Cloning a mutable member

Question

I have a class SBContainer which has a StringBuffer member mySB. I am implementing Cloneable for SBContainer like this -

SBContainer implements Cloneable {
    public StringBuffer mySB;
    public SBContainer() {
        mySB = new StringBuffer("This is a test string");
    }
    public Object clone() throws CloneNotSupportedException {
        SBContainer cloned = (SBContainer)super.clone();
        return cloned;
    }
}

Now, I create an object sbc1 for MyContainer and it's clone sbc2. It looks like the container mySB is not cloned; and sbc1.mySB and sbc2.mySB are pointing to the same StringBuffer object. I tested using the following class -

public class SBContainerTest {
    public static void main(String[] args) {
        SBContainer sbc1 = new SBContainer();
        SBContainer sbc2 = null;
        try {
            sbc2 = (SBContainer)sbc1.clone();
        } catch(CloneNotSupportedException e) {
            e.printStackTrace();
        }
        sbc1.mySB.append(" ...something appended");
        System.out.println(sbc2.mySB);
    }
}

EDIT: The output was: This is a test string ...something appended

So, I tried to clone mySB like this -

cloned.mySB = (StringBuffer)mySB.clone();

...and I get this error -

SBContainerTest.java:8: clone() has protected access in java.lang.Object
        cloned.mySB = (StringBuffer)mySB.clone();
                                        ^
1 error

So, how can I achieve this? How can I clone a mutable member of a class which implements Cloneable?

Thanks.

Answer 1

StringBuffer is not Cloneable, so you'll have to clone it manually. I suggest something like this in your clone method:

public Object clone() throws CloneNotSupportedException {
    SBContainer cloned = (SBContainer)super.clone();
    cloned.mySB = new StringBuffer(this.mySB);
    return cloned;
}