What is the Superclass of StringBuilder and StringBuffer

Their javadoc page shows they both extend from Object:

public final class StringBuilder
extends Object
implements Serializable, CharSequence

public final class StringBuffer
extends Object
implements Serializable, CharSequence
Is Object really the direct superclass of StringBuilder and StringBuffer? Since the two string-related classes are so similar (see my previous post for a short comparison), doesn't it make sense for them to have a common superclass?

Yes, they do share a common abstract superclass, java.lang.AbstractStringBuilder, as revealed in their source code:
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence {
/** use serialVersionUID for interoperability */
static final long serialVersionUID = 4383685877147921099L;
AbstractStringBuilder is an abstract class with package access type:
abstract class AbstractStringBuilder
implements Appendable, CharSequence {


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