In Java (String vs StringBuilder API), what is the output or error? String d = "bookkeeper"; d.substring(1, 7); d = "w" + d; d.append("woo"); /* Line 4 */ System.out.println(d);

Introduction / Context:
This snippet distinguishes between the APIs of immutable String and mutable StringBuilder/StringBuffer. It also highlights that ignoring the return value of substring does not change the original String, and that append is not a member of String at all.

Given Data / Assumptions:

• d is declared as String.
• d.substring(1,7) is called but its result is discarded.
• d = "w" + d; creates a new String.
• d.append("woo") is attempted on a String.

Concept / Approach:
Only StringBuilder and StringBuffer provide append. Invoking append on a String is a compile-time error: cannot find symbol: method append(java.lang.String). If a mutable sequence was intended, declare StringBuilder d = new StringBuilder("bookkeeper"); and then use d.append(...).

Step-by-Step Solution:

Verification / Alternative check:
If we replaced line 4 with d = d + "woo";, output would be wbookkeeperwoo. If we used StringBuilder, we could build efficiently without intermediate strings.

Why Other Options Are Wrong:

• They assume either append exists on String or that substring mutated the original; neither is true.

Common Pitfalls:
Treating Strings as mutable, or assuming substring changes the receiver. Always capture returned values from String operations.

Final Answer:
Compilation fails.