Why does Java StringBuilder have a constructor for CharSequence and another one for String?

Question

Knowing that String implements CharSequence interface, so why does StringBuilder have a constructor for CharSequence and another one for String? No indication in the javadoc !

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {...}

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

...

    /**
     * Constructs a string builder initialized to the contents of the
     * specified string. The initial capacity of the string builder is
     * {@code 16} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    /**
     * Constructs a string builder that contains the same characters
     * as the specified {@code CharSequence}. The initial capacity of
     * the string builder is {@code 16} plus the length of the
     * {@code CharSequence} argument.
     *
     * @param      seq   the sequence to copy.
     */
    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
...
}

Answer 1

Optimization. If I am not mistaken, there are two implementations of append. append(String) is more efficient than append(CharSequence) where CharSequence is a string. If I had to do some extra routine to check to make sure the CharSequence is compatible with String, convert it to String, and run the append(String), that would be longer than append(String) directly. Same result. Different speed.