CODEWITHSUNDEEP
javaarraylist
Pete855217
Pete855217

Reputation: 1602

Java memory space overlap?

I have a problem of adding elements into an ArrayList. Each time I do the 'add', the entire array content is replaced with the current element value. I end up with eg. 10 repeated element duplicates.

The classes are set up as follows:

public class BradfordReport {
    EmployeeRow _empRow = new EmployeeRow();
    ArrayList<EmployeeRow> _bradfordData = new ArrayList<EmployeeRow>();

    public void Run() {
       // processing to setup Employee row variables
       for (int x=0; x<10; x++) {
           // This next line in debug IS ADJUSTING THE ARRAYLIST DATA!!
           _empRow.EmpNum = x; // etc for other variable in EmployeeRow
           _bradfordData.add(er);
       }
    }
    // THE RESULT IN _bradfordData is 10 elements, all with EmpNum = 10!
 }

public class EmployeeRow {
    int EmpNum;
    string EmpNm; // etc.
 }

Am I getting Java memory allocation confused here? It appears that EmployeeRow variable and the ArrayList are sharing the same memory space - very peculiar. Thanks guys

Upvotes: 1

Views: 286

Answers (4)

user166390
user166390

Reputation:

Only one EmployeeRow object is every created.

Then it is modified. "It" being "the same object". If a new object is desired, then create a new object :)

Happy coding.

Upvotes: 2

rlperez
rlperez

Reputation: 1328

You aren't creating new rows so every element is the same and since the loop ends at ten the last object has a value of ten. 

public class BradfordReport {
EmployeeRow _empRow = new EmployeeRow();
ArrayList<EmployeeRow> _bradfordData = new ArrayList<EmployeeRow>();

public void Run() {
   // processing to setup Employee row variables
   for (int x=0; x<10; x++) {
       // This next line in debug IS ADJUSTING THE ARRAYLIST DATA!!
       _empRow = new EmployeeRow();
       _empRow.EmpNum = x; // etc for other variable in EmployeeRow
       _bradfordData.add(er);
   }
 }
 // THE RESULT IN _bradfordData is 10 elements, all with EmpNum = 10!
}

Upvotes: 1

Nico Huysamen
Nico Huysamen

Reputation: 10417

You are adding the same instance of the EmployeeRow class to the arraylist. Try something like:

public class BradfordReport {
    EmployeeRow _empRow = new EmployeeRow();
    ArrayList<EmployeeRow> _bradfordData = new ArrayList<EmployeeRow>();

    public void Run() {
       // processing to setup Employee row variables
       for (int x=0; x<10; x++) {
           // create a NEW INSTANCE of an EmployeeRow
           _empRow = new EmployeeRow();
           _empRow.EmpNum = x; // etc for other variable in EmployeeRow
           _bradfordData.add(_empRow);
       }
    }
    // THE RESULT IN _bradfordData is 10 elements, all with EmpNum = 10!
}

public class EmployeeRow {
    int EmpNum;
    string EmpNm; // etc.
 }

Upvotes: 4

dacwe
dacwe

Reputation: 43504

Yes when you do

 _empRow.EmpNum = x;

you are changing the objects internal variable. You need to construct a new object each time. Inside the loop do something like this:

EmployeeRow _empRow = new EmployeeRow();
 _empRow.EmpNum = x; 
 _bradfordData.add(_empRow);

Upvotes: 1

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