SCJP Java String Handling and Regular Expressions

Question 1

What is the result of the following code?
String s = "Hello";
s.concat(" World");
System.out.println(s);

Accepted Answer

Hello

Answer

String objects are immutable; concat() returns a new String but the result is not assigned back to s, so s remains "Hello".

Question 2

Which of the following creates a String object in the string pool?

Accepted Answer

String s = "hello"

Answer

String literals (String s = "hello") are stored in the string pool, while using new String() always creates an object on the heap.

Question 3

What does the == operator compare when used between two String objects?

Accepted Answer

The reference (memory address) of the objects

Answer

== compares object references, not content; use equals() to compare String content.

Question 4

Which class provides a thread-safe, mutable sequence of characters?

Accepted Answer

StringBuffer

Answer

StringBuffer is synchronized, making it thread-safe, whereas StringBuilder is faster but not thread-safe.

Question 5

What is the output of: System.out.println("abc".indexOf('c'));

Accepted Answer

2

Answer

indexOf() returns the zero-based index; 'a' is at 0, 'b' at 1, 'c' at 2.

SCJP - Sun Certified Java Programmer Practice Test