Top 10 Java Questions on Strings and Arrays

Lương Quốc Tây — Mon, 13 Jul 2026 19:17:04 +0000

What is the difference between String, StringBuilder, and StringBuffer?
📌 Answer:
• String: Immutable. Every modification creates a new object in memory.
• StringBuilder: Mutable, fast, and not thread-safe.
• StringBuffer: Mutable and thread-safe (synchronized), but slower than StringBuilder due to synchronization overhead.
Why is String in Java immutable?
📌 Answer:
• Security: Strings are frequently used as parameters for network connections, URLs, and class loaders. Immutability prevents malicious alteration.
• Efficient Hashing: The hash code is cached upon creation, making Strings highly efficient and safe to use as keys in HashMap or elements in HashSet.
• Thread-Safety: Multiple threads can safely share the same String instance without the need for synchronization.
• String Pool Support: Enables memory savings by allowing the JVM to reuse existing instances from the String constant pool.
What is the String pool in Java?
📌 Answer:
• It is a special memory region within the heap used to store String literals.
• When creating a string like "hello", the JVM first checks the pool. If the string already exists, it reuses the existing reference instead of creating a new object.
• The intern() method can be used to manually add a String object to the pool.
What is the difference between == and .equals() when comparing Strings?
📌 Answer:
• ==: Compares object references (memory addresses).
• .equals(): Compares the actual content (character sequence) of the Strings.
String a = "abc"; String b = new String("abc"); System.out.println(a == b); // false (different memory addresses) System.out.println(a.equals(b)); // true (same content)
How do you reverse a String in Java?
📌 Answer:
• Using StringBuilder (Recommended):
String s = "Java"; String reversed = new StringBuilder(s).reverse().toString();
How do you check if a string is a palindrome?
📌 Answer:
• A palindrome is a string that reads the same forward and backward.
String s = "madam"; boolean isPalindrome = s.equals(new StringBuilder(s).reverse().toString());
How do you convert a String to a character array (char[])?
📌 Answer:
• By using the toCharArray() method:
String s = "hello"; char[] arr = s.toCharArray();
How do you find the largest element in an integer array?
📌 Answer: Iterate through the array while keeping track of the maximum value:
int[] arr = {5, 9, 2, 7}; int max = arr[0]; for (int i = 1; i < arr.length; i++) { if (arr[i] > max) { max = arr[i]; } }
How do you sort an array in Java?
📌 Answer:
• For primitive or object arrays, use the built-in method: Arrays.sort(arr);
• For object arrays, you can use a custom comparator (e.g., to sort in descending order):
// Note: 'arr' must be an array of objects (e.g., Integer[]), not primitives (int[]) Arrays.sort(arr, (a, b) -> b - a); // sorts in descending order
What is the difference between an Array and an ArrayList in Java?
📌 Answer:
• Array:
◦ Fixed size (cannot be resized after creation).
◦ Can contain both primitive types (e.g., int, char) and objects.
• ArrayList:
◦ Dynamic size (automatically resizes as elements are added or removed).
◦ Can only contain objects (uses wrapper classes like Integer or Character for primitives).
◦ Provides a rich set of utility methods: add(), remove(), contains(), etc.

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Top 10 Java Questions on Strings and Arrays