I wrote this function which will attempt to store the map but its not working I think? I am using netbeans and each time i go to the directory of project in the java src i can't find the created file or anywhere else in the project. The map is surely valid because output comes out perfect when am not dealing with storage. btw I do implement seriliazable :)
Note: the map is of type TreeMap
public boolean storeMap(TreeMap<DateTime, Integer> map){
try{
f_out = new FileOutputStream("mapObject.data");
obj_out = new ObjectOutputStream (f_out);
obj_out.writeObject(map);
return true;
}catch(IOException ioe){
System.err.print(ioe);
return false;
}
}
is there a reason why the output file is not generated?
Thanks
I suggest to use absolute path, that is something like
f_out = new FileOutputStream("/home/username/mapObject.data");
or on windows
f_out = new FileOutputStream("c:\\work\\mapObject.data");
If there was no exception thrown (System.err.print(ioe); this line did not print anything) then the file was created somewhere.
SERIALIZE A HASHMAP:
This code is working fine , I have implemented and used in my app. Plz make ur functions accordingly for saving map and retrieving map.
Imp thing is, you need to make confirm that the objects you are putting as value in map must be serializable , means they should implement serailizbele interface. ex.
Map<.String,String> hashmap=new HashMap<.String,String>().. here in this line ...map and string both are implictly serializable , so we dont need to implement serializble for these explicitly but if you put your own object that must be serializable.
public static void main(String arr[])
{
Map<String,String> hashmap=new HashMap<String,String>();
hashmap.put("key1","value1");
hashmap.put("key2","value2");
hashmap.put("key3","value3");
hashmap.put("key4","value4");
FileOutputStream fos;
try {
fos = new FileOutputStream("c://list.ser");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(hashmap);
oos.close();
FileInputStream fis = new FileInputStream("c://list.ser");
ObjectInputStream ois = new ObjectInputStream(fis);
Map<String,String> anotherList = (Map<String,String>) ois.readObject();
ois.close();
System.out.println(anotherList);
} catch (FileNotFoundException e) {e.printStackTrace();
} catch (IOException e) {e.printStackTrace();
} catch (ClassNotFoundException e) {e.printStackTrace();
}
}
Try to call fulsh() method for outputStreams.
obj_out.flush();
f_out.flush();
and close them in finally statment.
Related
I am writing a small program that inserts customer details in ArrayList and write it in file.
The problem is with ObjectOutputStream I was able to append data in file with turning FileOutputStream("",true). But when I try to read data with ObjectInputStream it only reads data that was inserted at first instance. But data is being added to file.
Here is the code -
public void insertCustomer() throws IOException
{
Customer1=new customerDetails("1", "Moeen4", "654654", "asdf", "coding", "student", "65464", "3210");
Customer3=new customerDetails("3", "Moeen5", "888888", "asdf", "coding", "student2", "65464", "321022");
Customer4=new customerDetails("4", "Moeen6", "654654", "asdf", "coding", "student", "65464", "7890");
_list=new ArrayList<customerDetails>();
_list.add(Customer1);
_list.add(Customer3);
_list.add(Customer4);
customersList cl=new customersList();
cl.WriteObjectToFile(files._customers, _list);
ArrayList<customerDetails>li=new ArrayList<customerDetails>();
li= (ArrayList) cl.ReadObjectFromFile(files._customers);
for(int i=0;i<li.size();i++)
{ System.out.println(li.size());
System.out.println(li.get(i).Id);
System.out.println(li.get(i).name);
System.out.println(li.get(i).annual_Salary);
System.out.println(li.get(i).Company);
System.out.println(li.get(i).dateOfBirth);
System.out.println(li.get(i).phone_Number);
}
}
public void WriteObjectToFile(String filepath,Object serObj) {
try {
FileOutputStream fileOut = new FileOutputStream(filepath,true);
ObjectOutputStream objectOut = new ObjectOutputStream(fileOut);
objectOut.writeObject(serObj);
objectOut.close();
System.out.println("The Object was succesfully written to a file");
} catch (Exception ex) {
ex.printStackTrace();
}
}
public Object ReadObjectFromFile(String filepath) {
try {
FileInputStream fileIn = new FileInputStream(filepath);
ObjectInputStream objectIn = new ObjectInputStream(fileIn);
Object obj = objectIn.readObject();
objectIn.close();
System.out.println("The Object has been read from the file");
return obj;
} catch (Exception ex) {
ex.printStackTrace();
return null;
}
}
The real problem here is this:
FileOutputStream fileOut = new FileOutputStream(filepath, true);
ObjectOutputStream objectOut = new ObjectOutputStream(fileOut);
objectOut.writeObject(serObj);
You cannot append to an existing serialization like this. If you do get an exception when attempting to read any objects appended to a pre-existing (non-empty) file.
There is a trick / hack that allows you to append objects though; see Appending to an ObjectOutputStream. (The trick involves suppressing the writing of the object stream header. It is most easily done by overriding the method that does this.)
The other approach is to keep the ObjectOutputStream open between writeObject calls. However there are use-cases where that won't be possible.
Note that there is a semantic difference between these two approaches. The best way to explain it is that the first one behaves as if you called reset() each time you write an object; see the javadoc.
Another thing to note about your example is that your reader code only reads one object. If you want to read multiple objects, you need to call readObject in a loop. And that will only work if you have used the trick / hack above to avoid writing a spurious header.
As suggested the code is only reading the first object and you would need to use a loop to read all the objects from multiple writes.
However -
If you change the above code i.e. ReadObjectFromFile to use a loop this will lead to an StreamCorruptedException: invalid type code: AC. The ObjectOutputStream constructor writes a serialization stream header to the OutputStream i.e. the file, when it is closed and reopend using new ObjectOutputStream and new FileOutputStream(filepath, true) a new header will be written at the append point so you will get an exception as the header is only expected once at the beginning of the file
This will need to be handled e.g.
Use the same ObjectOutputStream for the duration
Override java.io.ObjectOutputStream.writeStreamHeader() to account for append to a file
Change the approach and use List<List<Object>> which you could read, add, write to as a whole.
Loop example would throw exception unless ObjectOutputStream approach is changed
public Object ReadObjectFromFile(String filepath) {
List<List<Object>> objects = new ArrayList<>();
FileInputStream fileIn = new FileInputStream(filepath);
ObjectInputStream objectIn = new ObjectInputStream(fileIn);
try {
while (true) {
List<Object> obj = (List<Object>) objectIn.readObject();
// This will throw StreamCorruptedException: invalid type code: AC
objects.add(obj);
System.out.println("The Object has been read from the file");
}
} catch (EOFException ex) {
// ENDS WHEN ALL READ
} finally {
fileIn.close();
objectIn.close();
}
return objects;
}
Sudo code List<List<Object>> approach -
public void readAndWrite() {
List<Object> customer = List.of(new CustomerDetails(...),
new CustomerDetails(...),
new CustomerDetails(...));
List<List<Object>> objects = readFromFile("existing-customer-file.txt");
objects.addAll(customer);
writeObjectToFile(objects);
}
I am building a basic bank application, although the usage of the java language is intermediate level.
There I am using file input and output a lots. Along the way some questions has popped up in my mind about the file-i/o in java.
1) What if I create two different text file for writing and reading objects? Does it make any difference?
2) How about the specifying path (or giving file name), what if I use // instead of \\?
3) Do I necessarily need to create a new file object like this: File file=new File("C://Users//Documents//NetBeansProjects//BankFile_assignment.txt"); in my specific case?
Last but not least if you may wonder about my file-i/o class:
public class ReaderWriter {
public void writeToFile(List<BankAccount> accounts) {
try {
File file = new File("C://Users//Documents//NetBeansProjects//BankFile_assignment.txt");
FileOutputStream fos = new FileOutputStream(file);
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(accounts);//take the arrayList
oos.flush();
oos.close();
fos.close();
} catch (Exception e) {
e.printStackTrace();
}
}
public List<BankAccount> readFromFile() {
List<BankAccount> readData = null;
try {
File file = new File("C://Users//Documents//NetBeansProjects//BankFile_assignment.txt");
FileInputStream fis = new FileInputStream(file);
ObjectInputStream ois = new ObjectInputStream(fis);
readData = (List<BankAccount>) ois.readObject();
ois.close();
fis.close();
} catch (Exception e) {
e.printStackTrace();
}
return readData;
}
}
1) Better way is to use databases (mySQL, SQLite,...) to access easily to all your datas without I/O worries.
2) If your application might work on different Operating Systems, a safe way to avoid any trouble with the specific symbol of system ( \ on Windows, / on Unix, Mac) is to use File.separator for example. More about this subject .
3) It must work on Windows, but fails on Unix. You can use (with adaptation for path) this instead of: File file = new File(System.getProperty("user.home")+ File.separator + BankFile_assignment.txt); See this .
Hi I have an issue when trying to append new objects to the existing file..
Once the android app has been lunched again I want to get the existing file and add a new objects then read the objects from the existing file ... Actually, when I'm trying to read the object, the code will read only the first objects .. You can find below the code .. Could you please help ? Thanks
using the following method to write an objects :
public void saveObject(Person p, File f){
try
{
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(f, true));
oos.writeObject(p);
oos.reset();
oos.flush();
oos.close();
}
catch(Exception ex)
{
Log.v("Serialization Save Error : ",ex.getMessage());
ex.printStackTrace();
}
}
Using the following method to read an objects :
public Object loadSerializedObject(File f)
{
try {
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(f));
try{
Object loadedObj = null;
while ((loadedObj = ois.readObject()) != null) {
Log.w(this.getClass().getName(), "ReadingObjects") ;
}
return objects;
}finally{
ois.close();
}
} catch (StreamCorruptedException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
return null;
}
Unfortunately you can't create a new ObjectOutputStream every time you want to append to the stream and then read everything back with a single stream. The constructor adds headers to the underlying stream before you start writing objects. You are probably seeing the java.io.StreamCorruptedException: invalid type code: AC exception, that's because the first header is 0xAC.
I don't know how many objects you are dealing with, but one option might be to read all your objects and then rewriting them all using a single ObjectOutputStream. That can get pricy if there are lots of objects. Alternatively, you might want to consider managing the serialization yourself manually through Externalizable. It can get painful though.
I have saved an arrayList into a binary file by using serialastion. How do I now retrieve this data from the binary file?
This is the code I have used for serialisation
public void createSerialisable() throws IOException
{
FileOutputStream fileOut = new FileOutputStream("theBkup.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(allDeps);
options();
}
and this the code I am trying to use for deserialization of the arrayList:
public void readInSerialisable() throws IOException
{
FileInputStream fileIn = new FileInputStream("theBKup.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
try
{
ArrayList readob = (ArrayList)oi.readObject();
allDeps = (ArrayList) in.readObject();
}
catch (IOException exc)
{
System.out.println("didnt work");
}
}
allDeps is the declared array list in the classes constructer. Im trying to save the arrayList from the file to the arrayList declared in this class.
Your code is mostly correct, but there is one mistake and a couple of things that might make it work better. I've highlighted them with asterisks (since, apparently, I can't make them bold in 'code' mode).
public void createSerialisable() throws IOException
{
FileOutputStream fileOut = new FileOutputStream("theBkup.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(allDeps);
**out.flush();** // Probably not strictly necessary, but a good idea nonetheless
**out.close();** // Probably not strictly necessary, but a good idea nonetheless
options();
}
public void readInSerialisable() throws IOException
{
FileInputStream fileIn = new FileInputStream("theBKup.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
try
{
**// You only wrote one object, so only try to read one object back.**
allDeps = (ArrayList) in.readObject();
}
catch (IOException exc)
{
System.out.println("didnt work");
**exc.printStackTrace();** // Very useful for findout out exactly what went wrong.
}
}
Hope that helps. If you still see a problem then make sure you post the stack trace and a complete, self-contained, compilable example that demonstrates the problem.
Note that I've assumed that allDeps contains objects that are actually Serializable and that your problem is in readInSerialisable rather than in createSerialisable. Again, a stack trace would be extremely helpful.
I have created a method in my java assignment to write into a file from a LinkedList (I used serialization) , then I have created another method to read the file into the inkedList. The following is my method's body:
try {
FileInputStream fin = new FileInputStream("c:\\Info.ser");
ObjectInputStream ois = new ObjectInputStream(fin);
Employee e = (Employee) ois.readObject();
linkP.add(e);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
but it doesnt work right. I think this part:
Employee e = (Employee) ois.readObject();
linkP.add(e);
reads only the first object of the file into the linkedlist and ignores other objects. I surrounded it for loop and while loop several times but it causes EOFException. How can I change my method to read all of the file's objects into the LinkedList?
If you used LinkedList for serialization you should expect a LinkedList to deserialize:
linkP= (LinkedList) ois.readObject();
instead of
Employee e = (Employee) ois.readObject();
linkP.add(e);
The easiest way is to include the size of the list as the first thing written to the file. When you read the file, the first thing you retrieve is the size. Then you can read the expected number of objects.
Are you sure that the serialized file contains all of the elements? It looks to me like you might only be serializing one.
Note: Please also add the code where you create the info.ser file, since you may have corrupted the ObjectOutputStream by closing/reopening it for each object.
But to answer your question, the proper way of doing it (without catching exceptions) would be:
#Test
public void testSerializingListByEntries() throws Exception {
List<Serializable> list = new ArrayList<Serializable>();
list.add(new Date());
list.add(new Date());
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeInt(list.size()); // Magic
for(Serializable o : list) {
oos.writeObject(o);
}
oos.close();
ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
ObjectInputStream ois = new ObjectInputStream(bais);
int count = ois.readInt();
List<Object> newList = new ArrayList<Object>();
for(int i = 0; i < count;i++) {
newList.add(ois.readObject());
}
ois.close();
assertEquals(list,newList);
}
Yes, you need to close the streams yourself of course. Omitted for readability.
Would probably need to see how you're writing in the first place but generally:
ObjectInputStream is = null;
try
{
is = new ObjectInputStream(new FileInputStream("c:/Info.ser"));
Object object = null;
while ((object = is.readObject()) != null)
{
linkP.add(object);
}
}
catch(Exception e)
{
//Whatever you need to do
}
finally
{
//Never forget to close your streams or you'll run into memory leaks
try
{
if (is != null)
{
is.close();
}
}
catch (Exception ex)
{
ex.printStackTrace();
}
}
Also, its probably better practice for you to handle the exceptions individually but I can't really tell what the streams throw so replace the (Exception e) with everything else.
surround Employee e = (Employee) ois.readObject();
linkP.add(e);
with a for loop as you suggested and surround the .readObject call with a try/catc(EOFException)
Just catch EOFException separately inside your reading loop and process it accordingly, i.e. break out of the loop.