Java DefaultListModel包含奇怪元素的问题

Question

I am writing a Java application with swing components. I have 2 JLists which get their data from a DefaultListModel, being these 2:

private DefaultListModel<Module> moduleListModel;
private DefaultListModel<Module> sem1ListModel;

Module is a dataclass I have created.

Now the problem is, I want to remove Modules from my moduleListModel, if they are also in the sem1ListModel. I know there are 3 modules in sem1ListModel which are also in the defaultListModel, and the code I use to check if it contains them is:

for(int i = 0; i < sem1ListModel.getSize(); i++){
        Module mod = sem1ListModel.getElementAt(i);
                    System.out.println(mod);
        if(moduleListModel.contains(mod)){
            System.out.println(mod);
            moduleListModel.removeElement(mod);
        }
    }

This code should be working, as the Modules are both exactly the same, yet it doesn't! When I print both DefaultListModel to the output window, I get this:

    System.out.println(sem1ListModel);
    System.out.println(moduleListModel);

sem1ListModel: [Software development 1 [7 credits], ICT Management 1 [7 credits], DOA I [6 credits]] moduleListModel: [DOA I [6 credits], Software development 1 [7 credits], ICT Management 1 [7 credits], Webdesign 2 [5 credits], Software 1 [5 credits]]

As you can see, Software development 1 [7 credits], ICT Management 1 [7 credits], DOA I [6 credits] are the 3 Modules which are in both DefaultListModels and should be deleted from moduleListModel, yet this doesn't happen.

What am I doing wrong?

Answer 1

I'll just make an answer from the comments so this doesn't hang about.

The problem is that the Module objects are in fact different instances, and did not have equals overridden. This means that, even though they printed the same thing via the overridden toString , the model treated them as distinct objects and did not perform the removal.

The standard way to fix this is to override equals (and hashCode in case you ever want to use a hashing data structure).