In Java (String immutability and concatenation), what is printed? String s = "ABC"; s.toLowerCase(); s += "def"; System.out.println(s);

Introduction / Context:
Here we check whether calling toLowerCase() without assignment changes a String and how concatenation with += works for immutable Strings in Java. The operation order determines the final printed value.

Given Data / Assumptions:

• s is initially "ABC".
• s.toLowerCase() result is ignored.
• s += "def" concatenates and reassigns to s.

Concept / Approach:
Strings are immutable; methods return new objects rather than mutating the original. If you do not store the returned String from toLowerCase(), s remains unchanged ("ABC"). The += operation creates a new String "ABCdef" and assigns it to s.

Step-by-Step Solution:

Verification / Alternative check:
If you wanted "abcdef", you would need s = s.toLowerCase(); before concatenation or write ("ABC".toLowerCase() + "def").

Why Other Options Are Wrong:

• "ABC" ignores the concatenation.
• "abc"/"abcdef" require assigning the lowercase result.
• Compile Error: code is valid.

Common Pitfalls:
Expecting String methods to mutate the instance; they never do. Always store the returned value when transformation is desired.

Final Answer:
ABCdef