Size-limited queue that holds last N elements in Java - collections

A very simple & quick question on Java libraries: is there a ready-made class that implements a Queue with a fixed maximum size - i.e. it always allows addition of elements, but it will silently remove head elements to accomodate space for newly added elements.
Of course, it's trivial to implement it manually:
import java.util.LinkedList;
public class LimitedQueue<E> extends LinkedList<E> {
private int limit;
public LimitedQueue(int limit) {
this.limit = limit;
}
#Override
public boolean add(E o) {
super.add(o);
while (size() > limit) { super.remove(); }
return true;
}
}
As far as I see, there's no standard implementation in Java stdlibs, but may be there's one in Apache Commons or something like that?

Apache commons collections 4 has a CircularFifoQueue<> which is what you are looking for. Quoting the javadoc:
CircularFifoQueue is a first-in first-out queue with a fixed size that replaces its oldest element if full.
import java.util.Queue;
import org.apache.commons.collections4.queue.CircularFifoQueue;
Queue<Integer> fifo = new CircularFifoQueue<Integer>(2);
fifo.add(1);
fifo.add(2);
fifo.add(3);
System.out.println(fifo);
// Observe the result:
// [2, 3]
If you are using an older version of the Apache commons collections (3.x), you can use the CircularFifoBuffer which is basically the same thing without generics.
Update: updated answer following release of commons collections version 4 that supports generics.

Guava now has an EvictingQueue, a non-blocking queue which automatically evicts elements from the head of the queue when attempting to add new elements onto the queue and it is full.
import java.util.Queue;
import com.google.common.collect.EvictingQueue;
Queue<Integer> fifo = EvictingQueue.create(2);
fifo.add(1);
fifo.add(2);
fifo.add(3);
System.out.println(fifo);
// Observe the result:
// [2, 3]

I like #FractalizeR solution. But I would in addition keep and return the value from super.add(o)!
public class LimitedQueue<E> extends LinkedList<E> {
private int limit;
public LimitedQueue(int limit) {
this.limit = limit;
}
#Override
public boolean add(E o) {
boolean added = super.add(o);
while (added && size() > limit) {
super.remove();
}
return added;
}
}

Use composition not extends (yes I mean extends, as in a reference to the extends keyword in java and yes this is inheritance). Composition is superier because it completely shields your implementation, allowing you to change the implementation without impacting the users of your class.
I recommend trying something like this (I'm typing directly into this window, so buyer beware of syntax errors):
public LimitedSizeQueue implements Queue
{
private int maxSize;
private LinkedList storageArea;
public LimitedSizeQueue(final int maxSize)
{
this.maxSize = maxSize;
storageArea = new LinkedList();
}
public boolean offer(ElementType element)
{
if (storageArea.size() < maxSize)
{
storageArea.addFirst(element);
}
else
{
... remove last element;
storageArea.addFirst(element);
}
}
... the rest of this class
A better option (based on the answer by Asaf) might be to wrap the Apache Collections CircularFifoBuffer with a generic class. For example:
public LimitedSizeQueue<ElementType> implements Queue<ElementType>
{
private int maxSize;
private CircularFifoBuffer storageArea;
public LimitedSizeQueue(final int maxSize)
{
if (maxSize > 0)
{
this.maxSize = maxSize;
storateArea = new CircularFifoBuffer(maxSize);
}
else
{
throw new IllegalArgumentException("blah blah blah");
}
}
... implement the Queue interface using the CircularFifoBuffer class
}

The only thing I know that has limited space is the BlockingQueue interface (which is e.g. implemented by the ArrayBlockingQueue class) - but they do not remove the first element if filled, but instead block the put operation until space is free (removed by other thread).
To my knowledge your trivial implementation is the easiest way to get such an behaviour.

You can use a MinMaxPriorityQueue from Google Guava, from the javadoc:
A min-max priority queue can be configured with a maximum size. If so, each time the size of the queue exceeds that value, the queue automatically removes its greatest element according to its comparator (which might be the element that was just added). This is different from conventional bounded queues, which either block or reject new elements when full.

An LRUMap is another possibility, also from Apache Commons.
http://commons.apache.org/collections/apidocs/org/apache/commons/collections/map/LRUMap.html

Ok I'll share this option. This is a pretty performant option - it uses an array internally - and reuses entries. It's thread safe - and you can retrieve the contents as a List.
static class FixedSizeCircularReference<T> {
T[] entries
FixedSizeCircularReference(int size) {
this.entries = new Object[size] as T[]
this.size = size
}
int cur = 0
int size
synchronized void add(T entry) {
entries[cur++] = entry
if (cur >= size) {
cur = 0
}
}
List<T> asList() {
int c = cur
int s = size
T[] e = entries.collect() as T[]
List<T> list = new ArrayList<>()
int oldest = (c == s - 1) ? 0 : c
for (int i = 0; i < e.length; i++) {
def entry = e[oldest + i < s ? oldest + i : oldest + i - s]
if (entry) list.add(entry)
}
return list
}
}

public class ArrayLimitedQueue<E> extends ArrayDeque<E> {
private int limit;
public ArrayLimitedQueue(int limit) {
super(limit + 1);
this.limit = limit;
}
#Override
public boolean add(E o) {
boolean added = super.add(o);
while (added && size() > limit) {
super.remove();
}
return added;
}
#Override
public void addLast(E e) {
super.addLast(e);
while (size() > limit) {
super.removeLast();
}
}
#Override
public boolean offerLast(E e) {
boolean added = super.offerLast(e);
while (added && size() > limit) {
super.pollLast();
}
return added;
}
}

Related

JavaFX ListChangeListener: getPermutation() not working

I use ListChangeListener to listen to changes in Tab Pane.
private final TabPane tabBar = new TabPane();
...
tabBar.getTabs().addListener(this::tabsChanged);
I'm trying to listen to tab move events with the following code:
private void tabsChanged(ListChangeListener.Change<? extends Tab> change) {
while (change.next()) {
if (change.wasPermutated()) {
for (int i = change.getFrom(); i < change.getTo(); i++) {
System.out.println(i + " -> " + change.getPermutation(i));
}
}
}
}
As JavaFX documentation says:
In order to get the new position of an element, you must call:
change.getPermutation(oldIndex). Returns: the new index of the same
element.
But in my case change.getPermutation(i) always returns just i.
For example, I have 4 tabs.
Their indexes are: 0, 1, 2, 3.
Then I move the 4th tab to the first position.
I expect the following output:
0 -> 1
1 -> 2
2 -> 3
3 -> 0
But I get:
0 -> 0
1 -> 1
2 -> 2
3 -> 3
How can I make it work as I need?
As already noted in the comments: the behavior you observe is a bug just reported as JDK-8278062 - the doc and your expectation based on the doc is correct, the notification (implemented in the internal class TabObservableList) is wrong.
Normally, if we want to find the newIndex, a listChangeListener would do something like:
for (int oldIndex = c.getFrom(); oldIndex < c.getTo(); ++oldIndex) {
int newIndex = c.getPermutation(oldIndex);
...
}
To work around the issue, we could manually keep a copy of the tabs, lookup the tab at the old index and find its new index in the re-ordered tabs:
for (int oldIndex = c.getFrom(); oldIndex < c.getTo(); ++oldIndex) {
Tab tab = copy.get(oldIndex);
int newIndex = c.getList().indexOf(tab);
...
}
// update the copy
Or we could have some fun and implement a TransformationList around the original tabs that does the work for us :) It jumps in when it detects a permutation and fires the correct notification. Note that the only internal class used below is SourceChangeAdapter, we either need to relax encapsulation or c&p its content (it is doing nothing but pass on notifications on behalf of the wrapper)
public class TabObservableListWrapper extends TransformationList<Tab, Tab> {
// copy of source used to build the correct permutation
private ObservableList<Tab> copy = FXCollections.observableArrayList();
public TabObservableListWrapper(ObservableList<Tab> source) {
super(source);
updateCopy();
}
#Override
protected void sourceChanged(Change<? extends Tab> c) {
// TBD: cope with a change that has
// - a mixture of permutation and other subchanges
// - multiple subchanges of type permutation
boolean isPermutation = false;
// check if the change is a permutation
while (c.next()) {
if (c.wasPermutated()) {
isPermutation = true;
break;
}
}
c.reset();
if (isPermutation) {
beginChange();
updatePermutation(c);
endChange();
} else {
// assuming other change type notifications are correct, just delegate
fireChange(new SourceAdapterChange<>(this, c));
}
// keep copy sync'ed to source
updateCopy();
}
/**
* Converts the incorrect permutation notification from source
* into a correct one and let super fire the appropriate change.
*
* Note: this method must be called inside a begin/endChange block.
* #param c a change with a single subChange of type wasPermutated
*/
private void updatePermutation(Change<? extends Tab> c) {
c.next();
int from = c.getFrom();
int to = c.getTo();
int permSize = to - from;
int[] perm = new int[permSize];
// fill the perm
for(int i = 0; i < permSize; i++) {
int oldIndex = from + i;
Tab tab = copy.get(oldIndex);
perm[i] = c.getList().indexOf(tab);
}
nextPermutation(from, to, perm);
}
// keep copy sync'ed
private void updateCopy() {
copy.setAll(getSource());
}
// implement public methods by delegating 1:1 to source
#Override
public int getSourceIndex(int index) {
return index;
}
#Override
public int getViewIndex(int index) {
return index;
}
#Override
public Tab get(int index) {
return getSource().get(index);
}
#Override
public int size() {
return getSource().size();
}
}
To use, wrap it around a tabPane's tab list and listen to the wrapper instead of directly to original list, something like:
TabObservableListWrapper wrapper = new TabObservableListWrapper(tabPane.getTabs());
wrapper.addListener((ListChangeListener<Tab>)change -> {
while (change.next()) {
if (change.wasPermutated()) {
System.out.println("from wrapper:");
for (int oldIndex = change.getFrom(); oldIndex < change.getTo(); oldIndex++) {
System.out.println(oldIndex + " -> " + change.getPermutation(oldIndex));
}
}
}
});

Qt novice: base class for QLineEdit and QTextEdit

Is there another class besides QWidget which holds all generic functions for both? Something like QEdit...
As an example I'd like to reference cut(), copy() and paste(), but it looks like I have to dynamic cast the QWidget. Is there any other way?
There is no other way besides QWidget. The reason is that QLineEdit is inherited directly from QWidget. You can see the full hierarchy of Qt classes here
You don't have to dynamic-cast anything: this is typically a sign of bad design. Qt generally has very few interface classes - they usually have the word Abstract somewhere in the name, and are not really pure interfaces as they have non-abstract base classes, like e.g. QObject. Thus there was no pattern to follow, and no need for abstracting out the edit operations into an interface.
There are several approaches to overcome this:
Leverage the fact that the methods in question are known by the metaobject system. Note that invokeMethod takes a method name, not signature.
bool cut(QWidget * w) {
return QMetaObject::invokeMethod(w, "cut");
}
bool copy(QWidget * w) {
return QMetaObject::invokeMethod(w, "copy");
}
//...
You can use the free-standing functions such as above on any widget that supports the editing operations.
As above, but cache the method lookup not to pay its costs repeatedly. Note that indexOfMethod takes a method signature, not merely its name.
static QMetaMethod lookup(QMetaObject * o, const char * signature) {
return o->method(o->indexOfMethod(signature));
}
struct Methods {
QMetaMethod cut, copy;
Methods() {}
explicit Methods(QMetaObject * o) :
cut(lookup(o, "cut()")),
copy(lookup(o, "copy()")) {}
Methods(const Methods &) = default;
};
// Meta class names have unique addresses - they are effectively memoized.
// Dynamic metaobjects are an exception we can safely ignore here.
static QMap<const char *, Methods> map;
static const Methods & lookup(QWidget * w) {
auto o = w->metaObject();
auto it = map.find(o->className());
if (it == map.end())
it = map.insert(o->className(), Methods(o));
return *it;
}
bool cut(QWidget * w) {
lookup(w).cut.invoke(w);
}
bool copy(QWidget * w) {
lookup(w).copy.invoke(w);
}
//...
Define an interface and provide implementations specialized for widget types. This approach's only benefit is that it's a bit faster than QMetaMethod::invoke. It makes little sense to use this code for clipboard methods, but it could be useful to minimize overhead for small methods that are called very often. I'd advise not to over-engineer it unless a benchmark shows that it really helps. The previous approach (#2 above) should be quite sufficient.
// Interface
class IClipboard {
public:
virtual cut(QWidget *) = 0;
virtual copy(QWidget *) = 0;
virtual paste(QWidget *) = 0;
};
class Registry {
// all meta class names have unique addresses - they are effectively memoized
static QMap<const char *, IClipboard*> registry;
public:
static void register(const QMetaObject * o, IClipboard * clipboard) {
auto name = o->className();
auto it = registry.find(name);
if (it == registry.end())
registry.insert(name, clipboard);
else
Q_ASSERT(it->value() == clipboard);
}
static IClipboard * for(QWidget * w) {
auto it = registry.find(w->metaObject()->className());
Q_ASSERT(registry.end() != it);
return it->value();
}
static void unregister(const QMetaObject * o) {
registry.remove(o->className());
}
};
template <class W> class ClipboardWidget : public IClipboard {
Q_DISABLE_COPY(ClipboardWidget)
public:
cut(QWidget * w) override { static_cast<W*>(w)->cut(); }
copy(QWidget * w) override { static_cast<W*>(w)->copy(); }
paste(QWidget * w) override { static_cast<W*>(w)->paste(); }
ClipboardWidget() {
Registry::register(&W::staticMetaObject(), this);
}
~ClipboardWidget() {
Registry::unregister(&W::staticMetaObject());
}
};
// Implementation
QMap<const char *, IClipboard*> Registry::registry;
static ClipboardWidget<QTextEdit> w1;
static ClipboardWidget<QLineEdit> w2;
void yourCode() {
//...
Registry::for(widget)->cut(widget);
}

Sum up the tree nodes using Java 8 Streams

Is it possible to sum up the nodes of a tree using Java 8 streams, if possible in a one liner ?
Here is a node class
public class Node
{
private int nodeNum;
ArrayList<Node> children = new ArrayList<>();
public Node(int num)
{
this.nodeNum = num;
}
public int getNodeNum()
{
return nodeNum;
}
public boolean addNode(Node node)
{
return children.add(node);
}
public ArrayList<Node> getNodes()
{
return this.children;
}
}
Normal way to solve this is using a recursion and sum up the node , like the code below.
int getNodeSum(Node node)
{
int total = 0;
if(node.children.isEmpty())
return node.getNodeNum();
else
{
for(Node tempNode:node.children)
{
total+= getNodeSum(tempNode);
}
return total+node.getNodeNum();
}
}
We can use streams to sum up the immediate child nodes but I'm not getting how to move deep and do it recursively using Streams.
This code only solves the problem to a single level. Any ideas?
total = list.stream().filter(Node -> node.children.isEmpty()).map(Node:: getNodeNum).reduce(node.getNodeNum(), (a,b) -> a+b);
One solution to your problem would be to use recursion along with Stream.flatMap.
First, you'd need to add the following helper method to your Node class:
public Stream<Node> allChildren() {
return Stream.concat(
Stream.of(this),
this.children.stream().flatMap(Node::allChildren)); // recursion here
}
This returns a Stream<Node> whose elements are this node and all its descendant nodes.
Then, you could rewrite your getNodeSum method as follows:
int getNodeSum(Node node) {
return node.allChildren()
.mapToInt(Node::getNodeNum)
.sum();
}
This uses the above defined Node.allChildren method along with the Stream.mapToInt and IntStream.sum methods to calculate the total sum.
Alternatively, you could have a Function<Node, Stream<Node>> descendants attribute in your Node class that performs the recursion in place:
private Function<Node, Stream<Node>> descendants =
node -> Stream.concat(
Stream.of(node),
node.children.stream()
.flatMap(this.descendants)); // recursion here: function invoked again
This is a recursive lambda expression, since the function you are defining is at both sides of the = sign. This kind of lambda expressions are allowed only as attributes of a class, i.e. you cannot assign a recursive lambda expression to a local variable.
With that recursive function in place, you could rewrite the allChildren method as follows:
public Stream<Node> allChildren() {
return descendants.apply(this);
}
Finally, the code for your getNodeSum method would be identical to the previous version:
int getNodeSum(Node node) {
return node.allChildren()
.mapToInt(Node::getNodeNum)
.sum();
}
Note: while this approach might result attractive for some people, it might have some drawbacks, i.e. now every instance of the Node class has the descendants attribute, despite not being needed at all. You could circumvect this i.e. by having a Tree class with this recursive function as an attribute, and Node being an inner class (with the descendants attribute removed).
You need to add recusive method for Node class, which wil be join child streams
public Stream<Node> recursiveConcat() {
return Stream.concat(
Stream.of(this),
children.stream().flatMap(Node::recursiveConcat));
}
Then do -
root.recusiveConcat().mapToInt(Node::getNodeNum).sum()
whole code
public class Node {
private int nodeNum;
ArrayList<Node> children = new ArrayList<>();
public Node(int num) {
this.nodeNum = num;
}
public int getNodeNum() {
return nodeNum;
}
public boolean addNode(Node node) {
return children.add(node);
}
public ArrayList<Node> getNodes() {
return this.children;
}
public Stream<Node> recursiveConcat() {
return Stream.concat(
Stream.of(this),
children.stream().flatMap(Node::recursiveConcat));
}
}
Node root = new Node(1);
Node node1 = new Node(2);
Node node2 = new Node(3);
Node node3 = new Node(4);
node2.addNode(node3);
node1.addNode(node2);
root.addNode(node1);
System.out.println(root.recursiveConcat().mapToInt(Node::getNodeNum).sum());

Retouching several images in several Task

Generalities : explanations about my program and its functioning
I am working on a photo-retouching JavaFX application. The final user can load several images. When he clicks on the button REVERSE, a Task is launched for each image using an Executor. Each of these Task executes the reversal algorithm : it fills an ArrayBlockingQueue<Pixel> (using add method).
When the final user clicks on the button REVERSE, as I said, these Task are launched. But just after these statements, I tell the JavaFX Application Thread to draw the Pixel of the ArrayBlockingQueue<Pixel> (using remove method).
Thus, there are parallelism and concurrency (solved by the ArrayBlockingQueue<Pixel>) between the JavaFX Application Thread and the Task, and between the Task themselves.
To draw the Pixel of the ArrayBlockingQueue<Pixel>, the JavaFX Application Thread starts an AnimationTimer. The latter contains the previously-mentionned remove method. This AnimationTimer is started for each image.
I think you're wondering yourself how this AnimationTimer can know to what image belongs the Pixel it has removed ? In fact, each Pixel has an attribute writable_image that specifies the image to what it belongs.
My problems
Tell me if I'm wrong, but my program should work. Indeed :
My JavaFX Application Thread is the only thread that change the GUI (and it's required in JavaFX) : the Task just do the calculations.
There is not concurrency, thanks to the BlockingQueue I use (in particular, there isn't possibility of draining).
The AnimationTimer knows to what image belongs each Pixel.
However, it's (obviously !) not the case (otherwise I wouldn't have created this question haha !).
My problem is that my JavaFX Application freezes (first problem), after having drawn only some reversed pixels (not all the pixels). On the last loaded image moreover (third problem).
A detail that could be the problems' cause
But I would need your opinion.
The AnimationTimer of course doesn't draw the reversed pixels of each image directly : this is animated. The final user can see each pixel of an image being reversed, little by little. It's very practical in other algorithms as the creation of a circle, because the user can "look" how the algorithm works.
But to do that, the AnimationTimer needs to read a variable called max. This variable is modified (writen) in... each Task. But it's an AtomicLong. So IF I AM NOT WRONG, there isn't any problem of concurrency between the Task themselves, or between the JavaFX Application Thread and these Task.
However, it could be the problem : indeed, the max's value could be 2000 in Task n°1 (= in image n°1), and 59 in Task n°2 (= in image n°2). The problem is the AnimationTimer must use 2000 for the image n°1, and 59 for the n°2. But if the Task n°1 et n°2 have finished, the only value known by the AnimationTimer would be 59...
Sources
When the user clicks on the button REVERSE
We launch the several Task and start several times the AnimationTimer. CLASS : RightPane.java
WritableImage current_writable_image;
for(int i = 0; i < this.gui.getArrayListImageViewsImpacted().size(); i++) {
current_writable_image = (WritableImage) this.gui.getArrayListImageViewsImpacted().get(i).getImage();
this.gui.getGraphicEngine().executor.execute(this.gui.getGraphicEngine().createTask(current_writable_image));
}
for(int i = 0; i < this.gui.getArrayListImageViewsImpacted().size(); i++) {
current_writable_image = (WritableImage) this.gui.getArrayListImageViewsImpacted().get(i).getImage();
this.gui.getImageAnimation().setWritableImage(current_writable_image);
this.gui.getImageAnimation().startAnimation();
}
The Task are part of the CLASS GraphicEngine, which contains an Executor :
public final Executor executor = Executors.newCachedThreadPool(runnable -> {
Thread t = new Thread(runnable);
t.setDaemon(true);
return t ;
});
public Task createTask(WritableImage writable_image) {
int image_width = (int) writable_image.getWidth(), image_height = (int) writable_image.getHeight();
Task ret = new Task() {
protected Void call() {
switch(operation_to_do) {
case "reverse" :
gui.getImageAnimation().setMax(image_width*image_height); // USE OF "MAX" VARIABLE
reverseImg(writable_image);
break;
}
return null;
}
};
return ret;
}
The same CLASS, GraphicEngine, also contains the reversal algorithm :
private void reverseImg(WritableImage writable_image) {
int image_width = (int) writable_image.getWidth(), image_height = (int) writable_image.getHeight();
BlockingQueue<Pixel> updates = gui.getUpdates();
PixelReader pixel_reader = writable_image.getPixelReader();
double[] rgb_reversed;
for (int x = 0; x < image_width; x++) {
for (int y = 0; y < image_height; y++) {
rgb_reversed = PhotoRetouchingFormulas.reverse(pixel_reader.getColor(x, y).getRed(), pixel_reader.getColor(x, y).getGreen(), pixel_reader.getColor(x, y).getBlue());
updates.add(new Pixel(x, y, Color.color(rgb_reversed[0], rgb_reversed[1], rgb_reversed[2], pixel_reader.getColor(x, y).getOpacity()), writable_image));
}
}
}
Finally, here is the code of the CLASS AnimationTimer. There is nothing particular. Note the variable max is used here too (and in the CLASS GraphicEngine : setMax).
public class ImageAnimation extends AnimationTimer {
private Gui gui;
private AtomicLong max, speed, max_delay;
private long count, start;
private WritableImage writable_image;
ImageAnimation (Gui gui) {
this.gui = gui;
this.count = 0;
this.start = -1;
this.max = new AtomicLong(Long.MAX_VALUE);
this.max_delay = new AtomicLong(999_000_000);
this.speed = new AtomicLong(this.max_delay.get());
}
public void setMax(long max) {
this.max.set(max);
}
public void setSpeed(long speed) { this.speed.set(speed); }
public double getMaxDelay() { return this.max_delay.get(); }
#Override
public void handle(long timestamp) {
if (start < 0) {
start = timestamp ;
return ;
}
ArrayList<Pixel> list_sorted_pixels = new ArrayList<>();
BlockingQueue<Pixel> updates = this.gui.getUpdates();
for(Pixel new_pixel : updates) {
if(new_pixel.getWritableImage() == writable_image) {
list_sorted_pixels.add(new_pixel);
}
}
while (list_sorted_pixels.size() > 0 && timestamp - start > (count * this.speed.get()) / (writable_image.getWidth()) && !updates.isEmpty()) {
Pixel update = list_sorted_pixels.remove(0);
updates.remove(update);
count++;
if (update.getX() >= 0 && update.getY() >= 0) {
writable_image.getPixelWriter().setColor(update.getX(), update.getY(), update.getColor());
}
}
if (count >= max.get()) {
this.count = 0;
this.start = -1;
this.max.set(Long.MAX_VALUE);
stop();
}
}
public void setWritableImage(WritableImage writable_image) { this.writable_image = writable_image; }
public void startAnimation() {
this.start();
}
}

Setting up TableColumns Value using Generic Types

I wanted to program a TableBrowser for a MYSQl Database in JavaFX.
My first problem is: i dont know which types i get back from the Database.
So i decided to wrap those types with a Wrapper-class.
To show these values on the GUI, i used the TableColumns setCellValueFactory-method, which
needs a value, that implements ObservableValue.
So i tried to implement the ObservableValue-interface.
But when i run the program it doesnt show the right Values.
TableBrowser after connecting to the Database
Has anyone an idea where i did wrong or knows a more recommended way to implement it ?
Here is the Part of the Code from the TableBrowser
/*
* this variable is used to iterate over the tableview's columns.
* It is a class variable, because it is not possible (for some reasons)
* to use a local variable while working with it in the context of Lambda-expressions
*/
int t = 0;
// those two variables are defined in the class Body
private final TableView<Entry> tableview = new TableView<>();
private final ObservableList<Entry> columndata = FXCollections.observableArrayList();
// the following Code is inside the Button's Actionlistener
for(int i = 1; i <= maxcol; i++) // adds a new TableColum for every colum in the DB
{
tableview.getColumns().add(new TableColumn<Entry, String>rsmd.getColumnName(i)));
}
// iterates over the ResultSet
while(rs.next())
{
// this is the dataset i put in my TableView
Entry row = new Entry(maxcol);
// for each Column i add the columnvalue to the current dataset
for(int i = 1; i <= maxcol; i++)
{
int type = rsmd.getColumnType(i);
Object value = rs.getObject(i);
row.setCellValue(i-1, type, value);
}
// adds a new dataset to the ObservableList<Entry>
columndata.add(row);
}
// puts all datasets in the TableView
tableview.setItems(columndata);
// iterates over all Columns
for(t = 0; t < tableview.getColumns().size(); t++)
{
// should set the CellValueFactory for each Column so it shows the data
/*
* I apologise if there a horrible mistake.
* I never worked with Lamda before and just copied it form an example page :)
*/
tableview.getColumns().get(t).setCellValueFactory(celldata -> celldata.getValue().getCellValue(t-1));
}
This is my Entry class, which is an inner Class in TableBrowserclass
/*
* should represent a Dataset.
* Has an array, which holdes every columnvalue as a WrapperType
*/
private class Entry
{
WrapperType<?>[] columns;
private Entry(int columncount)
{
columns = new WrapperType[columncount];
}
private WrapperType<?> getCellValue(int col)
{
return columns[col];
}
private void setCellValue(int col, int type, Object value)
{
columns[col] = MySQLTypeWrapper.getInstance().wrapType(type, value);
}
}
Here is the MySQLTypeWrapper class, which holds the WrapperType as an inner class
public class MySQLTypeWrapper
{
public WrapperType<?> wrapType(int type, Object Value)
{
Class<?> typeclass = toClass(type);
return new WrapperType<>(typeclass.cast(Value));
}
/*
* returns the appropriate class def for every database type
* Expl: VARCHAR returns String.class
*/
private static Class<?> toClass(int type) {...}
/*
* I copied the content of the of the overridden Methods from StringPropertyBase
* as i have clue how to implement ObservableValue
*/
class WrapperType<T> implements ObservableValue<WrapperType<T>>
{
private T value;
private ExpressionHelper<WrapperType<T>> helper = null;
private WrapperType(T value)
{
this.value = value;
}
#Override
public void addListener(InvalidationListener listener)
{
helper = ExpressionHelper.addListener(helper, this, listener);
}
#Override
public void removeListener(InvalidationListener listener)
{
helper = ExpressionHelper.removeListener(helper, listener);
}
#Override
public void addListener(ChangeListener<? super WrapperType<T>> listener)
{
helper = ExpressionHelper.addListener(helper, this, listener);
}
#Override
public void removeListener(ChangeListener<? super WrapperType<T>> listener)
{
helper = ExpressionHelper.removeListener(helper, listener);
}
#Override
public WrapperType<T> getValue()
{
return this;
}
public String toString()
{
return value.toString();
}
}
}
Thanks for your help in advance :)
As mentioned in the comments, your first problem was not using the TableView's Items property.
For the second part - one solution would be to create a helper method along the lines of
private <T> Callback<TableColumn.CellDataFeatures<Entry,T>,ObservableValue<T>> createCellFactory(int columnIndex) {
return celldata -> celldata.getValue().getCellValue(columnIndex);
}
and then change the loop to
// Now t can be a local variable, as it is not directly passed to the lambda.
for(int t = 0; t < tableview.getColumns().size(); t++)
{
// should set the CellValueFactory for each Column so it shows the data
tableview.getColumns().get(t).setCellValueFactory(createCellFactory(t));
}
Note that this time the variable passed to the lambda is a local effectively-final variable and not an instance variable, so the lambda is created with the correct value every time.
One last word of advice - are you sure you need this amount of generality? What I mean is - it is usually better to create a class to directly represent your DB structure with proper getters and setters, then you can use PropertyValueFactory.

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