I'm solving a Java problem which needs a java implementation of a tree with N leaves. For that, i decided to use XML Dom tree to represent the problem.
Is that possible with Dom4j ?
The problem is basically a tree of game which represents all moves to calculate the minimum number of moves required to win the game. Is that any useful sample for Dom4j ? Thanks.
I will attempt to answer but with some considerations in mind:
First of all to directly answer your question about whether it is possible to use a XML Dom tree to represent the problem? - Yes, it would be possible but in my opinion using an XML DOM tree to represent a data structure is not so natural (in fact I could also call it an overkill) unless you want to serialise that data structure to the disk or across the network as a result of an API call.
Examples are present here : http://dom4j.sourceforge.net/dom4j-1.6.1/guide.html
A cleaner and more simpler alternative would be to define a Node class like below:
class Node
{
int value; //assuming the node holds an integer value
List<Node> childNodes; // these are the N child nodes.
}
Your game algorithm can then iterate through the list of child nodes to perform it's computation logic.
One limitation that may appear with the above definition is that in case you want to search for a particular child node you need to iterate over the list - while in case you use DOM4j you could use XPath. But using DOM and XPath have their own limitations in terms of memory consumption - refer to this for details.
The simplistic data structure mentioned however above would not have the same memory implications - also it would be easier to manipulate the data structure during the computation process.
Hope this helps.
Related
I am trying to parse a java file using Eclipse JDT's AST. ASTVisitor provides a nice API to traverse all the nodes and work with the node which we want. Now what I want is, can we go to a target node, let say of type MethodDeclaration or all the nodes of that type, instead of traversing all the nodes? Because this reduces time if I have to get all the nodes of a particular type in a whole package. Thanks in advance.
Finding all nodes of a given type inherently is traversing. ASTVisitor is suitable for this exact task.
If you are concerned about unnecessary traversal below the node you are interested in, just return false from the corresponding visit() method, and the visitor will not descend into children of the current node.
I'd be surprised, though, if traversing actually were a performance bottleneck. Creating the AST in the first place is more expensive than that.
If you only want to address few nodes (identified, e.g., by a name pattern), then performing a search (which relies on an index) could perhaps be faster, but this probably pays off only if a significant number of files can be skipped entirely.
Finally, as you mention MethodDeclaration: perhaps you don't even need AST but the Java Model (which is much more light weight) is sufficient for your task?
In Java, is the following Tree class with List<Tree> children in each node the fastest way for further navigation of the tree?
public class Tree {
private int data;
private List<Tree> children = new ArrayList<Tree>();
// operations to insert and navigate Tree nodes
}
I just add new Tree objects to the appropriate array of children and so the entire tree expands. Don't use deletion of elements because I use them to navigate the tree all the time through the algorithm.
And how should this Tree data structure change if I do insertion and navigation as above plus deletion of nodes? Then maybe change ArrayList to LinkedList?
There lots of examples elsewhere but there is no comparison and so without enough experience I can not trust to choose any Tree implementation. That's why I'm asking some advice to find the best data structure (for fastest execution, memory is irrelevant) for two described situations (1. insertion/navigation; 2. insertion/navigation/removal).
EDIT: Can you say what you are not using the built in TreeMap or TreeSet? Because I'm studying Algorithms in general and it is hard to read builtin Java classes as they contain lots of code (checks, conversion, useless functions) that is not needed in understanding the basic operations of the algorithms.
What is the usecase? If you want add-remove-traverse and not any fancy tree functionality, just use a ArrayList! Why Tree?
I'm writing a Java external function that is supposed to be called from an XSLT stylesheet. I intend to return a modified node passed to my function as a parameter. However, I cannot deep copy a node since I'm getting The Saxon DOM cannot be updated. (I think the exception is a bit misleading since I'm not trying to modify a Saxon node). So my question is: what's the best way to utilize external function input nodes in order to return a modified version of it (e.g. adding an additional text node as a child)
Thanks in advance!
My first reaction would be that manipulating XML node trees is much better done in XSLT than in Java, so calling extension functions in order to do node manipulation seems very strange. In particular, "adding an additional text node as a child" is what XSLT was designed to do, so it seems very odd to call Java to do it.
Any code you do write in Java to manipulate nodes is going to be dependent on the tree model used. From the error message, it sounds as if you're probably building the tree initially using Saxon's TinyTree model, and then wrapping the TinyTree nodes in a DOM wrapper for the benefit of your Java code. The DOM wrapper will give you a DOM interface for retrieval/navigation, but not for update, because the TinyTree is immutable.
Unfortunately, the following code fails:
DOMWriter w = new DOMWriter();
w.setNode(programmaticallyCreatedNode);
tinyTreeNode.copy(w, CopyOptions.ALL_NAMESPACES, 0);
with (I'm using Saxon 9.1.0.8)
java.lang.NullPointerException
at net.sf.saxon.dom.DOMWriter.startElement(DOMWriter.java:103)
at net.sf.saxon.tinytree.TinyElementImpl.copy(TinyElementImpl.java:280)
...
I copied the node with:
DOMWriter w = new DOMWriter();
w.setNode(parentElementProgramaticallyCreated);
Element toCopy = ...;
((ElementOverNodeInfo)toCopy).getUnderlyingNodeInfo().copy(w, NodeInfo.ALL_NAMESPACES, false, 0);
Brief look at the code showed me that the NPE is because of not initialized namePool field in DOMWriter. Maybe I'm doing something completely wrong, but my intuition told me that copying a node should be somewhat simple.
Btw, I'm not tied to using DOM with Saxon, anything that would allow me receive a list of elements, read basic info from them (I was not able to quickly figure out how to get a list of attributes with their values from TinyElementImpl) and construct/copy nodes will serve my purpose. Again, I'm doing some complicated logic with attribute values in Java and depending on the result return a rearranged XML tree. That would be really painful to do it in plain XSLT.
My class is currently doing Binary Trees as part of our data structures unit. I understand that the Node class must have 3 parameters (value, left node, right node) in its constructor. As part of a requirement, we must have a Tree class. What is the purpose of a tree class? Is it for managing the entire set of Nodes? Does it simply contain the functions necessary to insert, remove and search for specific Nodes?
Thank you in advance.
My Node class:
class Node {
protected int data;
protected leftNode;
protected rightNode;
Node (int data, Node leftNode, Node rightNode){
this.data = data;
this.leftNode = leftNode;
this.rightNode = rightNode;
}
}
Yes, it is supposed to give the functional interface of a Tree by encapsulating all the behavior and algorithms related to the internal structure.
Why this is good?
Because you will define something that just provide some functionality and that works in a stand-alone way so that everyone should be able to use your tree class without caring about nodes, algorithms and whatever.
Ideally the class should be parametric so that you'll have Tree<T> and you'll be able to have generic methods for example
T getRoot()
Basically you'll have to project it so that it will allow you to
insert values
delete values
search values
visit the whole tree
whatever
As I see it, a Tree class should hold a reference to the root node of the tree, and to methods and attributes that operate on the tree as a whole, as opposed to methods that belong to each node, like getLeft(), getValue().
For example, I'd define a size attribute in the Tree class, and the methods that add or remove nodes to the tree (which also happen to be in this class) would be responsible for keeping the size up to date.
The purpose of any data structure is to give you a way to hold onto collections of related values and manipulate them in meaningful ways.
A tree is a special kind of data structure. It's a good choice for data that is hierarchical: those with natural parent-child relationships. A binary tree has, at most, two children for every parent node.
One other feature of trees that deserves special mention is the fact that it's self-similar: every Node in a tree is itself the root of a sub-tree. Recursion exploits this fact.
Yes, those are good methods to start with. You might want to have a look at the java.util.Map interface for others that might be useful.
The assumption is, as you suspect, a little bit flawed. The Node type that you have defined is a perfectly fine instance of a binary tree.
Jack mentions that you want to have a Tree type around the whole set of nodes in order to provide operations like getRoot, inserts, deletes and so on. This is of course not a bad idea, but it is by no means a requirement. Many of these operations could go on Node itself, and you don't necessarily have to have all of these operations; for example, there are arguments both for and against having a getRoot operation. An argument against it is that if you have an algorithm that at one point only needs a subtree, then the Tree object that holds on to the root Node prevents garbage collection of Nodes that your algorithm no longer needs.
But more generally, the real question that needs to be asked here is this: which of the things you're dealing are interface, and which are implementation? Because it's one thing if you want to expose binary trees as an interface to callers, and another if you want to provide some sort of finite map as the interface, and the binary tree is just an implementation detail. In the former case, the client would "know" that a tree is either null or a Node with a value and branches, and would be allowed to, for example, recurse on the structure of the tree; in the latter case, all that the client would "know" is that your class supports some form of put, get and delete operations, but it's not allowed to rely on the fact that this is stored as a search tree. In many variants of the latter case you indeed would use a Tree type as a front end to hide the nodes from the clients. (For example, Java's built-in java.util.TreeMap class does this.)
So the shortest answer is, really, it depends. The slightly longer answer is it depends on what the contract is between the binary tree implementation and its users; what things are the callers allowed to assume about the trees, what details of the binary tree are things that the author expects to be able to change in the future, etc.
"Is it for managing the entire set of Nodes?"
Yes.
"Does it simply contain the functions necessary to insert, remove and search for specific Nodes?"
Basically, yes. For that reason, it should contain at least a reference to the root node.
I'm looking for any implementation of a pure Tree data structure for Java (that aren't graphical ones in java.awt), preferably generic.
With a generic tree I'd like to add elements that are not supposed to be sorted and do something like this:
TreeNode anotherNode = new TreeNode();
node.add(anotherNode);
…and then I'd like to traverse the nodes (so that I can save and preserve the structure in a file when I load the tree from the same file again).
Anyone knows what implementations exist or have any other idea to achieve this?
You can use the DefaultMutableTreeNode defined in the javax.swing.tree package. It contains methods getUserObject() and setUserObject(Object) allowing you to attach data to each tree node. It allows an arbitrary number of child nodes for each parent and provides methods for iterating over the tree in a breadth-first or depth-first fashion (breadthFirstEnumeration() / depthFirstEnumeration()).
Also note that despite being resident in the javax.swing.tree package this class does not contain any UI code; It is merely the underlying model of JTree.
Scala has a nice Tree data structure. It's a "General Balanced Tree." It's not exactly Java, but it's close, and can serve as a good model.
It is hard to believe, given how much is in the base Java libraries, but there is no good generic Tree structure.
For a start, the TreeSet and TreeMap in the runtime are red-black-tree implementations.
Assuming you don't want to store arbitrary Java objects on the nodes, you could use the W3C DOM. It even comes with its own serialization format (I forget what it's called :-).