Java String Interview Questions 2026: 27 Q&A With Code

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~15 min

Strings appear in nearly every Java interview because they touch immutability, memory, hashing, and performance all at once. Candidates report that the string pool, == versus equals, and StringBuilder versus StringBuffer are guaranteed questions. This guide covers 27 String questions with full answers, runnable code, and predict-the-output traps. Behaviour reflects the official Oracle java.lang.String documentation; confirm any method-level detail on the official docs.

Pair this with Java Interview Questions 2026 and Java Collections Interview Questions 2026.

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Table of Contents

1. Immutability and the Pool (Q1 to Q9)
2. Comparison and Methods (Q10 to Q18)
3. StringBuilder and Performance (Q19 to Q24)
4. Advanced (Q25 to Q27)
5. Predict the Output
6. Quick Revision Table
7. Frequently Asked Questions

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Immutability and the Pool

Q1. Why is String immutable in Java? Medium

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Q2. What is the string constant pool? Medium

String a = "hi";   // goes to pool
String b = "hi";   // reuses the same pool reference
System.out.println(a == b);   // true

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Q3. What is the difference between String literal and new String()? Medium

String x = new String("hi");
System.out.println(x == "hi");          // false
System.out.println(x.intern() == "hi"); // true

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Q4. What does the intern() method do? Hard

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Q5. How many objects are created by String s = new String("hi")? Hard

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Q6. Is String thread-safe? Medium

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Q7. Can you extend the String class? Easy

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Q8. Why is String a popular choice for HashMap keys? Medium

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Q9. What is the size of the string pool and where does it live? Hard

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Comparison and Methods

Q10. What is the difference between == and equals for strings? Easy

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Q11. What is the difference between equals and equalsIgnoreCase? Easy

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Q12. What does compareTo return for strings? Medium

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Q13. What is the difference between substring in older and newer Java? Hard

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Q14. How do you reverse a string in Java? Easy

String reversed = new StringBuilder("abc").reverse().toString(); // "cba"

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Q15. How do you check if two strings are anagrams? Medium

boolean anagram(String a, String b) {
    if (a.length() != b.length()) return false;
    int[] count = new int[256];
    for (char c : a.toCharArray()) count[c]++;
    for (char c : b.toCharArray()) if (--count[c] < 0) return false;
    return true;
}

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Q16. What does String.split do with a trailing empty string? Hard

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Q17. What is the difference between length() for String and length for arrays? Easy

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Q18. How do you convert a String to a char array and back? Easy

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StringBuilder and Performance

Q19. What is the difference between String, StringBuilder, and StringBuffer? Medium

Type	Mutable	Thread-safe	Speed
String	No	Yes (immutable)	n/a
StringBuilder	Yes	No	Fast
StringBuffer	Yes	Yes	Slower

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Q20. Why is string concatenation in a loop inefficient? Medium

StringBuilder sb = new StringBuilder();
for (int i = 0; i < n; i++) sb.append(i);
String result = sb.toString();

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Q21. How does the compiler optimise simple string concatenation? Hard

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Q22. Is StringBuilder thread-safe? Easy

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Q23. How do you efficiently join a list of strings? Medium

String csv = String.join(",", List.of("a", "b", "c")); // "a,b,c"

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Q24. What are text blocks in Java? Medium

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Advanced

Q25. How does the JVM handle String concatenation with primitives? Medium

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Q26. What is the hashCode formula for String? Hard

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Q27. What is the difference between isEmpty and isBlank? Easy

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Predict the Output

Snippet 1

String a = "java";
String b = "ja" + "va";        // compile-time constant
String c = new String("java");
System.out.println(a == b);
System.out.println(a == c);

Output:

true
false

Why: "ja" + "va" is folded into the literal "java" at compile time and interned, so a == b is true. new String creates a distinct heap object, so a == c is false.

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Snippet 2

String s = "abc";
s.concat("def");
System.out.println(s);

Output:

abc

Why: Strings are immutable, concat returns a new string that is discarded here. s is unchanged. You must assign the result.

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Snippet 3

System.out.println("5" + 3 + 2);
System.out.println(5 + 3 + "2");

Output:

532
82

Why: Evaluation is left to right. In line 1, the first operand is a string, so everything concatenates. In line 2, 5 + 3 computes 8 first, then concatenates with "2".

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Quick Revision Table

Concept	Key takeaway
Immutability	Enables pool, thread safety, safe keys
== vs equals	reference vs content
Literal vs new	pooled vs fresh heap object
Loop concat	use StringBuilder, not +
StringBuilder vs Buffer	unsynchronised vs synchronised
substring leak	fixed since Java 7u6
hashCode	31-based polynomial, cached

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Frequently Asked Questions

Why is String immutable in Java?

Immutability enables safe string-pool sharing, thread safety without locks, safe use as HashMap keys with a cached hash code, and security for sensitive values like file paths. It is the single most asked String concept.

What is the difference between == and equals for strings?

== compares references, equals compares character content. Because of the string pool, == can be accidentally true for literals but breaks for new String. Always use equals for value comparison.

When should I use StringBuilder over String concatenation?

Use StringBuilder whenever you build a string in a loop. Repeated + creates a new String each iteration giving O(n squared) behaviour, while StringBuilder mutates one buffer in O(n).

How do I reverse a string in an interview?

The one-liner is new StringBuilder(s).reverse().toString(), but be ready to do it manually with a two-pointer swap on a char array, since some interviewers want to see the mechanics rather than the library call.

What is the difference between isEmpty and isBlank?

isEmpty is true only for a zero-length string, while isBlank (Java 11+) is also true for strings that contain only whitespace. Use isBlank to treat " " as effectively empty.

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Related Articles

• Java Interview Questions 2026, the master 50-question set
• Java Collections Interview Questions 2026, hashing and keys
• Java OOPs Interview Questions 2026, OOP foundations
• Java Output Prediction Questions 2026, more tricky snippets

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Confirm any method-level behaviour on the official Oracle java.lang.String documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.