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Create copy of a list

Sometimes we want to pass a list as parameter to a method, we will do some work on the list but don’t want to affect the origin list.

The code below is not good, as we are passing the orginal reference to the method, all changes will affect the orginal list


Instead, we should do something like:

     DoSomeWork(new ArrayList(SelectedPerson.getAllAddresses()));

Pleaes be awared that the second example passed a copy of the list to DoSomeWork, but the elements in the copy still hold reference to the orginal elements. Modifying any object element in the copy list will also change the element in the original.


hibernate criteria on composite id

public class TeacherStudentPK  implements{
    @JoinColumn(name = "STUDENT_ID")
	private Student student;
    @JoinColumn(name = "TEACHER_ID")
public class TeacherStudent implements {

	/* primay key*/
	private TeacherStudentPK teacherStudentPK;
      public class Student{
	private Collection teachers = new ArrayList();

The crieteria is shown below:

criteria = em.createCriteria(Student.class, "student")
		   .createAlias("student.teachers", "teacherStudent")
.add(Restrictions.eq("teacherStudent.teacherStudentPK.teacher", selectedTeacher))

JMS queue listener

Let’s say we are saving a note along some pics into the database. We will need to resize the pics and only save thumbnails. The resizing images part could take a couple seconds and we don’t want user to wait on the page, so we do it asynchronously.

The workflow would be: 1 save note;  2 flush session; 3 send resizing images request to jms ; 4 JMS queue listener receive the msg and find service action to create thumbnail.

During the resizing image process, I need to link the resized images to the note and then save the thumbnail. I had some code like below:

    public void createThumbnail(Note note){
        Note note = em.merge(note);

This doesn’t work because the process is asynchronous, when the resizing starts in JMS the note may not be in the database yet. What I had in my case the createThumbnail method then create another record for note, so I have duplicate record in the database.

To solve this problem, we should use find to look up the record in database before perform any action. If we can’t find the record in database then we should rollback and retry in JMS.

so the code goes:

    public void createThumbnail(Note note){
        Note newNote = em.find(note);
        if(newNote == null){


singtonMap and singletonList allow you quickly create an unmodifiable collection:

methodTakesAMap(Collections.singletonMap(key, value));

rather than this:

Map<KeyType, ValueType> m = new HashMap<KeyType, ValueType>();
m.put(key, value);

singletonList example:

List sampleList = collections.singletonList("theOnlyOption");

JQuery notes

  1. How to add listener for html content loaded by ajax call?

    Bind on a non-dynamic parent container using .on()

    $('.some-parent-class').on('click', '.element', function() {
      // DO STUFF!
  2. AddClass function

      AddClass function always check exisitng classes before adding new one, so we don’t need to check duplicate classes

     3. Find function

       The .find() and .children() methods are similar, except that the latter only travels a single level down the DOM tree

     4. text() and textarea

       The .text() method cannot be used on form inputs or scripts. To set or get the text value of input or textarea elements, use the .val() method

      5. checkbox and prop

$('.myCheckbox').prop('checked', true);
$('.myCheckbox').prop('checked', false);

      6. toggle

Quick way to switch show/hide, checked/unchecked state

$('#checkBoxId').click(function() {

       7. event.preventDefault()

Cancel the default action (navigation) of the click, this is used when we only want to trigger a js function but don’t want the page navagation.


access javascript object property using variable

I want to assign data to a js object as a property, but the new property name is dynamic.

The bracket notation should be used here:

var apiDynamicString = "dynamicStringFromOtherFunction";

var apiDynamicData ="somedata...";[apiDynamicString] = apiDynamicData ;

Then you can access the property this way:


OR (if you know the variable value)


javascript onclick event not working in chrome

The onclick event for select option is not triggered in chrome. We need to fire the onchange event on the select component.

<select class="select-one-group-select">
 <option class="select-one-group-option" value="option1">option 1</option>
 <option class="select-one-group-option"  value="option2">option 2</option>
 <option class="select-one-group-option"  value="option3">option 3</option>

This doesn’t work:

	$(".select-one-group-option").click(function() {


This solve the problem:

	$(".select-one-group-select").change(function() {

JPA native query with parameter

I tried ?name but couldn’t get it working. I end up with using number (?1 ,?2) to build up my dynamic query and it works properly.
Query query = em.createNativeQuery(“SELECT * FROM OP_CHECKLIST c WHERE c.DATE BETWEEN ?1 AND ?2 “);

Query query = em.createNativeQuery("SELECT * FROM OP_CHECKLIST c WHERE c.DATE BETWEEN ?1 AND ?2 ");
query.setParameter("1", sdf.format(fromDate));
query.setParameter("2", sdf.format(endDate));

Set Content Security Policy for Cordova app

I’m getting some error when debuging my cordova app on Android device.  The error msg is

Refused to load the image 'data:image/png;base64,....' because it violates the following Content Security Policy directive: "img-src 'self' data".

I googled it and found that I need to set content security policy meta tag.

This link gives detailed example and helped me solve the problem.

MySQL innodb file too large

We had problem when altering one of our largest table in MySQL server. Eventually we managed to apply the change, but MySQL wouldn’t release the disk space it took for temp table. MySQL mark the data as deleted, but it still holding the space. This link gives good answer on this problem:

Optimize innodb table when innodb_file_per_table disable