Question

How can I retrieve size of folder or file in Java?

Answer 1

java.io.File file = new java.io.File("myfile.txt");
file.length();

This returns the length of the file in bytes or 0 if the file does not exist. There is no built-in way to get the size of a folder, you are going to have to walk the directory tree recursively (using the listFiles() method of a file object that represents a directory) and accumulate the directory size for yourself:

public static long folderSize(File directory) {
    long length = 0;
    for (File file : directory.listFiles()) {
        if (file.isFile())
            length += file.length();
        else
            length += folderSize(file);
    }
    return length;
}

WARNING: This method is not sufficiently robust for production use. directory.listFiles() may return null and cause a NullPointerException. Also, it doesn’t consider symlinks and possibly has other failure modes. Use this method.