Question

I have a Java class as

class Students{
    private String fName;   
    private String lName;
    private String uName;
    
    
    public Students(String fName, String lName, String uName) {
            this.fName = fName;
            this.lName = lName;
            this.uName = uName;
     }      
    
    public String getuName() {
        return uName;
    }
    public void setuName(String uName) {
        this.uName = uName;
    }
    public String getfName() {
        return fName;
    }
    public void setfName(String fName) {
        this.fName = fName;
    }
    public String getlName() {
        return lName;
    }
    public void setlName(String lName) {
        this.lName = lName;
    }
    
}

Also I call this using

public class TestClass {
public static void main(String[] args) {

    Students students1 = new Students("xyz","abc","xyzAbc");
    Students students2 = new Students("poi","son","poison");
    Students students3 = new Students("yog","jos","yogjos");
    
    Students students4 = new Students("xyz","abc","xyzAbc");
    Students students5 = new Students("pon","son","xyzAbc");
    Students students6 = new Students("yog","jos","someotherUName");
    Students students7 = new Students("yog","jos","someotherUName2");
    
    List studentList1 = new ArrayList();
    List studentList2 = new ArrayList();
    
    studentList1.add(students1);
    studentList1.add(students2);
    studentList1.add(students3);
    
    studentList2.add(students4);
    studentList2.add(students5);
    studentList2.add(students6);
}
}

Now I want a filtered list which would contain only unique "uName" values. Thus I want the comparison between "uName" field of each list and remove common ones.

At the end I would want 2 filtered list for studentList1 and studentList2.

I read about the removeAll method, but it seems to work with List of Integer/String data and not with List of Objects (as in my case).

Answer 1

You can still use removeAll if the Objects in the List implement equals() properly.

AbstractCollection, which is the base for most kind of List implementations (including ArrayList) uses contains() in its implementation of removeAll. ArrayList‘s implementation of contains relies on indexOf(), which lastly uses equals().

You could implement equals() in your Student class to specify that an Student is equal to another if and only their uName fields are equal.

Please note that equals has associated semantics (see its javadoc), and you should be careful when choosing how to implement it. Consider if two student instances really represent the same student when their uNames are equal. In my opinion, this sounds like a very specific requirement of how to sort these things out and should not impact the semantics of the class.

You’ll be much better off with @AlexR or @KumarVivekMitra‘s approach.