First acquaintance of file

File Class

file （file）： Files are used to store data .
route （path）： Represents a location in a computer , This location can be a folder , It can also be a document .
stay Java in , There is a class called File, This class can represent 【 A file or folder in a computer 】
Be careful ,File The original meaning of this word is document , But in Java in , It's not just about documents , It can also represent a folder . We can go through File Class to operate on a file or folder .

Relative path and absolute path

Absolute path ： It's a very detailed path , The absolute path starts with the drive letter . such as ： D:\demo\HelloWorld.java

Relative paths ： It's a short path , The relative path does not start with a drive letter , The relative path is idea The middle refers to the content under the current project . such as ：Demo.java

File Construction method of

File Construction method of class ：

• File(String pathname)： Create a... Based on the path of a string File object . This path can be a file , It could be a folder .
• File(String parent, String child)： Create a... Based on the combination of parent and child paths File object .

​ HelloWorld.java File path ：D:\demo\HelloWorld.java

​ HelloWorld.java Parent path ：D:\demo\

​ HelloWorld.java Subpath ：HelloWorld.java

• File(File parent, String child): Create a... Based on the combination of parent and child paths File object .
• File Represents a file or folder in a computer , This file or folder can exist , It can also be nonexistent .

File The method to get the class

File Class is used to obtain the method

• String getAbsolutePath()： Used to get File The absolute path represented by the object .
• String getPath()： obtain File The path represented by the object .
• String getName()： obtain File Object represents the name of the file or folder .
• long length()： obtain File Object represents the size of the file in bytes . Can't get folder size , If File Object represents a folder , So called length The result of this method is an uncertain result .

If you're creating File When the object is constructed, the relative path is written in the construction method , call getPath What you get is the relative path
If you're creating File When the object is constructed, the absolute path is written in the construction method , call getPath What you get is the absolute path

File Class judgment method

File Class judgment method ：

• boolean exists()： Judge File Whether the file or folder represented by the object exists .
• boolean isDirectory()： Decide if it's a folder
• boolean isFile()： Determine if it's a file

File Class creation method

File Create method in class

• boolean createNewFile()： Create a file , If the file already exists , So creation failed . If the creation is successful, return true, Otherwise return to false.
• boolean mkdir()： Create a folder , If the folder already exists , So creation failed . If the creation is successful, return true, Otherwise return to false.
• boolean mkdirs()： Create a folder , If the folder already exists , So creation failed . If the creation is successful, return true, Otherwise return to false.
• mkdir Only single level directories can be created , mkdirs You can create multi-level directories .
• boolean mkdirs()： Create a folder , If the folder already exists , So creation failed
  ​ Be careful ： If you use mkdirs When creating a file , The superior directory does not exist , Then we will create the superior directory together .
• boolean mkdir()： Create a folder , If the folder already exists , So creation failed . Be careful ： If you create a file , The superior directory does not exist , Then the creation fails , But there is no error .
• boolean createNewFile()： Create a file , If the file already exists , So creation failed . Be careful ： If you create a file , The superior directory does not exist , Then the creation fails . And throw an exception .

File Class deletion method

File Delete method in class
​ boolean delete()： Delete File The file or folder represented by an object , If the deletion succeeds, return true.
​ File Medium delete Method if you delete a folder , Only empty folders can be deleted .

Use delete Method to delete the content does not go to the recycle bin

File Class traversal method

Methods for traversal ：

• String[] list()： Get all files and folders in the specified directory , And put it into the string array to return .
• File[] listFiles()： Get all the files and folders in the specified directory , And into the File Array returns .

Be careful ：

• If File Object represents a file , So called listFiles The result is a null value .
• If File Object representation does not exist in the computer , So called listFiles The result is also a null value
• If File There is nothing in the folder represented by the object , So called listFiles The result is an empty array .