Develop Reference

JTree displaying drive does not show up at all

My task is to show a tree of all directories/files of a PC drive,
I have a class DirectoryNode that extends DefaultMutableTreeNode with File field directoryPath. I build nodes recursively:
public void buildDirectoryTree(){
if(!directoryPath.isDirectory()){
return;
}
for(File f : directoryPath.listFiles()){
if(f.isHidden() || !f.exists()) continue;
DirectoryNode newChild = new DirectoryNode(f);
add(newChild);
newChild.buildDirectoryTree();
}
}
It works fine for concrete directories, but when I try to use it for whole drive, or some large directories, JTree with this node does not show up at all
I think it encounters a problem with specific directories. I've add exists and is Hidden checks to skip this problem roots, but it didn't help.
In addition, exists, isHidden and isDirectory return false for some of my valid directories directories (I am using Windows 10).

File.listFiles() is one of those ancient methods that violates Java's convention/good practice to never return null from a method that returns an array. So you have to check for null.
From the docs:
An array of abstract pathnames denoting the files and directories in
the directory denoted by this abstract pathname. The array will be
empty if the directory is empty. Returns null if this abstract
pathname does not denote a directory, or if an I/O error occurs.
I have changed your code to make it a little safer. If it's called from the EDT, you might want to add some log message or the like instead of throwing the exception into nirvana.
public void buildDirectoryTree() throws IOException {
if (!directoryPath.isDirectory()) {
return;
}
final File[] files = directoryPath.listFiles();
if (files != null) {
for (File f : files) {
if (f.isHidden() || !f.exists()) continue;
DirectoryNode newChild = new DirectoryNode(f);
add(newChild);
newChild.buildDirectoryTree();
}
} else {
throw new IOException("Failed to list files for " + directoryPath);
}
}
As others have pointed out, there are more modern APIs and they have been introduced for good reasons. I recommend to read up on NIO2 and the Path APIs for better solutions.

Recursion: Checking for files in Directories and reading them

Before you speculate something like "This guy is asking for homework help", I'll go ahead and clear any doubts you may have and say yes, this is related to homework. However, I hope that does not take away from the learning that this question provides to me and/or anyone who reads this in the future.
Background: We're currently working on recursion and our assignment asks that we write a program that uses command arguments to check a directory and its file contents for a string(that is also a command argument). We must use recursion for this.
-I want to make this clear that I UNDERSTAND WHAT THE ASSIGNMENT IS ASKING
I am simply asking, how would this work recursively because I just don't get it.
We did a problem where we had to find the size of a directory and it made sense, but I don't get how to check if something is a directory or file and based on that we read its contents or go deeper into the directory until we find a file.
Here's what I've currently done. Not too sure how wrong this is as I'm basing entirely off of the 'check the size of a directory' assignment we previously did:
The folder that I'm checking is something like this:
Directory ---> files --inside main directory --->> Two directories ----> files within both of those directories
public class SearchingForStrings {
public static void main(String[] args) {
String path = "."; // default location of this project
File sf = new File(path);
String mysteriesDirectory = args[0];
String keyString = args[1];
countLinesWithString(sf, mysteriesDirectory, keyString);
}
public static int countLinesWithString(File startPath, String mysteriesDirectory, String keyString) {
if(!startPath.exists()) {
throw new IllegalArgumentException("File " + startPath + " does not exist!");
} else if(startPath.isFile()) {
return Integer.parseInt(startPath.getAbsolutePath()); // Just to show where the file is I located the parsing is just to stop an error from flagging on this part; Going to ask professor if it's okay with him
// this is where we would begin reading the contents of the files
} else if(startPath.isDirectory()) {
// This is where our recursion would take place: essentially
// we will be going 'deeper' into the directory until we find a file
//File[] subFiles = startPath.listFiles();
countLinesWithString(startPath, mysteriesDirectory, keyString);
} else {
throw new IllegalStateException("Unknown file type: " + startPath);
}
}
}
In short: Could someone explain how recursion would work if you wanted to go deeper into a director(y/ies)?

I'll give this a try. It's something that is easier to explain than to understand.
The recursive method, on which you have made a decent start, might be documented as follows:
"For a given directory: for each file in the directory, count all the lines which contain a given string; for each directory in the directory, recurse".
The recursion is possible - and useful - because your original target is a container, and one of the types of things it can contain is another container.
So think of the counting method like this:
int countLines(dir, string) // the string could be an instance variable, also, and not passed in
{
var countedLines = 0;
for each item in dir:
if item is file, countedLines += matchedLinesInFile(item, string);
else if item is dir, countedLines += countLines(item, string);
else throw up; // or throw an exception -- your choice
}
then call countLines from an exterior method with the original dir to use, plus the string.
One of the things that trips people up about recursion is that, after you get it written, it doesn't seem possible that it can do all that it does. But think through the above for different scenarios. If the dir passed in has files and no dirs, it will accumulate countedLines for each file in the dir, and return the result. That's what you want.
If the dir does contain other dirs, then for each one of those, you're going to call the routine and start on that contained dir. The call will accumulate countedLines for each file in that dir, and call itself for each dir recursively down the tree, until it reaches a dir that has no dirs in it. And it still counts lines in those, it just doesn't have any further down to recurse.
At the lowest level, it is going to accumulate those lines and return them. Then the second-lowest level will get that total to add to its total, and start the return trips back up the recursion tree.
Does that explain it any better?

Just help you get started with recursion check this :
It will recursively go from base directory printing all the folders and files.
Modify this to your requirements. Try and let us know.
import java.io.File;
public class Test {
public static void getResource(final String resourcePath) {
File file = new File(resourcePath);
if (file.isFile()) {
System.out.println("File Name : " + file.getName());
return;
} else {
File[] listFiles = file.listFiles();
if (listFiles != null) {
for (File resourceInDirectory : listFiles) {
if (!resourceInDirectory.isFile()) {
System.out.println("Folder "
+ resourceInDirectory.getAbsolutePath());
getResource(resourceInDirectory.getAbsolutePath());
} else {
getResource(resourceInDirectory.getAbsolutePath());
}
}
}
}
}
public static void main(String[] args) {
final String folderPath = "C:/Test";
getResource(folderPath);
}
}

"Simple" Trie Implementation

I need to implement a Trie (in Java) for a college project. The Trie should be able to add and remove Strings (for phase 1).
I have spent several hours each day (for the last few days) trying to figure out how to do this and FAILED miserably each time.
I require some help, the examples on the internet and my textbook (Data Structures and Algorithms in Java By Adam Drozdek) are not helping.
Information
Node classes I am working with:
class Node {
public boolean isLeaf;
}
class internalNode extends Node {
public String letters; //letter[0] = '$' always.
//See image -> if letter[1] = 'A' then children[1] refers to child node "AMMO"
//See image -> if letter[2] = 'B' then children[2] refers to internal node "#EU"
public TrieNode[] children = new TrieNode[2];
public TrieInternalNode(char ch)
{
letters = "#" + String.valueOf(ch);//letter[0] = '$' always.
isLeaf = false;
}
}
class leafNode extends Node
{
public String word;
public TrieLeafNode(String word)
{
this.word = new String(word);
isLeaf = true;
}
}
And here is the pseudo code for insert that I need to follow: (warning it is very vague)
trieInsert(String K)
{
i = 0;
p = the root;
while (not inserted)
{
if the end of word k is reached
set the end-of-word marker in p to true;
else if (p.ptrs[K[i]] == 0)
create a leaf containing K and put its address in p.ptrs[K[i]];
else if reference p.ptrs[K[i]] refers to a leaf
{
K_L = key in leaf p.ptrs[K[i]]
do
{
create a nonleaf and put its address in p.ptrs[K[i]];
p = the new nonleaf;
} while (K[i] == K_L[i++]);
}
create a leaf containing K and put its address in p.ptrs[K[--i]];
if the end of word k is reached
set the end-of-word marker in p to true;
else
create a leaf containing K_L and put its address in p.ptrs[K_L[i]];
else
p = p.ptrs[K[i++]];
}
}
I need to implement the following methods.
public boolean add(String word){...}//adds word to trie structure should return true if successful and false otherwise
public boolean remove(String word){...}//removes word from trie structure should return true if successful and false otherwise
I cant find pseudo code for remove, but if insert does not work delete wont help me.
Here is a image of how the Trie that I need to implement should look like.
I am aware that the Trie will still be inefficient if implemented like this, but at the moment I need not worry about this.
The book provides an implementation that is similar to what I need to do but doesn't use the end of word char ('$') and only stores the words without their prefixes in the child nodes http://mathcs.duq.edu/drozdek/DSinJava/SpellCheck.java
Constraints
I need to implement the trie in JAVA.
I may not import or use any of Java's built-in data structures. (ie. no Map, HashMap, ArrayList etc)
I may use Arrays, Java primitive Types and Java Strings.
The Trie must use a $ (dollar) symbol to indicate a end-of-word. (see the image below )
I may asume that now word containing the $symbol will be inserted.
I need to implement the Trie it in the same style as the book does.
Case of words doesn't matter ie. all words will be considered to be lowercase
The Trie should only store the end-of-word character and the characters applicable to a word and not the entire alphabet(like some implementations).
I do not expect anyone to do the implementation for me(unless they have one lying around :P) I just really need help.

First of all, I don't think you should make leaf nodes and internal nodes separate classes. I recommend making a universal node class with an isLeaf() method. This method would return true if a node has no children.
Here is some higher-level pseudocode for the functions you need to implement. For simplicity, I assume the existence of a method called getIndex() which returns the index corresponding to a character.
Insert(String str)
Node current = null
for each character in str
int index = getIndex(character)
if current.children[index] has not been initialized
initialize current.children[index] to be a new Node
current = current.children[index]
You can easily augment this pseudocode to fit your needs. For example, if you want to return false whenever insertion isn't successful:
Return false if the input string is null
Return false if the input string contains invalid characters
Now, here is some higher-level pseudocode for remove.
Remove(String str)
Node current = null
for each character in str
int index = getIndex(character)
current = current.children[index]
// At this point, we found the node we want to remove. However, we want to
// delete as many ancestor nodes as possible. We can delete an ancestor node
// if it is not need it any more. That is, we can delete an ancestor node
// if it has exactly one child.
Node ancestor = current
while ancestor is not null
if ancestor has 2 or more children
break out of loop
else if ancestor has less than 2 children
Node grandAncestor = ancestor.parent
if grandAncestor is not null
reinitialize grandAncestor.children // this has the effect of removing ancestor
ancestor = ancestor.parent
At a very high level, we follow the input string to the node we want to remove. After this, we traverse up the tree following parent pointers and delete every node with 1 child (since it is no longer needed). Once we reach a node with 2 children, we stop.
Like Insert, we can easily augment this pseudocode to return false whenever deletion isn't successful:
Return false if the input string is null
Return false if the input string contains invalid characters
Return false if the input string leads to a Node which doesn't exist
It is easiest to implement delete if your Node class has a parent field. However, it is possible to implement the method without parent points, but it is more difficult. You can see an example of the trickier implementation here.

Method returning a NullPointerException when it should be returning a -1

The following code works perfectly fine as long as the name I input is in the directory. If the name doesn't exist in the directory, it returns a NullPointerException. I don't understand cos if the name doesn't exist in the directory, it should just return the -1. Why the exception? Thanks for any guidance.
public class Direct{
//Directory is a class that contain a name and get/set methods for it.
private Directory[] directory = new Directory[100];
public int find(String name){
for (int x=0; x < directory.length; x++){
if (directory[x].getName().equals(name)){ //exception refers to this line to hold the error
return x;
}
}
return -1;
}
}
//Testing on main method
Direct direct = new Direct();
//This works cos the name John is in the directory.
System.out.println(direct.find("John"));
This returns an error cos x is not present in the directory.
System.out.println(direct.find("x"));

When you create a Directory array of length 100, it starts out with 100 null references. You reached past all existing Directory objects that are filled in (if any), and you have reached a null reference before reaching the end of the array.
Test for directory[x] being null before accessing getName(). It's up to you whether to immediately return -1 on a null array element or to continue searching the array.

directory[x].getName() is null. First check if it is null or not and then do the .getName()

How can I get element from List<File>?

In Java, I have a following declaration:
public List<File> getAllFiles(){
return Collections.unmodifiableList(createdFiles);
}
I want to access the object (file) that is first on that list. How can I best do it?

getAllFiles().iterator().next();
or
getAllFiles().get(0);

File file = getAllFiles().get(0);

Remember to check whether the list is empty.
List<File> allFiles = getAllFiles();
if (allFiles.size() == 0) {
// handle this situation as you like.
} else {
File file = allFiles.get(0);
// ...
}
Since you want the first element I think you should avoid to the iterator approach as, depending on the implementation, you would create a object unnecessary: the iterator.