Question

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.

Answer 1

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.

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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)

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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");