBLE_QUOTE) {