I have an Android app which keeps a persistent log of events in a custom object that extends ArrayList. It is kept in a singleton so that various activities all point to the same one log. The main activity reloads the list in the onResume.
Problem is, when I reload the log, all the UI elements (like the ArrayAdapter) lose the reference and I need to set them up again. It works. But, it seems like a lot of work.
How can I reload the object into the original object's instance so that I don't have to do all that setup again? Using a singleton should make it easier not harder.
I know it's all a problem with pass-by-reference. But I just can't get my head around it.
Java is not my first language... Thanks.
Activity onCreate:
dataLog = DataLog.getInstance();
logView = (ListView) findViewById(R.id.logView);
dataLogAdapter = new ArrayAdapter<String>(this,R.layout.log_row, dataLog);
logView.setAdapter(dataLogAdapter);
Activity onResume:
dataLog = Persister.recoverLog(this, "datalog.dat");
dataLogAdapter = new ArrayAdapter<String>(this,R.layout.log_row, dataLog);
logView.setAdapter(dataLogAdapter);
Persister:
public static DataLog recoverLog(Context context, String fileName){
File file = context.getFileStreamPath(fileName);
DataLog obj = new DataLog ();
if(file.exists() && file.length()>0){
FileInputStream fis = null;
ObjectInputStream ois = null;
try {
fis = new FileInputStream(file);
ois = new ObjectInputStream(fis);
obj = (DataLog) ois.readObject();
ois.close();
fis.close();
} catch (IOException ex) {
ex.printStackTrace();
} catch (ClassNotFoundException ex) {
ex.printStackTrace();
}
}
return obj;
}
DataLog:
public class DataLog extends ArrayList<String> implements Serializable {
private static final long serialVersionUID = 0L;
public DataLog() {}
private static DataLog _instance;
public static DataLog getInstance() {
if(_instance==null){
_instance = new DataLog();
}
return _instance;
}
public boolean add(String entry) {
super.add(entry);
return true;
}
public void add(int index, String entry) {
if (index > 0)
super.add(index, entry);
else
super.add(entry);
}
public void clear() {
super.clear();
}
}
Your datalog isn't a true singleton. If it were, no new reference would be created after it was created the very first time.
This:
dataLog = Persister.recoverLog(this, "datalog.dat");
should not return a Datalog but some internal data DataLog uses (an ArrayList of Strings) and should be called within DataLog
Then only pass the dataLog instance around. If you ever reassign it you'll have problems. When you reload data log you'll need to reload the structures relying on DataLog's array instance.
Handle reloading your DataLog data in onResume if necessary, but do it like so:
DataLog.getInstance().load();
So that your data log is never reassigned. That's the key. DataLog is a wrapper around your data. If DataLog's internal state is changed, no references to DataLog are lost.
Here's an example:
public class DataLog implements Serializable {
//internalize this data.
private ArrayList<String> data;
private static final long serialVersionUID = 0L;
//singleton's constructor should be private.
private DataLog() {}
private static DataLog _instance;
public static DataLog getInstance() {
if(_instance==null){
_instance = new DataLog();
}
return _instance;
}
public ArrayList<String> getData(){
//copy this if you're worried about it being modified.
//DO NOT pass this reference around. Pass a reference to data log,
//reload GUI relying on this reference as needed.
return data;
}
public void load(){
data = //reload string array from disk.
}
public boolean add(String entry) {
super.add(entry);
return true;
}
public void add(int index, String entry) {
if (index > 0)
data.add(index,entry);
else
data.add(entry);
}
public void clear() {
data.clear();
}
}
Related
My Goal: I need to keep the App state exactly in same sate after shutdown, lets say it's equivalent to "suspend" state.
My Problem : I do know that serialization mechanism doesn't save transient variables neither static variables. However I need to maintain the static variables in exactly same state after App suspension/shut down.
Approach-1 : I could save the state of static variable(s) into a different file, using my "file format", and serialize the objects into a different one.
a) Is this the "normal" approach?
Approach-2 : If I extend the ObjectInputStream/ObjectOutputStreamand override the methods readStreamHeader/writeStreamHeaderI can write whatever I want. So I can also write my static variables.
b) Am I doing something I should not?
Here's the code I've written testing approach-2, and seams to work fine. Please note, I'm not a Java programmer, so for it's very important to understand best practices, if there's any in this particular case.
#SuppressWarnings("serial")
class SequenceIdentifier implements Serializable
{
protected static long seqIdentifier_ = 1L; //This variable MUST NOT be reseted.
private long id_; //Object variable to be serialised.
private SequenceIdentifier(long id)
{ id_ = id;
}
#Override
public String toString()
{ return ("Id : " + id_ + " of " + seqIdentifier_);
}
public static SequenceIdentifier newInstance()
{ return new SequenceIdentifier(seqIdentifier_++);
}
}
final class OOStream extends ObjectOutputStream
{
public OOStream(OutputStream out) throws IOException
{ super(out);
}
#Override
protected void writeStreamHeader() throws IOException
{ super.writeLong(SequenceIdentifier.seqIdentifier_);
}
}
final class OIStream extends ObjectInputStream
{
public OIStream(InputStream in) throws IOException
{ super(in);
}
#Override
protected void readStreamHeader() throws IOException
{ SequenceIdentifier.seqIdentifier_ = super.readLong();
}
}
public class Main
{
public static void dump(ArrayList<SequenceIdentifier> ids)
{
for (SequenceIdentifier id : ids)
System.out.println(id);
}
public static void saveData()
{
ArrayList<SequenceIdentifier> ids = new ArrayList<>(Arrays.asList(SequenceIdentifier.newInstance(),
SequenceIdentifier.newInstance(),
SequenceIdentifier.newInstance(),
SequenceIdentifier.newInstance()));
try (OOStream oOut = new OOStream(new FileOutputStream("foo.bin")))
{ oOut.writeObject(ids);
} catch (Exception e)
{ System.err.println(e);
}
dump(ids);
}
#SuppressWarnings("unchecked")
public static void loadData()
{
ArrayList<SequenceIdentifier> ids = null;
try (OIStream oIn = new OIStream(new FileInputStream("foo.bin")))
{ ids = (ArrayList<SequenceIdentifier>)oIn.readObject();
} catch (Exception e)
{ System.err.println(e);
}
dump(ids);
}
public static void main(String[] args)
{
saveData();
System.out.println("Counter at this point " + SequenceIdentifier.seqIdentifier_);
SequenceIdentifier.seqIdentifier_ = 0;
loadData();
System.out.println("Counter at this point " + SequenceIdentifier.seqIdentifier_);
}
}
I would create a separate Memento-class containing all the relevant data as fields and de-/serialize that.
class MyClassWithStaticFields1 {
private static String field;
}
class MyClassWithStaticFields2 {
private static String field;
}
class StaticMemento {
String field1;
String field2;
}
// serialization
StaticMemento mem = new StaticMemento();
mem.field1 = MyClassWithStaticFields1.field;
mem.field2 = MyClassWithStaticFields2.field;
outputStream.writeObject(mem);
// deserialize
StaticMemento mem = outputStream.readObject();
MyClassWithStaticFields1.setField(mem.field1);
MyClassWithStaticFields2.setField(mem.field2);
So basically your Approach-1.
Several possibilities.
Make it non-static.
Write complementary readObect()/writeObject() methods that call defaultReadObject() and defaultWriteObject() respectively and then serialize/deserialize the field.
Write complementary writeReplace()/readResolve() methods that substitute a proxy object that does contain this member as a non-transient non-static member.
Make the object Externalizable and take complete control of the serialization process yourself in the associated methods.
Review your requirement.
I'm writing a messaging system to queue actions for my program to execute. I need to be able to pass various objects by the messages. I currently have a Msg object that accepts (Action enum, Data<?>...object). The Data object is intended to be a wrapper for any object I might pass.
Currently the Data object uses this code, with generics:
public class Data<T> {
private T data;
public Data(T data){
this.data = data;
}
public T getData(){
return data;
}
}
The Msg object takes Data<?>... type, so Msg has a Data<?>[] field.
If getData() is called on a Data<?> object, it returns the Object type. Obviously not ideal.
I need to be able to pass, say, Image objects as well as String objects. I'm certain there's a better way of passing arbitrary data.
The reason you're having trouble is that you're trying to get the static typing system of Java to do something that it can't. Once you convert from a Data<T> to a Data<?>, whatever T was is effectively lost. There's no clean way to get it back.
The quickest way to get it to work (from what you have right now) is to start throwing casts everywhere, like this:
Data<?> d = new Data("Hello");
String contents = (String)d.getData();
This is kind of a terrible idea, so let's go back to the drawing board.
If (ideally), you have all of the types you could ever need ahead of time (i.e. every Data is either a String or an Image or an Integer), then you can pretty easily (though it's a bit tedious) define a Sum type (aka a union if you're coming from C) of the different types of data you'll have to handle. As a class invariant, we assume that exactly one of the fields is non-null, and the rest are null. For this example I'll assume it can be either a String, an Image, or an Integer, but it's fairly simple to add or remove types from Data as necessary.
public class Data {
private Image imgData;
private String stringData;
private Integer intData;
public Data(Image img) {
this.imgData = img;
}
public Data(String stringData) {
this.stringData = stringData;
}
public Data(Integer intData) {
this.intData = intData;
}
public boolean isImage() {
return imageData != null;
}
public boolean isInteger() {
return intData != null;
}
public boolean isString() {
return stringData != null;
}
public Image asImage() {
if(! isImage()) throw new RuntimeException();
return imgData;
}
public Image asString() {
if(! isString()) throw new RuntimeException();
return stringData;
}
public Image asInt() {
if(! isInt()) throw new RuntimeException();
return intData;
}
}
One necessary side effect is that we cannot wrap null without causing exceptional behavior. Is this is desired, it isn't too difficult to modify the class to allow for it.
With this Data class, it's pretty easy to do if-else logic to parse it.
Data d = ....... //Get a data from somewhere
if(d.isImage()) {
Image img = d.asImage();
//...
} else if (d.isString()) {
String string = d.asString();
//...
} else if (d.isInteger()) {
Integer i = d.asInt();
//...
} else {
throw new RuntimeException("Illegal data " + d + " received");
}
If you call getData().getClass() you will get the class or type that was passed, which doesn't seem to me to be the same as an Object. You might not know what you are getting, but you can either find out or define a common interface for everything you might pass. You could for example, call toString() or getClass() on anything passed. Your question is that you are passing any conceivable object, so my question is what are you going to do with it? If you are going to serialize it into a database you don't need know anything about what type it is, otherwise you can test it or call a common interface.
public class PlayData {
class Msg {
private List<Data<?>> message = new ArrayList<Data<?>>();
public void addData(Data<?> datum) { message.add(datum); }
public void printTypes() { for ( Data<?> datum: message ) { System.out.println(datum.getData().getClass()); } }
}
class Data<T> {
private T value;
public Data(T value) { this.value = value; }
public T getData() { return value; }
}
class Listener {
public void receive(Msg msg) { msg.printTypes(); }
}
class Sender {
private Listener listener;
public Sender(Listener listener) { this.listener = listener; }
public void send(Msg msg) { listener.receive(msg); }
}
class MyPacket {
int i;
public MyPacket(int i) { this.i = i; }
}
public static void main(String[] args) throws Exception { new PlayData().run(); }
public void run() throws Exception {
Sender sender = new Sender(new Listener());
Msg msg = new Msg();
msg.addData(new Data<String>("testing") );
msg.addData(new Data<MyPacket>(new MyPacket(42)) );
sender.send(msg);
}
}
I'm trying to use a singleton (PhotoStorage) to provide an arrayList of Photo objects, but it seems that the PhotoStorage instance is not behaving as a singleton (two instances).
I am using dagger to inject this singleton into a class named PhotoInteractor. The objectGraph seems A-OK up to this point.
The same PhotoInteractor instance is used in three fragments in a viewpager. These fragments are all instantiated at runtime:
RecentFragment:
HistoryFragment:
Notice how the instance #4067 of the PhotoInteractor is the same for both fragments.
Also:
mAppContext#4093: same
photoStorage#4094: same
When I click a photo object (grid image) from RecentFragment, the PhotoStorage.addPhoto(url) method is called. This correctly adds the photo object to the photoStorage instance array (4094). That much is OK.
Problem:
When I close the applicaton, it is intended that the PhotoStorage.savePhotosToFile method serialzes this arrayList object into JSON on the filesystem.
The following method is called from the same PhotoInteractor instance:
#Override
public void savePhotos(){
photoStorage.get(mAppContext).savePhotosToFile();
}
When I debug the application, the PhotoStorage.get method already has a singleton instance, but what appears to be a 2nd instance!
//Singleton enforcement
public static PhotoStorage get(Context c){
if(sPhotoStorage == null){
sPhotoStorage = new PhotoStorage(c.getApplicationContext());
}
return sPhotoStorage;
}
This means that the ArrayList of photos will always be empty since it is a new instance of PhotoStorage. Iām not sure where it is instantiating itself from.
Edit - Added PhotoStorage.class:
public class PhotoStorage{
private ArrayList<Photo> mPhotos;
private PhotoJSONer mSerializer;
private static PhotoStorage sPhotoStorage;
private static Context mAppContext;
private static final String PHOTOS_DATABASE = "photos.json";
public static final String TAG = PhotoStorage.class.getSimpleName();
public PhotoStorage(Context appContext){
mSerializer = new PhotoJSONer(appContext, PHOTOS_DATABASE);
try{
mPhotos = mSerializer.loadPhotos();
}catch(Exception e){
mPhotos = new ArrayList<Photo>();
}
}
//Singleton enforcement
public static PhotoStorage get(Context c){
if(sPhotoStorage == null){
sPhotoStorage = new PhotoStorage(c.getApplicationContext());
}
return sPhotoStorage;
}
public ArrayList<Photo> getPhotos(){
return mPhotos;
}
public Photo getPhoto(String url){
for(Photo p: mPhotos){
if(p.getUrl() == url)
return p;
}
return null;
}
public void deletePhoto(String url){
Log.i(TAG, "deleted photo");
mPhotos.remove(url);
}
public void addPhoto(Photo photo){
Log.i(TAG, "added photo");
mPhotos.add(photo);
}
public boolean savePhotosToFile(){
try{
mSerializer.savePhotos(mPhotos);
return true;
}catch (Exception e){
return false;
}
}
}
You are not executing Singletton pattern in the correct way,
The Singleton design pattern addresses all of these concerns. With the Singleton design pattern you can:
Ensure that only one instance of a class is created
Provide a global point of access to the object
In your case, we don't see PhotoStorage class but this call comes from an instance, what is not allowed by Singletton pattern:
photoStorage.get(mAppContext).savePhotosToFile();
//ā instance call WRONG!!
This line works, but as your get method is static is not a good practice as Karakuri pointed and also breaks the Singletton pattern definition.
public static PhotoStorage get(Context c){
SOLUTION
To make photoStorage.get() invalid and create a correct Singletton pattern you must:
declare the getInstance() method static in the class (here PhotoStorage)
hide default constructor to avoid instances of the class
create private constructors if necessary
call getInstance() it in a static way:
class PhotoStorage {
// hidding default constructor
private PhotoStorage () {};
// creating your own constructor but private!!!
private PhotoStorage(Context appContext){
mSerializer = new PhotoJSONer(appContext, PHOTOS_DATABASE);
try{
mPhotos = mSerializer.loadPhotos();
}catch(Exception e){
mPhotos = new ArrayList<Photo>();
}
}
//Singleton enforcement
public synchronized static PhotoStorage get(Context c){
if(sPhotoStorage == null){
sPhotoStorage = new PhotoStorage(c.getApplicationContext());
}
return sPhotoStorage;
}
}
Then you can make static call from everywhere the class scope allows:
#Override
public void savePhotos(){
PhotoStorage.get(mAppContext).savePhotosToFile();
//ā static call CORRECT!!
}
UPDATE: if your app have several threads and singleton getInstance requests may overlapp, there is a double check syncronized singletton pattern you can apply:
//Singleton enforcement
public synchronized static PhotoStorage get(Context c){
if(sPhotoStorage == null){
synchronized(PhotoStorage.class) {
if(sPhotoStorage == null) {
sPhotoStorage = new PhotoStorage(c.getApplicationContext());
}
}
}
}
In a previous post Creating a ToolTip Managed bean
I was able to create a manged bean to collect and display tooltip text with only a single lookup and store them in an Application Scope variable. This has worked great.
I am on the rather steep part of the JAVA learning curve so please forgive me.
I have another managed bean requirement to create a HashMap Application Scope but this time it needs to be of a type String, Object. The application is where I have a single 'master' database that contains most of the code, custom controls, and XPages. This Master Database will point to One or More application databases that will store the Notes Documents specific to the application in question. So I have created in the Master a series of Application Documents that specify the RepIDs of the Application, Help and Rules databases specific to the Application along with a number of other pieces of information about the Application. This should allow me to reuse the same custom control that will open the specific DB by passing it the Application Name. As an example the Master Design DB might point to "Purchasing", "Customer Complaints", "Travel Requests" etc.
The code below works to load and store the HashMap, but I am having trouble retrieving the the data.
The compiler shows two errors one at the public Object get(String key) method and the other at mapValue = this.internalMap.get(key); in the getAppRepID method I think that it is mainly syntax but not sure. I have commented the error in the code where it appears.
/**
*This Class makes the variables that define an application within Workflo!Approval
*available as an ApplicationScope variable.
*/
package ca.wfsystems.wfsAppUtils;
import lotus.domino.Base;
import lotus.domino.Session;
import lotus.domino.Database;
import lotus.domino.View;
import lotus.domino.NotesException;
import lotus.domino.ViewColumn;
import lotus.domino.ViewEntry;
import lotus.domino.ViewEntryCollection;
import lotus.domino.Name;
import java.io.Serializable;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Vector;
import com.ibm.xsp.extlib.util.ExtLibUtil;
/**
* #author Bill Fox Workflo Systems WFSystems.ca
* July 2014
* This class is provided as part of the Workflo!Approval Product
* and can be reused within this application.
* If copied to a different application please retain this attribution.
*
*/
public abstract class ApplicationUtils implements Serializable, Map<String, Object> {
private static final long serialVersionUID = 1L;
private Session s;
private Name serverName;
private String repID;
private String thisKey;
private ViewEntryCollection formVECol;
private Vector formNames;
private Database thisDB;
private Database appDB;
private View appView;
private View formView;
private ViewEntry formVE;
private ViewEntry tFormVE;
private ViewEntry ve;
private ViewEntry tVE;
private ViewEntryCollection veCol;
private final Map<String, Object> internalMap = new HashMap<String, Object>();
public ApplicationUtils() {
this.populateMap(internalMap);
}
private void populateMap(Map<String, Object> theMap) {
try{
s = ExtLibUtil.getCurrentSession();
//serverName = s.createName(s.getServerName());
thisDB = s.getCurrentDatabase();
appView = thisDB.getView("vwWFSApplications");
veCol = appView.getAllEntries();
ve = veCol.getFirstEntry();
ViewEntry tVE = null;
while (ve != null) {
rtnValue mapValue = new rtnValue();
tVE = veCol.getNextEntry(ve);
Vector colVal = ve.getColumnValues();
thisKey = colVal.get(0).toString();
mapValue.setRepID(colVal.get(2).toString());
// ...... load the rest of the values .......
theMap.put(thisKey, mapValue);
recycleObjects(ve);
ve = tVE;
}
}catch(NotesException e){
System.out.println(e.toString());
}finally{
recycleObjects(ve, veCol, appView, tVE);
}
}
public class rtnValue{
private String RepID;
private String HelpRepID;
private String RuleRepID;
private Vector FormNames;
public String getRepID() {
return RepID;
}
public void setRepID(String repID) {
RepID = repID;
}
public String getHelpRepID() {
return HelpRepID;
}
public void setHelpRepID(String helpRepID) {
HelpRepID = helpRepID;
}
public String getRuleRepID() {
return RuleRepID;
}
public void setRuleRepID(String ruleRepID) {
RuleRepID = ruleRepID;
}
public Vector getFormNames() {
return FormNames;
}
public void setFormNames(Vector formNames) {
FormNames = formNames;
}
}
public void clear() {
this.internalMap.clear();
this.populateMap(this.internalMap);
}
public boolean containsKey(Object key) {
return this.internalMap.containsKey(key);
}
public boolean containsValue(Object value) {
return this.internalMap.containsValue(value);
}
public Set<java.util.Map.Entry<String, Object>> entrySet() {
return this.internalMap.entrySet();
}
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) {
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
rtnValue newMap = new rtnValue();
return newMap;
}
}
public boolean isEmpty() {
return this.internalMap.isEmpty();
}
public Set<String> keySet() {
return this.internalMap.keySet();
}
public Object put(String key, Object value) {
return this.internalMap.put(key, value);
}
public Object remove(Object key) {
return this.internalMap.remove(key);
}
public int size() {
return this.internalMap.size();
}
public Collection<Object> values() {
return this.internalMap.values();
}
public void putAll(Map<? extends String, ? extends Object> m) {
this.internalMap.putAll(m);
}
public String getAppRepID(String key){
/*get the Replica Id of the application database
* not sure this is the correct way to call this
*/
rtnValue mapValue = new rtnValue();
mapValue = this.internalMap.get(key);
//error on line above Type Mismatch: can not convert Object to ApplicationUtils.rtnValue
String repID = mapValue.getRepID();
}
public static void recycleObjects(Object... args) {
for (Object o : args) {
if (o != null) {
if (o instanceof Base) {
try {
((Base) o).recycle();
} catch (Throwable t) {
// who cares?
}
}
}
}
}
}
For the get() method, the way I handle that kind of situation is create a variable of the correct data type as null, in my try/catch set the variable, and at the end return the variable. So:
Object retVal = null;
try....
return retVal;
For the other error, if you right-click on the error marker, it might give you the opportunity to cast the variable to rtnValue, so:
mapValue = (rtnValue) this.internalMap.get(key)
If you haven't got it, Head First Java was a useful book for getting my head around some Java concepts. It's also worth downloading the FindBugs plugin for Domino Designer from OpenNTF. It will identify errors as well as bad practices. Just ignore the errors in the "local" package!
The problem is that there is an execution path that do not return nothing
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) { // false
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
rtnValue newMap = new rtnValue();
return newMap;
}
}
if key is not present in the internalMap, nothing is thrown, then that method do not return anything.
To fix the problem, return the newMap at the end.
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) {
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
}
rtnValue newMap = new rtnValue();
return newMap;
}
You can inline the return to save the allocation (which is what the compiler will do anyway). I didn't do it just to make it clear in the example.
But still you have a compiler error in getAppRepID method. You are expecting a rtnValue but you send back an Object. You must cast there.
The appropriate way to handle this is, if you know that all values are of a given type, create the map with the proper type.
Have you tried making your internalMap a map of rtnValue instances (so )?
I have a question on copy construction in Java. Consider the following class;
In the copy constructor I can say new(Integer(other.id)) to get a new integer object being passed to the constructor, but I can't say new T(other.data) as the compiler will say cannot instantiate the type T. How can I make sure that when the generic item is copy constructed that it will not just pass a reference such that the 2 objects will share the underlying data.
Also, in the getLinks method it is doing a new and creating a new object of the list but is that going to deep copy and create new object of the items contained in the list or will it just contain references to the existing objects list items such that you have 2 lists both pointing to the same data. See below the comments / code. Thanks in advance for your expertise.
class DigraphNode<T>
{
Integer id;
T data;
ArrayList<DigraphNode<T> > links;
public DigraphNode(Integer i)
{
id = i;
links = new ArrayList<DigraphNode<T> >();
}
public DigraphNode(Integer i, T d)
{
id = i; data = d;
links = new ArrayList<DigraphNode<T> >();
}
public DigraphNode(DigraphNode<T> other)
{
other.id = new Integer(other.id);
other.data = other.data; // line in question
this.links=other.getLinks(); // also will create a new list with references
// or will it deep copy the items contained in the list?
// see getLinks() method below
}
public void setData (T d ) { data = d; }
public void addLink (DigraphNode<T> n) { links.add(n); }
public void addLinks (ArrayList<DigraphNode<T> > ns) { links.addAll(ns); }
public ArrayList<DigraphNode<T> > getLinks()
{
return new ArrayList<DigraphNode<T> >(links);
}
public void printNode()
{
System.out.print("Id: " + id + " links: ");
for ( DigraphNode<T> i : links )
{
System.out.print(i.id + " ");
}
System.out.println();
}
}
You can't instantiate new T(other.data) as you tried, but you can clone() other.data if T implements Cloneable
Every call to getLinks() will create a new list with reference to object contained to links, you have to different lists with same reference inside (so change one reference object property will reflect to other list object because they are the same object)
About ArrayList<> links = new ArrayList<>(); from Oracle doc:
Initializer blocks for instance variables look just like static
initializer blocks, but without the static keyword:
{
// whatever code is needed for initialization goes here
}
The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of
code between multiple constructors.
EDIT:
You can define a static method (copy) that try to use all possible strategies to copy generic object; the best approch is to define your own interface to separate your own stategy and simulate a sort of copy-constructor (you can reuse copy method if you want), else via serialization or, as last try, using cloning (but clone() is full of pitfall).
You can also use this libraries:
Cloning
Commons-SerializationUtils
interface MyCloneableInterface<T> {
T duplicate(T object) throws CopyException;
}
public static <T> T copy(T data) throws CopyException {
if(data == null) return null;
if(data instanceof MyCloneableInterface) {
return ((MyCloneabeInterface)data).duplicate(data);
}
if(data instanceof Serializable) {
try {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(this);
ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
ObjectInputStream ois = new ObjectInputStream(bais);
return (CloneExample) ois.readObject();
}
catch(...) {//rethrow}
}
if(data instanceof Cloneable) {
try {
return (T)data.clone();
}
catch(CloneNotSupportedException e) {//rethrow}
}
// else you can look for copy-constructor via reflection or
// cloning object field-by-field via reflection...
}
First Question: You cannot instantiate a generic instance (in other words call T's constructor). You should either define T implements Cloneable and call clone or use another interface of your own if T is always under your control. There are many pitfalls to this method, I'd suggest you first read about this interface and familiarize yourself with the pitfalls (you can find a great chapter on this, in "Effective Java" book). Also, it is not always that you can guarantee that this class will use T types which are Cloneable.
About links - you're instantiating it in the beginning and then override it in the constructor - Why? Remove the initialization. The way your getLinks works is not by creating a deep copy. Meaning - you'll get a new list, the list itself will be different from the original list, but the items will be shallow copies.
About your last question - as I already said, it's redundant. Remove the initialization at the beginning. You're creating an object, never use it and leave it for garbage collection. What you can do to avoid calling this in every constructor is something like this:
public DigraphNode() {
links = new ArrayList<DigraphNode<T> >();
}
And have other constructors call this constructor, for example:
public DigraphNode(T val) {
this();
this.data = val;
}
Upvoted all helpful answers, but I am answering my own question below which shows the updated code. I wanted to see how someone would implement a copy for a generic but no one posted code for that so I rolled my own. See below my answer.
import java.lang.reflect.*;
import java.util.*;
class MissingDigraphNodeException extends Exception
{
private static final long serialVersionUID = 1000L;
public MissingDigraphNodeException(String message)
{
super(message);
}
}
class CopyException extends Exception
{
private static final long serialVersionUID = 2000L;
public CopyException(String message)
{
super(message);
}
}
class DigraphNode<T>
{
Integer id;
T data;
ArrayList<DigraphNode<T> > links;
public DigraphNode(Integer i)
{
id = i;
links = new ArrayList<DigraphNode<T> >();
}
public DigraphNode(Integer i, T d)
{
id = i; data = d;
links = new ArrayList<DigraphNode<T> >();
}
public DigraphNode(DigraphNode<T> other)
{
try
{
this.data = copy(other.data);
}
catch (CopyException e)
{
e.printStackTrace();
}
this.links=other.getLinks();
this.id = new Integer(other.id);
}
// is there a better way to copy a generic?
#SuppressWarnings("unchecked")
public T copy( T source ) throws CopyException
{
Class<?> clzz = source.getClass();
Method meth;
Object dupl = null;
try {
meth = clzz.getMethod("clone", new Class[0]);
dupl = meth.invoke(source, new Object[0]);
} catch (Exception e)
{
e.printStackTrace();
throw new CopyException("Error: Copying Generic of T");
}
return (T) dupl;
}
public void setData (T d ) { data = d; }
public void addLink (DigraphNode<T> n) { links.add(n); }
public void addLinks (ArrayList<DigraphNode<T> > ns) { links.addAll(ns); }
public ArrayList<DigraphNode<T> > getLinks()
{
// return a new copy of the list
ArrayList<DigraphNode<T> > l = new ArrayList<DigraphNode<T> >();
for ( DigraphNode<T> i : links )
{
i.links.add(new DigraphNode<T>(i)); // use copy constructor
}
return l;
}
public void printNode()
{
System.out.print("Id: " + id + " links: ");
for ( DigraphNode<T> i : links )
{
System.out.print(i.id + " ");
}
System.out.println();
}
}