Are custom classes in Java mutable by default?

Question

I've been reading up on encapsulation and was wondering; if I make a new class, is it by default mutable? If so, how would I go about making it an immutable class, if possible, without just doing defensive copying?

Thanks.

Answer 1

It depends on what you put in the class.

public class MutableClass {
    private String firstName;

    public MutableClass(String s) {
        firstName = s;
    }

    public String getFirstName() {
        return firstName;
    }

    // this allows mutation...
    public void setFirstName(String s) {
        firstName = s;
    }
}    

public class ImmutableClass {
    private String firstName;

    public MutableClass(String s) {
        firstName = s;
    }

    public String getFirstName() {
        return firstName;
    }
}

That doesn't account of things like setAccessible with reflection, but I expect that is not what you are concerned about.

I hope that helps.