mkdirs() also creates parent directories in the path this File represents.
javadocs for mkdirs():
Creates the directory named by this abstract pathname, including any necessary but nonexistent parent directories. Note that if this operation fails it may have succeeded in creating some of the necessary parent directories.
javadocs for mkdir():
Creates the directory named by this abstract pathname.
Example:
File f = new File("non_existing_dir/someDir");
System.out.println(f.mkdir());
System.out.println(f.mkdirs());
will yield false for the first [and no dir will be created], and true for the second, and you will have created non_existing_dir/someDir