The Java.iO.File document says the following words about its constructor which takes the pathname:
public File(String pathname)
Creates a new File instance by converting the given pathname string
into an abstract pathname. If the given string is the empty string,
then the result is the empty abstract pathname.
But what if the pathname points to a file which is already existing?
File file = new File(PATH_TO_AN_EXISTING_FILE);
Does the above file instance represent a fresh new file (with the existing one be deleted?) Or does it represent the existing file ?
What the documentation says is that it will create a new File instance. This mean it will create a new instance in memory of the File class.
This object will point to a file on you file system. However, if the file exists, it will not create a new file.
I think the documentation is a little confusing: creating a new File object in Java does not mean creation of a new file in your file system. The File object is merely an abstract representation of file and directory pathname, it may or may not represent a real file on disk or on a network storage.
It is more or less equivalent to a String representing an address of something: when you write
String str = "1600 Pennsylvania Ave NW, Washington, DC 20500";
you create a string with an address of an existing building. There is no other connection between the string str that you created and The White House that happens to be located at that address.
The only difference between a File created with an existing path name and a file created with a non-existent path name is that the call of exists() on the former will return true, while the same call on the later would return false.
A File is not directly linked to an actual file on the file system. If the file exists, it will point to that file. If the file doesn't exist, it will not create it. exist() will return false.
This is a very confusingly named class.
A File object represents a file path, not an actual file. So when you create a File object you do not change anything on the filing system. Conceptually, it's no different to a String.
In java.nio, the class has been renamed to (the much more intuitive) Path.
The java.io.File class represents a path on some file system. It is not directly bound to a file. You are not opening a file when you create a File instance.
A File object is merely an object on the heap. Yes, it does have fields and methods that imply that this object represents a real file (or a directory: see the ambiguity?).
You can create File objects for files/directories that do not exist: nothing will happen to the file system; the File instances will be created. After all, a File is just a descriptor.
Furthermore, you can create several File objects with different paths (esp. when one is absolute and others are relative from different parent paths), but they will all point to the same file/directory when they are actually evaluated (by opening a file with In/OutputStream, Reader/Writer; when checking with exists() or creating: createFile(), createDirectory().
File f=new File("C://Existing_file")
above line indicates already existed file not the new one to be created file.
File class instance always refers to IO operations and also it always refers to already consisted file
By creating new instance
File f= new File("ABC.txt");
This new object of file will point to a file named ABC.txt in your system, if present. If the ABC.txt file is not there, then the file object simply does not point to any file.
When a file is stored in a computer. The information related to the file is also stored( you can check it in the properties by right clicking on file). These are those information that is about the file.
So the File class object does nothing except represents the information about the file.
The File class object only provides you with information about the file and the same is stated in its definition.
Related
Is it posibble to save class File object in other directory?
I send this class File object from Client directory to server directory by objectOutputStream.
My question is how to create in server directory this class File object.
File f=(File)objectInputStream.readObject();
Path path = FileSystems.getDefault().getPath(".").toAbsolutePath();
path1=Paths.get(path+name); //name is a directory for this special client name
File a= new File(path1.toString());
That's not what File does.
File is, basically, a euphemism for 'String'. A file object contains 1 field, of type string, with the full path to the field. It represents 'a way to read that file' and not the file itself. If you try to serialize it via ObjectOutputStream, all that you're transferring is the file's name. Not the contents.
You're going to have to transfer the actual bytes across the wire if you want to transfer the contents.
In java, when we create a file, we create files using the name of the extension.
For example :
File file = new File("D:/light.txt");
I would like to know what type of file format do we get when we create a file without the file extension type.
For example :
File file = new File("D:/light");
This answer assumes you're doing more than just creating a File object - that you're actually creating a file on the file system. (A File object is just a logically representation of a file system entry which may or may not exist.) If you're really just creating a File object, read EJP's answer - at that point, you've basically just got a name. That doesn't have a "type" or a "format".
The extension is just part of the name. The operating system may try to use that to display a different icon, or launch a specific application when you double-click on the icon, or whatever - but it's really just part of the name.
Fundamentally, a file consists of:
The name you specify when you create it
The bytes you write in it
Metadata such as access control
Unless you deliberately add metadata, it's typically just inherited (default permissions etc).
You can write any data in any file - just because a file has an extension of .txt doesn't mean it's definitely a text file. It could have content which is actually MP3-encoded audio data, for example. Whether the OS uses the file extension or the content to work out what to do with the file is up to the OS.
What type of File is created when we create new File without the extension?
No file is created at all, and there is only one type of File.
In java, when we create a file, we create files using the name of the extension.
Or not.
For example: File file = new File("D:/light.txt");
I would like to know what type of file format do we get when we create a file without the file extension type.
You don't. You don't get any file format at all, because you don't get a file: only a File object in memory.
For example: File file = new File("D:/light");
You can produce all the examples you want, but no file is created, and no file format.
In any case Java doesn't care about filename extensions. Your operating system might, but that's a different story.
I need to copy a File from A to B but keep the directory structure.
for example
C:\folder\second folder\myFile.txt
to
C:\new folder\my second folder\myFile.txt
so that if I the new destination does not exists it will get created
I have tried this example but it copies the whole directory not just the file I specified.
Make use of the File.mkdirs() function: 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.
Prior to reading and writing the file, you can check whither the file path exists, if not then create it. For example:
String s = "c:\\A Dir\\B Dir\\myFile.txt";
File f = new File(s);
if(!f.getParentFile().exists())
f.getParentFile().mkdirs(); // create the parent directory "c:\\A Dir\\B Dir\\"
I have this very basic question.
Does File file = new File("fileName"); actually create a file if one does not exist?
What happens if the file already exists in that location?
Are there any good tutorials you can point me to so I can read more about it?
No it does not. The File object represents an abstract notion of a file, which may exist, but doesn't need to. Note that the File object can also point to a directory (which may or may not exist).
Usually you can find out information about java in the api
http://docs.oracle.com/javase/7/docs/api/java/io/File.html
No, if you want to create an empty file, use createNewFile
File myFile = new File("test.txt");
myFile.createNewFile();
No, calling the objects constructor simply creates an instance of the File-Class.
Read the documentation:
File(File parent, String child):
Creates a new File instance from a parent abstract pathname and a child pathname string.
The call of the createNewFile()- Method writes the file to disk.
Atomically creates a new, empty file named by this abstract pathname
if and only if a file with this name does not yet exist.
You can simply check it by creating a File-object with a non existing file path and calling the File.exists(); method.
if (!file.exists()) {
//File does not exist
}
I was unable to understand the following file constructors.
public File(String parent, String child) and
public File(File parent, String child)
What do the parameters parent and child mean for the file? When can I use these? I have done few programs related to file but I have never used these. I usually use
public File(String pathname)
I have read Javadoc but I could not figure out when and how to use these constructors. Could someone please explain and give examples.
Explanation
The parent parameter is the parent directory of the child file name or relative file path.
Where parent is a File instance, it is a directory file. Where parent is a String, it's simply that directory in pathname terms.
Examples
Consider the following partial file system:
Documents
Homework
Classwork
Tests
Rather than declaring each new file with "Documents\Subdir", you can declare the Documents directory as a file, and use it as the parent File of the other File instances, like so:
File documents = new File("Documents");
File tests = new File("Documents/Tests"); // new File(String);
File homework = new File(documents, "Homework"); // new File(File, String)
File classwork = new File("Documents", "Classwork"); // new File(String, String)
Real-world application
In my experience, I've used applications that provide an API containing a method that returns the directory file in which third-party "plugins" are allowed to save/read files. Without the File(File, String) constructor, I would need to convert the directory file into an absolute path and append my target file to it.
In the following example, Environment.getProgramDirectory() returns the directory file in which permissions are granted.
File settingsFile = new File(Environment.getProgramDirectory(), "settings.txt");
Let's explain with some examples:
Assuming that you have the following structure:
/dir1
dir11
The constructor that you usually use new File("/dir1/dir11") is equivalent to
new File("/dir1", "dir11") (constructor taking 2 String as arguments)
and also equivalent to
new File(new File("/dir1"), "dir11") (constructor using a File as first argument).
"The parent abstract pathname is taken to denote a directory, and the child pathname string is taken to denote either a directory or a file. " As specified on the API
Parent will point to the Directory
Child will be its Contents..