So my problem is that im currently trying to use java's reflection to traverse a tree like structure. The problem is the only thing i know about each structure is that it can contain one of three things. Strings (the leaf's) Other Objects, Or Lists of other objects. Using reflection i want to do a DFS of the tree until i find a node that im looking for. My problem seems to be that when i use reflection to get a field that happens to be of type List i get back List and i am unable to down cast the the correct type. here are some samples i have tried.
Using Fields
Object returnObj = new Object();
Field field = object.getClass().getDeclaredField(fieldClassName);
field.setAccessible(true);
List<DistributionPartnerRoleType> test = (List<DistributionPartnerRoleType>) field.get(object);
And using Methods
String methodName = "get" + Character.toUpperCase(fieldClassName.charAt(0)) + fieldClassName.substring(1);
Method[] getters = object.getClass().getMethods();
Method getter = getMethod(getters, methodName);
Type returnType = getter.getGenericReturnType();
if(returnType instanceof ParameterizedType){
Type actualType = ((ParameterizedType) returnType).getActualTypeArguments()[0];
Class actualClass = (Class) actualType;
returnObj = getter.invoke(object, null);
List<Object> newList = new ArrayList<Object>();
for(Object obj : (List<Object>)returnObj){
newList.add(actualClass.cast(obj));
}
returnObj = newList;
}
Im aware that the problem is that the objects are truly of type Object but the function and fields are explicitly of type List as declared in the code
protected List<DistributionPartnerRoleType> distributionPartnerRole;
public List<DistributionPartnerRoleType> getDistributionPartnerRole() {
if (distributionPartnerRole == null) {
distributionPartnerRole = new ArrayList<DistributionPartnerRoleType>();
}
return this.distributionPartnerRole;
}
If anyone knows of a solution for this problem that would be great, Or if i need to go about a different method other then reflection.
To sum up my problem. Invoke is returning a List But the objects inside the list are not actually of the type this function returns they are of type java.lang.Object Is there any way to get around this or is dynamic access of the lists objects not possible?
I don't think use of reflection is a good idea in this case. In my view there are very few appropriate use cases. It's often a sign that you need to rethink your approach.
In this case I suggest you look at the Visitor design pattern. The visitor itself can include the logic for a depth-first search as well as dealing with the various types of data in the nodes.
In your case the pattern might look something like:
interface Node {
void accept(NodeVisitor visitor);
}
class StringNode implements Node {
public String getValue();
public void accept(NodeVisitor visitor) {
visitor.visit(this);
}
}
class IntegerNode implements Node {
public int geValue();
public void accept(NodeVisitor visitor) {
visitor.visit(this);
}
}
class CompositeNode implements Node {
public void forEachChild(Consumer<Node> action);
public void accept(NodeVisitor visitor) {
visitor.visit(this);
}
}
interface NodeVisitor {
default void visit(StringNode node) {}
default void visit(IntegerNode node) {}
default void visit(CompositeNode node) {}
}
Now your search algorithm might look like:
class IntegerSearch implements NodeVisitor {
private final int target;
private final List<IntegerNode> results = new ArrayList<>();
void visit(IntegerNode node) {
if (node.getValue() == target)
results.add(node);
}
}
void visit(CompositeNode node) {
node.forEachChild(child -> child.accept(this));
}
}
No reflection, casting or other dodgy idioms!
This might not exactly match your case (e.g. perhaps the nodes with values are also composites?) but hopefully you see the general pattern for avoiding reflection.
There is a deeper problem here, As stated by Louis Wasserman, reflection should not be causing this and there seems to be a problem with my code in another location.
Say I've got a Java file with 500 custom objects of type Item. Now, say I want users to be able to add/remove any one of those objects to that list in the program. I want to try to avoid doing something like this:
public class Inventory {
public static ArrayList<Item> inv = new ArrayList<>();
public static void addItem1 {
inv.add(Item.Item1); //Pulling from Item class
} //addItem1()
public static void removeItem1 {
inv.remove(Item.Item1);
} //removeItem1()
public static void addItem 2 {
. . .
}
. . .
}
By doing that, I'd have to make an add and a remove function for every single item. There will be hundreds of items, so I sincerely hope there's a better way for the user to be able to do so from inside of the program. This would further be awful because of how it would require some massive nested switch statements to swap out everything.
Instead I'd hope to implement a single adder method and a single remover method, that could take the user's input (a String with the literal name of the Item they are trying to add), and somehow find/select the Item. Implementation thoughts?
How about using generic class ?
public class Inventory<T> {
public static ArrayList<Item> inv = new ArrayList<>();
public void addItem (T item){
inv.add((Item)item); // where item can be anything from Item.item1 to Item.item500
}
public void removeItem (T item){
inv.remove((Item)item);
}
In that case, to see if your item is in fact an item do something similar to this: System.out.println(item.getClass().getName()); //should return Item
Perhaps use:
public void setItem(Item item){
inv.add(item);
}
Then use the same concept for removal.
I have an object that has a variable that I want to be able to hold either a queue or a stack. Anything that has an add and a remove with the appropriate logics. I think this can be done with an interface but the two in java.util doesn't have the same interface or even the same name for the two operations.
My plan right now is to create a wrapper to make them fit what I want, but this seems inelegant. Is there a better way?
I want something like:
Something<E> steps;
So that I can call step.pop() and step.push() or whatever method names without having to know if steps implements queue logic or stack logic.
You might want either ArrayDeque or LinkedList depending on your needs.
Both implement Deque (double ended queue).
From the Javadoc on ArrayDeque: "This class is likely to be faster than Stack when used as a stack, and faster than LinkedList when used as a queue."
Elements can be added or removed from either end of a Deque.
A Deque can be used as a queue by calling addLast and removeFirst, and can also be used by a stack by using addLast and removeLast.
If you really want it to behave like either one, you can keep a boolean flag and write helper methods, or you can write a class:
public class QueueOrStack<E> implements Iterable<E> {
private Deque<E> container = new ArrayDeque<E>();
private boolean isQueue;
public QueueOrStack(boolean isQueue) {
this.isQueue = isQueue;
}
public E pop() {
return isQueue ? container.removeFirst() : container.removeLast();
}
public void push(E element) {
container.addLast(element);
}
public void pushAll(E... element) {
for (E e : element)
container.addLast(e);
}
public boolean isQueue() {
return isQueue;
}
public void setQueue(boolean isQueue) {
this.isQueue = isQueue;
}
public boolean toggleQueue() {
return isQueue = !isQueue;
}
#Override
public Iterator<E> iterator() {
return container.iterator();
}
}
Here's the test:
QueueOrStack<String> strings = new QueueOrStack<>(true);
strings.pushAll("hello", ", " , "world\n");
for(String s : strings)
System.out.print(s); //"hello, world"
System.out.println(strings.pop()); //"hello"
strings.toggleQueue();
System.out.println(strings.pop()); //"world"
I have existing codebase that sometimes uses ArrayList or LinkedList and I need to find a way to log whenever add or remove is called to track what has been either added or removed.
What is the best way to make sure I have logging in place?
So for example.
ArrayList<Integer> list = new ArrayList<Integer>();
list.add(123);
and
LinkedList<Integer> anotherNewList = new LinkedList<Integer>();
anotherNewList.add(333);
Not sure if I can intercept add method to achieve this or create overriding class that implements java.util.List interface then use it instead. Either way I'm looking for a good solution that requires minimum intervention and prefrerrably without using any third party packages...
I would use the so called Decorator Pattern to wrap your lists.
This would be a simple example code just to give you an idea:
private static class LogDecorator<T> implements Collection<T> {
private final Collection<T> delegate;
private LogDecorator(Collection<T> delegate) {this.delegate = delegate;}
#Override
public int size() {
return delegate.size();
}
#Override
public boolean isEmpty() {
return delegate.isEmpty();
}
#Override
public boolean contains(Object o) {
return delegate.contains(o);
}
#Override
public Iterator<T> iterator() {
return delegate.iterator();
}
#Override
public Object[] toArray() {
return delegate.toArray();
}
#Override
public <T1> T1[] toArray(T1[] a) {
return delegate.toArray(a);
}
#Override
public boolean add(T t) {
// ADD YOUR INTERCEPTING CODE HERE
return delegate.add(t);
}
#Override
public boolean remove(Object o) {
return delegate.remove(o);
}
#Override
public boolean containsAll(Collection<?> c) {
return delegate.containsAll(c);
}
#Override
public boolean addAll(Collection<? extends T> c) {
return delegate.addAll(c);
}
#Override
public boolean removeAll(Collection<?> c) {
return delegate.removeAll(c);
}
#Override
public boolean retainAll(Collection<?> c) {
return delegate.retainAll(c);
}
#Override
public void clear() {
delegate.clear();
}
}
There is not really a simple way to get there.
Those classes are part of the "standard libraries"; so you can't change their behavior. You could create your own versions of them; and use class path ordering to get them used; but this really dirty hack.
The only other option: extend those classes; #Override the methods you want to be logged; and make sure all your sources use your own versions of those classes. Or if you prefer composition over inheritance you go for the decorator pattern; as suggested by JDC's answer.
The "third" option is really different - you turn to aspect oriented programming (for example using AspectJ) and use such tools to manipulate things on a bytecode level. But that adds a whole new layer of "complexity" to your product; thus I am not counting it as real option.
EDIT on your answer: it seems that you don't understand the difference between interface and implementation?! An interface simply describes a set of method signatures; but in order to have real code behind those methods, there needs to be an implementing class. You see, when you do
List<X> things = new ArrayList<>();
the real type of things is ArrayList; but you rarely care about that real type; it is good enough to know that you can all those List methods on things. So, when you create some new implementation of the List interface ... that doesn't affect any existing
... = new ArrayList ...
declarations at all. You would have to change all assignments to
List<X> things = new YourNewListImplementation<>();
JDC has given a good way to follow.
I would like bring important precisions.
The decorator pattern allows to create a class which decorates another class by adding or removing dynamically a new responsibility to an instance.
In your case, you want to add responsibility.
Decorator is not an intrusive pattern but the decorator class have to conform to the class that it decorates.
So in your case, having a decorator which derives from the Collection interface is not conform to the decorated object since List has methods that Collection has not.
Your need is decorating List instances, so decorator should derive from the List type.
Besides, the decorator class can do, according its needs, a processing before and or after the operation of the class that it decorates but it is also responsible to call the original operation of the decorated class.
In your case, you want to know if an element was added or in or removed from the List. To achieve it, as the method result has consequences on whether you log or not the information, it is preferable to delegate first the processing to the decorated object and then your decorator can perform its processings.
Sometimes, you don't need to decorate a method, don't do it but don't forget to delegate suitably to the decorated object.
import java.util.Iterator;
import java.util.List;
public class DecoratorList<T> implements List<T> {
private static final Tracer tracer = ....;
private List<T> decorated;
private DecoratorList(List<T> decorated) {
this.decorated=decorated;
}
// no decorated methods
....
#Override
public int size() {
return this.decorated.size();
}
#Override
public boolean isEmpty() {
return this.decorated.isEmpty();
}
#Override
public boolean contains(Object o) {
return this.decorated.contains(o);
}
#Override
public Iterator<T> iterator() {
return this.decorated.iterator();
}
....
// end no decorated methods
// exemple of decorated methods
#Override
public void add(int index, T element) {
tracer.info("element " + element + " added to index " + index);
this.decorated.add(index,element);
}
#Override
public boolean remove(Object o) {
final boolean isRemoved = this.decorated.remove(o);
if (isRemoved){
tracer.info("element " + o + " removed");
}
return isRemoved;
}
}
As explained, a decorator is not intrusive for the decorated objects.
So the idea is not changing your code that works but add the decorating operation just after the list be instantiated.
If don't program by interface when you declare your list variables, that is you declare ArrayList list = new ArrayList() instead of List list = new ArrayList() , of course you should change the declared type to List but it doesn't break the code, on the contrary.
Here is your example code :
ArrayList<Integer> list = new ArrayList<Integer>();
list.add(123);
LinkedList<Integer> anotherNewList = new LinkedList<Integer>();
anotherNewList.add(333);
Now, you could do it :
List<Integer> list = new ArrayList<Integer>();
list = new DecoratorList<Integer>(list); // line added
list.add(123);
List<Integer> anotherNewList = new LinkedList<Integer>();
anotherNewList = new DecoratorList<Integer>(anotherNewList); // line added
anotherNewList.add(333);
To ease the task and make it safer, you could even create a util method to apply the decoration on the list :
private static <T> List<T> decorateList(List<T> list) {
list = new DecoratorList<T>(list);
return list;
}
and call it like that :
List<Integer> list = new ArrayList<Integer>();
list = decorateList(list); // line added
list.add(123);
You can use Aspects - but it will log every add and remove call:
#Aspect
public class ListLoggerAspect {
#Around("execution(* java.util.List.add(..))")
public boolean aroundAdd(ProceedingJoinPoint joinPoint) throws Throwable {
boolean result = (boolean) joinPoint.proceed(joinPoint.getArgs());
// do the logging
return result;
}
}
You'll need to configure the aspect in META-INF/aop.xml :
<aspectj>
<aspects>
<aspect name="com.example.ListLoggerAspect"/>
</aspects>
</aspectj>
An easy way to accomplish this is wrapping your source list in a ObservableList and use that as base list. You can simply add an listener to this list to catch every modification (and log out if you wish)
Example:
List obs = FXCollections.observableList(myOriginalList);
obs.addListener(c -> {
for(Item it : c.getRemoved())
System.out.println(it);
for(Item it : c.getAddedSubList())
System.out.println(it);
});
See the javafx documentation on how to add a good listener
Your List is the source here. You need to keep track of the changes to the source. This is a good and natural example of the Observer pattern. You can create an Observable which is your list. Then create some Observers and register them to the Observable. When the Observable is changed, notify all the registered Observers. Inside the Observer you can log the changes using the input event. You should literally implement some ObservableCollection here. You can use Java Rx to get this work done. Please find the sample code given below.
package com.test;
import java.util.ArrayList;
import java.util.List;
import rx.Observable;
import rx.subjects.PublishSubject;
public class ObservableListDemo {
public static class ObservableList<T> {
protected final List<T> list;
protected final PublishSubject<T> onAdd;
public ObservableList() {
this.list = new ArrayList<T>();
this.onAdd = PublishSubject.create();
}
public void add(T value) {
list.add(value);
onAdd.onNext(value);
}
public Observable<T> getObservable() {
return onAdd;
}
}
public static void main(String[] args) throws InterruptedException {
ObservableList<Integer> observableList = new ObservableList<>();
observableList.getObservable().subscribe(System.out::println);
observableList.add(1);
Thread.sleep(1000);
observableList.add(2);
Thread.sleep(1000);
observableList.add(3);
}
}
Hope this helps. Happy coding !
We need a little more information to find the right solution. But I see a number of options.
You can track changes, using a decorator.
You can copy the collection and calculate the changes
You can use aspects to 'decorate' every List in the JVM
Change the existing codebase (a little bit)
1) works if you know exactly how the list is used, and once it is returned to your new code, you are the only user. So the existing code can't have any methods that add to the original list (because would invoke add/remove on the delegate instead of the decorated collection).
2) This approach is used when multiple classes can modify the list. You need to be able to get a copy of the list, before any modifications begin, and then calculate what happened afterwards. If you have access to Apache Collections library you can use CollectionUtils to calculate the intersection and disjunction.
3) This solution requires some for of weaving (compile or load time) as this will create a proxy for every List, so it can add callback code around the method calls. I would not recommend this option unless you have a good understanding of how aspects work, as this solution has a rather steep learning curve, and if something goes wrong and you need to debug you code, it can be a bit tricky.
4) You say existing codebase, which leads me to believe, that you could actually change the code if you really wanted. If this is at all possible, that is the approach I would choose. If the user of the List needs to be able to track changes, then the best possible solution is that the library returns a ChangeTrackingList (interface defining methods from tracking), which you could build using decoration.
One thing you have to be aware of when decorating, is that List has a removeAll() and a addAll(), these methods may or may not call the add() and remove(), this depends on the list implementation. If you are not aware of how these methods are invoked internally you could end up seeing an object as removed twice (unless you can use a set).
I have a class named "Pencilcase", where the only instance variable is an ArrayList<Pencil>.
I have a method in it like this:
public int qwerty(Pencilcase p)
I've tried to do something like this: for (Pencil pen : p)
But I get an error, which says "for-each not applicable to expression type. Required: array or java.lang.Iterable; found Pencilcase".
Since it says that "Required: java.lang.iterable", I tried to use iterator, but still could not solve the problem. What am I doing wrong? Thanks
To use the for-each of Java you need to use java.lang.Iterable as the error message said. This means that the class you want to iterate needs to implement this interface. You have two ways to go about this.
First Solution: Access the internal list directly.
public class Pencilcase {
public ArrayList<Pencil> list;
}
And iterate it like this:
public int qwerty(Pencilcase p) {
for (Pencil pen : p.list) {
....
}
}
Now this solution uses a public class variable. That is a bad thing. Also it doesn't look very fancy. So we have a
Second Solution: Implement Iterable
Now for that you need to alter your Pencilcase class to implement Iterable
public class Pencilcase implements java.lang.Iterable<Pencil> {
private ArrayList<Pencil> list;
#Override
public Iterator<Pencil> iterator() {
return list.iterator();
}
}
Using this class you can iterate over the Pencilcase instance directly.
This works because the for-loop basically fetches the iterator with the iterator function and uses this to fetch all the entries in the list.
So with this your loop can look like this:
public int qwerty(Pencilcase p) {
for (Pencil pen : p) {
....
}
}