问为什么我会收到未经检查的强制转换警告？

EN

import java.util.*;
import javax.swing.tree.*;

public class Test {
    public static void main(String[] args) {
        DefaultMutableTreeNode node = null;

        Enumeration<DefaultMutableTreeNode> en1 = 
            (Enumeration<DefaultMutableTreeNode>)node.postorderEnumeration();
        Enumeration<DefaultMutableTreeNode> en2 = 
            (Enumeration<DefaultMutableTreeNode>)node.pathFromAncestorEnumeration(null);
    }
}

c:\Users\Jon\Test>javac -Xlint Test.java
Test.java:8: warning: [unchecked] unchecked cast
        Enumeration<DefaultMutableTreeNode> en1 = (Enumeration<DefaultMutableTre
eNode>)node.postorderEnumeration();

                                ^
  required: Enumeration<DefaultMutableTreeNode>
  found:    Enumeration
Test.java:9: warning: [unchecked] unchecked cast
        Enumeration<DefaultMutableTreeNode> en2 = (Enumeration<DefaultMutableTre
eNode>)node.pathFromAncestorEnumeration(null);

                                       ^
  required: Enumeration<DefaultMutableTreeNode>
  found:    Enumeration
2 warnings