I beg to differ.  JDK 1.4.2-11 (copy on write)

StringBuffer:

    public String toString() {
    return new String(this);
    }

    final void setShared() { shared = true; }
    final char[] getValue() { return value; }

String:

    public String (StringBuffer buffer) {
        synchronized(buffer) {
            buffer.setShared();
            this.value = buffer.getValue();
            this.offset = 0;
            this.count = buffer.length();
        }
    }

JDK 1.5.0-6 (always copy)

StringBuffer:

    public synchronized String toString() {
    return new String(value, 0, count);
    }

    public String(char value[], int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > value.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }
        char[] v = new char[count];
        System.arraycopy(value, offset, v, 0, count);
        this.offset = 0;
        this.count = count;
        this.value = v;
    }

Regards

    robert