String immutability in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code involving String immutability?

Consider the following Java code snippet. What will be printed when it runs?

Java

public class Test {
    public static void main(String[] args) {
        String s = "hello";
        s.toUpperCase();
        System.out.println(s);
    }
}

A HELLO

B Compilation error

C null

D hello

💡 Hint

Remember that Strings in Java cannot be changed once created.

The method toUpperCase() returns a new String object with uppercase letters but does not change the original String. Since s is not reassigned, it still holds "hello".

❓ Predict Output

intermediate

What does this code print after String concatenation?

Look at this Java code. What will be printed?

Java

public class Test {
    public static void main(String[] args) {
        String s = "cat";
        s += "dog";
        System.out.println(s);
    }
}

A dog

B cat

C catdog

D Compilation error

💡 Hint

String concatenation creates a new String object.

The += operator creates a new String "catdog" and assigns it back to s. So s now points to "catdog".

🔧 Debug

advanced

Which option causes a compilation error due to String immutability?

Which of the following Java code snippets will cause a compilation error?

A

String s = "hello";
s.charAt(0) = 'H';

B

String s = "hello";
s = s.toUpperCase();

C

String s = "hello";
System.out.println(s.length());

D

String s = "hello";
String t = s + " world";

💡 Hint

Think about whether you can assign a character to a position in a String.

Strings are immutable in Java. You cannot assign a character to an index like s.charAt(0) = 'H'; causes a compilation error.

🧠 Conceptual

advanced

How does Java handle String immutability internally?

Which statement best describes how Java implements String immutability?

A Java uses a special String object that allows changes only through reflection.

B Java stores String characters in a final char array that cannot be changed after creation.

C Java copies the String every time it is passed to a method to prevent changes.

D Java uses a mutable char array internally but prevents changes by restricting access.

💡 Hint

Think about what 'final' means in Java and how it relates to immutability.

Java stores String characters in a final char array inside the String object. This array cannot be changed after the String is created, ensuring immutability.

❓ Predict Output

expert

What is the output of this Java code involving String references?

Analyze the code below. What will be printed?

Java

public class Test {
    public static void main(String[] args) {
        String a = "abc";
        String b = a;
        a = a + "d";
        System.out.println(b);
    }
}

A abc

B abcd

C Compilation error

D null

💡 Hint

Consider what happens to the original String reference when you concatenate.

When a is concatenated with "d", a points to a new String "abcd". But b still points to the original String "abc".

Practice

1. What does it mean that a String in Java is immutable?

easy

A. Once created, the string cannot be changed.

B. The string can be changed anytime.

C. The string changes automatically when printed.

D. The string can only be changed inside loops.

Solution

1. Step 1: Understand the meaning of immutability

   Immutability means the object cannot be changed after creation.

2. Step 2: Apply this to Java Strings

   Java strings cannot be modified once created; any change creates a new string.

3. Final Answer:

   Once created, the string cannot be changed. -> Option A

4. Quick Check:

   String immutability = cannot change after creation [OK]

Hint: Immutable means no change allowed after creation [OK]

Common Mistakes:

• Thinking strings can be changed directly
• Confusing immutability with constant variables
• Assuming strings change when printed

2. Which of the following is the correct way to create a new string by changing a character in an existing string str?

easy

A. str.charAt(0) = 'A';

B. str[0] = 'A';

C. str = "A" + str.substring(1);

D. str.replaceChar(0, 'A');

Solution

1. Step 1: Check syntax for changing characters in strings

   Strings are immutable, so direct assignment like str.charAt(0) = 'A' is invalid.

2. Step 2: Use substring to create a new string

   Concatenate "A" with the substring from index 1 to form a new string.

3. Final Answer:

   str = "A" + str.substring(1); -> Option C

4. Quick Check:

   Use substring + concat to change string [OK]

Hint: Use substring and concat to create new string [OK]

Common Mistakes:

• Trying to assign directly to string characters
• Using array-like indexing on strings
• Using non-existent methods like replaceChar

3. What is the output of the following code?

String s = "hello";
s.toUpperCase();
System.out.println(s);

medium

A. HELLO

B. hello

C. null

D. HELLOhello

Solution

1. Step 1: Understand that strings are immutable

   The method toUpperCase() returns a new string but does not change the original string s.

2. Step 2: Check what is printed

   Since s was not reassigned, printing s outputs the original string "hello".

3. Final Answer:

   hello -> Option B

4. Quick Check:

   Original string unchanged = hello [OK]

Hint: toUpperCase returns new string; original stays same [OK]

Common Mistakes:

• Assuming toUpperCase changes original string
• Expecting uppercase output without reassignment
• Confusing method effect with print output

4. Find the error in this code snippet:

String s = "test";
s.charAt(0) = 'T';
System.out.println(s);

medium

A. Cannot assign to charAt() result; strings are immutable.

B. Missing semicolon after declaration.

C. charAt() method does not exist for String.

D. System.out.println() cannot print strings.

Solution

1. Step 1: Check usage of charAt()

   The method charAt() returns a character but cannot be assigned a new value.

2. Step 2: Understand string immutability

   Strings cannot be changed by assigning to characters; this causes a compile error.

3. Final Answer:

   Cannot assign to charAt() result; strings are immutable. -> Option A

4. Quick Check:

   charAt() result is read-only [OK]

Hint: charAt() returns char, cannot assign to it [OK]

Common Mistakes:

• Trying to assign to charAt()
• Thinking strings are mutable
• Ignoring compile errors on assignment

5. You want to replace all occurrences of the letter 'a' with 'b' in a string input. Which code correctly does this considering string immutability?

hard

A. input.replace('a', 'b'); System.out.println(input);

B. input.charAt(0) = 'b'; System.out.println(input);

C. input.toCharArray()[0] = 'b'; System.out.println(input);

D. input = input.replace('a', 'b'); System.out.println(input);

Solution

1. Step 1: Understand replace() returns new string

   The replace() method returns a new string with replacements but does not change the original.

2. Step 2: Assign the new string back to input

   To update input, assign the result of replace() back to it before printing.

3. Final Answer:

   input = input.replace('a', 'b'); System.out.println(input); -> Option D

4. Quick Check:

   Assign replace result to variable to update string [OK]

Hint: Assign replace() result to variable to update string [OK]

Common Mistakes:

• Calling replace() without assignment
• Trying to assign to charAt() or array elements
• Expecting original string to change automatically