What is a pathname string's prefix and its length in JAVA?

Question

File.java uses a variable as:

private final transient int prefixLength;

And says, this is "abstract pathname's prefix".

File.java also have a constructor as:

public File(String pathname) {
        if (pathname == null) {
            throw new NullPointerException();
        }
        this.path = fs.normalize(pathname);
        this.prefixLength = fs.prefixLength(this.path);
    }

Here it is setting the variable prefixLength using fs.prefixLength() method.

Variable fs is defined in File.java as:

private static final FileSystem fs = DefaultFileSystem.getFileSystem();

Method getFileSystem() of DefaultFileSystem class returns object of UnixFileSystem. So method fs.prefixLength() actually calls prefixLength() method of UnixFileSystem. The prefixLength() method of UnixFileSystem is implemented as:

public int prefixLength(String pathname) {
        if (pathname.length() == 0) return 0;
        return (pathname.charAt(0) == '/') ? 1 : 0;
    }

Means this method will only return 0 or 1 Depending upon the length of the pathname or first character of the pathname.

My doubt is: What type of length it is, and what is its significance?

score 3 · Accepted Answer · answered Sep 01 '16 at 09:48

The idea behind prefixLength is to treat the part of file name indicating the location of the root of its path separately from the rest of the file name:

c:\quick\brown\fox.java
^^^

Above, prefix is c:\.

UNIX implementation is straightforward, because only two initial locations are possible - the root / and the current directory (no prefix).

Windows implementation, which supports \\, c:, c:\, and \ is shown below:

public int prefixLength(String path) {
    char slash = this.slash;
    int n = path.length();
    if (n == 0) return 0;
    char c0 = path.charAt(0);
    char c1 = (n > 1) ? path.charAt(1) : 0;
    if (c0 == slash) {
        if (c1 == slash) return 2;  /* Absolute UNC pathname "\\\\foo" */
        return 1;                   /* Drive-relative "\\foo" */
    }
    if (isLetter(c0) && (c1 == ':')) {
        if ((n > 2) && (path.charAt(2) == slash))
            return 3;               /* Absolute local pathname "z:\\foo" */
        return 2;                   /* Directory-relative "z:foo" */
    }
    return 0;                       /* Completely relative */
}

score 0 · Answer 2 · answered Sep 01 '16 at 09:49

It is not clear your questions, lets try to clarify:

1) What type of length it is?
2) What is its significance?

1)This length means if the filepath has the name with the bar or not. In the unix file systems it is important to know....

when "/user/some_folder" return 1
when "user/some_folder" return 0
when "" return 0

2) probably it is used when you try to access files inside this filepath to take the "/" into account..

What is a pathname string's prefix and its length in JAVA?

2 Answers2