Java Notes

String Interview Questions

Comprehensive collection of Java String interview questions covering conceptual, coding, output prediction, and tricky questions asked in technical interviews at all levels.

This comprehensive collection covers the most frequently asked Java String interview questions organized by difficulty level and topic.

Section 1: Fundamental Conceptual Questions

Q1: What is a String in Java?

Answer: String is a class in java.lang package that represents an immutable sequence of characters. It is declared as public final class String and implements Serializable, Comparable<String>, and CharSequence interfaces. Strings are objects stored in heap memory, with literals benefiting from the String Pool.

Q2: Why is String immutable in Java?

Answer: String is immutable for five key reasons:

String Pool sharing — Multiple variables can safely reference the same pool object
Security — Strings used in network connections, file paths, and class loading can't be maliciously altered
Thread safety — Immutable objects are inherently thread-safe without synchronization
HashCode caching — Since content never changes, hashCode is computed once and reused
Class loading — JVM uses strings for class names; mutable strings would compromise classloading security

Q3: What is the String Pool?

Answer: The String Pool (String Constant Pool) is a special memory area inside the heap (since Java 7) that stores unique string literals. When you create a literal like "Hello", JVM checks if this string exists in the pool:

If yes → returns the existing reference (no new object)
If no → creates a new string in the pool

This saves memory by sharing identical strings. Only string literals and intern()'d strings are in the pool.

Q4: Explain == vs .equals() for Strings.

Answer:

== compares references (memory addresses) — true only if both variables point to the exact same object
.equals() compares content (character by character) — true if both strings have identical characters

java example

String a = "Hello";       // Pool
String b = "Hello";       // Same pool object
String c = new String("Hello"); // New heap object

a == b;        // true (same pool object)
a == c;        // false (different objects)
a.equals(c);   // true (same content)

Rule: Always use .equals() for content comparison.

Q5: How many objects are created by `String s = new String("Hello")`?

Answer: Up to 2 objects:

"Hello" in String Pool — created from the literal (if not already present)
New String object in heap — created by the new keyword (always)

The variable s references the heap object (#2), not the pool object.

Q6: What is the intern() method?

Answer: intern() returns the canonical (pool) representation of a string:

If the string exists in the pool → returns the pool reference
If not → adds it to the pool and returns that reference

java example

String s1 = new String("Hello"); // Heap object
String s2 = s1.intern();          // Pool reference
String s3 = "Hello";              // Pool reference
System.out.println(s2 == s3);     // true

Q7: What is the difference between String, StringBuffer, and StringBuilder?

Answer:

Feature	String	StringBuffer	StringBuilder
Mutability	Immutable	Mutable	Mutable
Thread-safe	Yes (immutable)	Yes (synchronized)	No
Performance	Slow for modifications	Medium	Fast
Storage	String Pool + Heap	Heap	Heap
Since	JDK 1.0	JDK 1.0	JDK 1.5
Use case	Constants, keys	Multi-threaded building	Single-threaded building

Q8: Why is String final?

Answer: The final modifier on String class:

Prevents subclassing — no class can extend String and override methods
Preserves immutability — a subclass could add mutable state, breaking guarantees
Enables JVM optimizations — compiler knows methods won't be overridden
Security — ensures String behavior is predictable and unalterable

Q9: Can we use String in switch statements?

Answer: Yes, since Java 7. Strings in switch use .equals() internally:

Example

String command = "start"; switch (command) { case "start": System.out.println("Starting..."); break; case "stop": System.out.println("Stopping..."); break; default: System.out.println("Unknown command"); }

The switch compares using equals() and hashCode() for efficiency.

Q10: What is the String.format() method?

Answer: A static method that creates formatted strings using format specifiers:

%s — String
%d — Integer
%f — Float/Double (%.2f for 2 decimal places)
%c — Character
%b — Boolean
%n — Platform-specific newline

java example

String result = String.format("Name: %s, Age: %d, GPA: %.2f", "Alice", 25, 3.85);
// "Name: Alice, Age: 25, GPA: 3.85"

Section 2: Output Prediction Questions

Q11: What is the output?

java example

public class Output1 {
    public static void main(String[] args) {
        String s1 = "Hello";
        String s2 = "Hello";
        String s3 = new String("Hello");
        
        System.out.println(s1 == s2);
        System.out.println(s1 == s3);
        System.out.println(s1.equals(s3));
    }
}

Output

true
false
true

Explanation: s1 and s2 share the same pool object. s3 is a separate heap object. .equals() compares content.

Q12: What is the output?

java example

public class Output2 {
    public static void main(String[] args) {
        String s1 = "Java";
        String s2 = "Ja" + "va";
        String s3 = "Ja";
        String s4 = s3 + "va";
        
        System.out.println(s1 == s2);
        System.out.println(s1 == s4);
        System.out.println(s1.equals(s4));
    }
}

Output

true
false
true

Explanation: "Ja" + "va" is computed at compile time (constant folding) → same as "Java" in pool. But s3 + "va" involves a variable, so it's computed at runtime using StringBuilder → new heap object.

Q13: What is the output?

java example

public class Output3 {
    public static void main(String[] args) {
        String s = "Hello";
        s.toUpperCase();
        s.concat(" World");
        System.out.println(s);
    }
}

Output

Hello

Explanation: String is immutable. toUpperCase() and concat() return NEW strings, but the results are not assigned back to s. The original s remains "Hello".

Q14: What is the output?

java example

public class Output4 {
    public static void main(String[] args) {
        String s1 = "Hello";
        String s2 = new String("Hello");
        String s3 = s2.intern();
        
        System.out.println(s1 == s2);
        System.out.println(s1 == s3);
        System.out.println(s2 == s3);
    }
}

Output

false
true
false

Explanation: s2 is a heap object. s3 = s2.intern() returns the pool reference (same as s1). So s1 == s3 is true, but s2 is still the separate heap object.

Q15: What is the output?

java example

public class Output5 {
    public static void main(String[] args) {
        String s1 = "abc";
        String s2 = "abc";
        
        System.out.println(s1.hashCode() == s2.hashCode());
        System.out.println(System.identityHashCode(s1) == System.identityHashCode(s2));
    }
}

Output

true
true

Explanation: Both have same content so same hashCode. And since both are from pool, they're the same object — so identityHashCode (which is based on object identity) is also the same.

Q16: What is the output?

java example

public class Output6 {
    public static void main(String[] args) {
        String str = null;
        System.out.println("null".equals(str));
        System.out.println(str.equals("null"));
    }
}

Output

false
Exception in thread "main" java.lang.NullPointerException

Explanation: "null".equals(str) safely returns false (comparing "null" content with null reference). But str.equals("null") calls a method on null → NullPointerException.

Best Practice: Put the literal on the left: "expected".equals(variable).

Section 3: Coding Questions

Q17: How to count occurrences of a character in a string?

java example

public class CharCount {
    public static void main(String[] args) {
        String str = "Hello World";
        char target = 'l';
        
        long count = str.chars().filter(c -> c == target).count();
        System.out.println("'" + target + "' appears " + count + " times in \"" + str + "\"");
    }
}

Output

'l' appears 3 times in "Hello World"

Q18: Write a program to check if a string is a valid email.

Example

public class EmailValidation { public static void main(String[] args) { String[] emails = {"user@example.com", "invalid@", "@domain.com", "user.name@domain.co.in", "no-at-sign"}; for (String email : emails) { boolean valid = email.matches("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}"); System.out.println(email + " → " + (valid ? "Valid" : "Invalid")); } } }

Output

user@example.com → Valid
invalid@ → Invalid
@domain.com → Invalid
user.name@domain.co.in → Valid
no-at-sign → Invalid

Q19: Find all permutations of a string.

Example

public class StringPermutations { static void permute(String str, int l, int r) { if (l == r) { System.out.print(str + " "); return; } for (int i = l; i <= r; i++) { str = swap(str, l, i); permute(str, l + 1, r); str = swap(str, l, i); // backtrack } } static String swap(String s, int i, int j) { char[] arr = s.toCharArray(); char temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; return new String(arr); } public static void main(String[] args) { String str = "ABC"; System.out.println("Permutations of \"" + str + "\":"); permute(str, 0, str.length() - 1); System.out.println(); } }

Output

Permutations of "ABC":
ABC ACB BAC BCA CBA CAB

Q20: Longest common prefix among an array of strings.

Example

public class LongestCommonPrefix { public static void main(String[] args) { String[] strs = {"flower", "flow", "flight"}; String prefix = strs[0]; for (int i = 1; i < strs.length; i++) { while (!strs[i].startsWith(prefix)) { prefix = prefix.substring(0, prefix.length() - 1); if (prefix.isEmpty()) break; } } System.out.println("Strings: " + java.util.Arrays.toString(strs)); System.out.println("Longest common prefix: \"" + prefix + "\""); } }

Output

Strings: [flower, flow, flight]
Longest common prefix: "fl"

Section 4: Advanced Questions

Q21: How does String concatenation work internally?

Answer: The + operator for strings is compiled to StringBuilder operations:

java example

// Source code:
String result = "Hello" + " " + name + "!";

// Compiled to (approximately):
String result = new StringBuilder()
    .append("Hello").append(" ").append(name).append("!")
    .toString();

Exception: If ALL operands are compile-time constants (literals or final variables), the compiler performs concatenation at compile time (constant folding).

Q22: What is the difference between `String.valueOf()` and `Object.toString()`?

Answer:

String.valueOf(null) → returns the String "null" (safe)
null.toString() → throws NullPointerException
String.valueOf(obj) internally calls obj.toString() but handles null

Q23: Why shouldn't we use String as a password storage?

Answer:

Strings are immutable — cannot be cleared from memory after use
Strings may reside in String Pool for extended periods
String content appears in heap dumps
Use char[] instead — can be zeroed out after use: Arrays.fill(password, '0')

Q24: How is String hashCode calculated?

Answer: Using the formula: s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

Example

// Simplified implementation: int hash = 0; for (int i = 0; i < str.length(); i++) { hash = 31 * hash + str.charAt(i); }

31 is used because: it's a prime (reduces collisions), it's odd (avoids overflow issues with even numbers), and 31 * x can be optimized to (x << 5) - x by the JIT compiler.

Q25: What are compact strings (Java 9+)?

Answer: Before Java 9, Strings stored characters in a char[] (2 bytes per character). Since Java 9, strings that contain only Latin-1 characters use a byte[] with 1 byte per character (LATIN1 encoding). Strings with other characters use 2 bytes per character (UTF-16). This reduces memory usage by approximately 30-40% for typical applications.

Common Mistakes

Using == for content comparison — always returns wrong results for non-literal strings
Not assigning method results — s.trim() without s = s.trim()
Calling methods on null — always check or put literal first
String concatenation in loops — use StringBuilder
Ignoring case sensitivity — use equalsIgnoreCase when appropriate
Not understanding String Pool — leads to confusion about == behavior

Best Practices

Always use .equals() for String comparison
Put literals first in equals: "text".equals(variable)
Use StringBuilder for repeated concatenation
Use String.isEmpty() or isBlank() instead of s.equals("")
Consider StringJoiner or String.join() for joining collections
Use char[] for passwords — not String
Set initial StringBuilder capacity when size is known
Use switch on Strings (Java 7+) instead of if-else chains
Prefer String.format() for complex formatting
Understand intern() but don't overuse it — let the JVM manage the pool

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for String Interview Questions.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

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Java Notes

String Interview Questions

Comprehensive collection of Java String interview questions covering conceptual, coding, output prediction, and tricky questions asked in technical interviews at all levels.

This comprehensive collection covers the most frequently asked Java String interview questions organized by difficulty level and topic.

Section 1: Fundamental Conceptual Questions

Q1: What is a String in Java?

Q2: Why is String immutable in Java?

Answer: String is immutable for five key reasons:

String Pool sharing — Multiple variables can safely reference the same pool object
Security — Strings used in network connections, file paths, and class loading can't be maliciously altered
Thread safety — Immutable objects are inherently thread-safe without synchronization
HashCode caching — Since content never changes, hashCode is computed once and reused
Class loading — JVM uses strings for class names; mutable strings would compromise classloading security

Q3: What is the String Pool?

If yes → returns the existing reference (no new object)
If no → creates a new string in the pool

This saves memory by sharing identical strings. Only string literals and intern()'d strings are in the pool.

Q4: Explain == vs .equals() for Strings.

Answer:

== compares references (memory addresses) — true only if both variables point to the exact same object
.equals() compares content (character by character) — true if both strings have identical characters

java example

String a = "Hello";       // Pool
String b = "Hello";       // Same pool object
String c = new String("Hello"); // New heap object

a == b;        // true (same pool object)
a == c;        // false (different objects)
a.equals(c);   // true (same content)

Rule: Always use .equals() for content comparison.

Q5: How many objects are created by `String s = new String("Hello")`?

Answer: Up to 2 objects:

"Hello" in String Pool — created from the literal (if not already present)
New String object in heap — created by the new keyword (always)

The variable s references the heap object (#2), not the pool object.

Q6: What is the intern() method?

Answer: intern() returns the canonical (pool) representation of a string:

If the string exists in the pool → returns the pool reference
If not → adds it to the pool and returns that reference

java example

String s1 = new String("Hello"); // Heap object
String s2 = s1.intern();          // Pool reference
String s3 = "Hello";              // Pool reference
System.out.println(s2 == s3);     // true

Q7: What is the difference between String, StringBuffer, and StringBuilder?

Answer:

Feature	String	StringBuffer	StringBuilder
Mutability	Immutable	Mutable	Mutable
Thread-safe	Yes (immutable)	Yes (synchronized)	No
Performance	Slow for modifications	Medium	Fast
Storage	String Pool + Heap	Heap	Heap
Since	JDK 1.0	JDK 1.0	JDK 1.5
Use case	Constants, keys	Multi-threaded building	Single-threaded building

Q8: Why is String final?

Answer: The final modifier on String class:

Prevents subclassing — no class can extend String and override methods
Preserves immutability — a subclass could add mutable state, breaking guarantees
Enables JVM optimizations — compiler knows methods won't be overridden
Security — ensures String behavior is predictable and unalterable

Q9: Can we use String in switch statements?

Answer: Yes, since Java 7. Strings in switch use .equals() internally:

Example

The switch compares using equals() and hashCode() for efficiency.

Q10: What is the String.format() method?

Answer: A static method that creates formatted strings using format specifiers:

%s — String
%d — Integer
%f — Float/Double (%.2f for 2 decimal places)
%c — Character
%b — Boolean
%n — Platform-specific newline

java example

String result = String.format("Name: %s, Age: %d, GPA: %.2f", "Alice", 25, 3.85);
// "Name: Alice, Age: 25, GPA: 3.85"

Section 2: Output Prediction Questions

Q11: What is the output?

java example

public class Output1 {
    public static void main(String[] args) {
        String s1 = "Hello";
        String s2 = "Hello";
        String s3 = new String("Hello");
        
        System.out.println(s1 == s2);
        System.out.println(s1 == s3);
        System.out.println(s1.equals(s3));
    }
}

Output

true
false
true

Explanation: s1 and s2 share the same pool object. s3 is a separate heap object. .equals() compares content.

Q12: What is the output?

java example

public class Output2 {
    public static void main(String[] args) {
        String s1 = "Java";
        String s2 = "Ja" + "va";
        String s3 = "Ja";
        String s4 = s3 + "va";
        
        System.out.println(s1 == s2);
        System.out.println(s1 == s4);
        System.out.println(s1.equals(s4));
    }
}

Output

true
false
true

Q13: What is the output?

java example

public class Output3 {
    public static void main(String[] args) {
        String s = "Hello";
        s.toUpperCase();
        s.concat(" World");
        System.out.println(s);
    }
}

Output

Hello

Explanation: String is immutable. toUpperCase() and concat() return NEW strings, but the results are not assigned back to s. The original s remains "Hello".

Q14: What is the output?

java example

public class Output4 {
    public static void main(String[] args) {
        String s1 = "Hello";
        String s2 = new String("Hello");
        String s3 = s2.intern();
        
        System.out.println(s1 == s2);
        System.out.println(s1 == s3);
        System.out.println(s2 == s3);
    }
}

Output

false
true
false

Explanation: s2 is a heap object. s3 = s2.intern() returns the pool reference (same as s1). So s1 == s3 is true, but s2 is still the separate heap object.

Q15: What is the output?

java example

public class Output5 {
    public static void main(String[] args) {
        String s1 = "abc";
        String s2 = "abc";
        
        System.out.println(s1.hashCode() == s2.hashCode());
        System.out.println(System.identityHashCode(s1) == System.identityHashCode(s2));
    }
}

Output

true
true

Explanation: Both have same content so same hashCode. And since both are from pool, they're the same object — so identityHashCode (which is based on object identity) is also the same.

Q16: What is the output?

java example

public class Output6 {
    public static void main(String[] args) {
        String str = null;
        System.out.println("null".equals(str));
        System.out.println(str.equals("null"));
    }
}

Output

false
Exception in thread "main" java.lang.NullPointerException

Explanation: "null".equals(str) safely returns false (comparing "null" content with null reference). But str.equals("null") calls a method on null → NullPointerException.

Best Practice: Put the literal on the left: "expected".equals(variable).

Section 3: Coding Questions

Q17: How to count occurrences of a character in a string?

java example

public class CharCount {
    public static void main(String[] args) {
        String str = "Hello World";
        char target = 'l';
        
        long count = str.chars().filter(c -> c == target).count();
        System.out.println("'" + target + "' appears " + count + " times in \"" + str + "\"");
    }
}

Output

'l' appears 3 times in "Hello World"

Q18: Write a program to check if a string is a valid email.

Example

Output

user@example.com → Valid
invalid@ → Invalid
@domain.com → Invalid
user.name@domain.co.in → Valid
no-at-sign → Invalid

Q19: Find all permutations of a string.

Example

Output

Permutations of "ABC":
ABC ACB BAC BCA CBA CAB

Q20: Longest common prefix among an array of strings.

Example

Output

Strings: [flower, flow, flight]
Longest common prefix: "fl"

Section 4: Advanced Questions

Q21: How does String concatenation work internally?

Answer: The + operator for strings is compiled to StringBuilder operations:

java example

// Source code:
String result = "Hello" + " " + name + "!";

// Compiled to (approximately):
String result = new StringBuilder()
    .append("Hello").append(" ").append(name).append("!")
    .toString();

Exception: If ALL operands are compile-time constants (literals or final variables), the compiler performs concatenation at compile time (constant folding).

Q22: What is the difference between `String.valueOf()` and `Object.toString()`?

Answer:

String.valueOf(null) → returns the String "null" (safe)
null.toString() → throws NullPointerException
String.valueOf(obj) internally calls obj.toString() but handles null

Q23: Why shouldn't we use String as a password storage?

Answer:

Strings are immutable — cannot be cleared from memory after use
Strings may reside in String Pool for extended periods
String content appears in heap dumps
Use char[] instead — can be zeroed out after use: Arrays.fill(password, '0')

Q24: How is String hashCode calculated?

Answer: Using the formula: s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

Example

// Simplified implementation: int hash = 0; for (int i = 0; i < str.length(); i++) { hash = 31 * hash + str.charAt(i); }

31 is used because: it's a prime (reduces collisions), it's odd (avoids overflow issues with even numbers), and 31 * x can be optimized to (x << 5) - x by the JIT compiler.

Q25: What are compact strings (Java 9+)?

Common Mistakes

Using == for content comparison — always returns wrong results for non-literal strings
Not assigning method results — s.trim() without s = s.trim()
Calling methods on null — always check or put literal first
String concatenation in loops — use StringBuilder
Ignoring case sensitivity — use equalsIgnoreCase when appropriate
Not understanding String Pool — leads to confusion about == behavior

Best Practices

Always use .equals() for String comparison
Put literals first in equals: "text".equals(variable)
Use StringBuilder for repeated concatenation
Use String.isEmpty() or isBlank() instead of s.equals("")
Consider StringJoiner or String.join() for joining collections
Use char[] for passwords — not String
Set initial StringBuilder capacity when size is known
Use switch on Strings (Java 7+) instead of if-else chains
Prefer String.format() for complex formatting
Understand intern() but don't overuse it — let the JVM manage the pool

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for String Interview Questions.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, fundamentals, strings, string

Continue learning this concept

PreviousString Class in Java NextString Methods

Section 1: Fundamental Conceptual Questions

Q1: What is a String in Java?

Q2: Why is String immutable in Java?

Q3: What is the String Pool?

Q4: Explain == vs .equals() for Strings.

Q5: How many objects are created by String s = new String("Hello")?

Q6: What is the intern() method?

Q7: What is the difference between String, StringBuffer, and StringBuilder?

Q8: Why is String final?

Q9: Can we use String in switch statements?

Q10: What is the String.format() method?

Section 2: Output Prediction Questions

Q11: What is the output?

Q12: What is the output?

Q13: What is the output?

Q14: What is the output?

Q15: What is the output?

Q16: What is the output?

Section 3: Coding Questions

Q17: How to count occurrences of a character in a string?

Q18: Write a program to check if a string is a valid email.

Q19: Find all permutations of a string.

Q20: Longest common prefix among an array of strings.

Section 4: Advanced Questions

Q21: How does String concatenation work internally?

Q22: What is the difference between String.valueOf() and Object.toString()?

Q23: Why shouldn't we use String as a password storage?

Q24: How is String hashCode calculated?

Q25: What are compact strings (Java 9+)?

Common Mistakes

Best Practices

Continue learning this concept

Section 1: Fundamental Conceptual Questions

Q1: What is a String in Java?

Q2: Why is String immutable in Java?

Q3: What is the String Pool?

Q4: Explain == vs .equals() for Strings.

Q5: How many objects are created by String s = new String("Hello")?

Q6: What is the intern() method?

Q7: What is the difference between String, StringBuffer, and StringBuilder?

Q8: Why is String final?

Q9: Can we use String in switch statements?

Q10: What is the String.format() method?

Section 2: Output Prediction Questions

Q11: What is the output?

Q12: What is the output?

Q13: What is the output?

Q14: What is the output?

Q15: What is the output?

Q16: What is the output?

Section 3: Coding Questions

Q17: How to count occurrences of a character in a string?

Q18: Write a program to check if a string is a valid email.

Q19: Find all permutations of a string.

Q20: Longest common prefix among an array of strings.

Section 4: Advanced Questions

Q21: How does String concatenation work internally?

Q22: What is the difference between String.valueOf() and Object.toString()?

Q23: Why shouldn't we use String as a password storage?

Q24: How is String hashCode calculated?

Q25: What are compact strings (Java 9+)?

Common Mistakes

Best Practices

Continue learning this concept

Q5: How many objects are created by `String s = new String("Hello")`?

Q22: What is the difference between `String.valueOf()` and `Object.toString()`?

Q5: How many objects are created by `String s = new String("Hello")`?

Q22: What is the difference between `String.valueOf()` and `Object.toString()`?