Sonar - Store a copy - Mutable members should not be stored or returned directly

Question

I have a list which is a private member in my class. I have used getter and setter to get and set the values. SOnar throws an error - Mutable members should not be stored or returned directly.

For example: ABC and DEF are two classes.

class ABC{
private List<DEF> defList;
public List<DEF> getDefList() { return defList; }
public void setDefList(List<DEF> defList) { this.defList = defList; }

After lot of googling and searching, I have understood that the getter can be changed as follows:

public List<DEF> getDefList() { return new ArrayList<>(defList); }

When i try to use setter similarly,

public void setDefList(List<DEF> defList) { this.defList.addAll(defList); }

then the variable starts showing

'private field 'defList' is never assigned.

May I know the correct way to do when it is a list, (a list of another class)

Note: Both the answers from Prasad Karunagoda and Leo Aso works. I cant mark both as accepted answer. So having a note here

Answer 1

The warning is because you did not give the field an initial value. This is how you should implement the code to ensure immutability using java.util.Collections.

class ABC {
    private List<DEF> defList = Collections.emptyList();

    public List<DEF> getDefList() { 
        return defList;
    }

    public void setDefList(List<DEF> defList) {
        // defensively copy, then make immutable
        defList = new ArrayList<>(defList);
        this.defList = Collections.unmodifiableList(defList);
    }