String immutability in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new string by adding " World" to the original string.

Java

String greeting = "Hello";
String newGreeting = greeting[1]" World";

Drag options to blanks, or click blank then click option'

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using arithmetic operators like '-' or '*' instead of '+' for strings.

In Java, strings are concatenated using the + operator.

2fill in blank

medium

Complete the code to convert the string to uppercase.

Java

String text = "hello";
String upperText = text.[1]();

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AtoUpperCase

BtoLowerCase

Ctrim

Dsubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using toLowerCase() instead of toUpperCase().

The toUpperCase() method returns a new string with all letters in uppercase.

3fill in blank

hard

Fix the error in the code to correctly replace "cat" with "dog" in the string.

Java

String animal = "cat";
String newAnimal = animal.[1]("cat", "dog");

Drag options to blanks, or click blank then click option'

AreplaceAll

BreplaceWith

CreplaceFirst

Dreplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent method replaceWith().

Using replaceAll() which expects regex, but replace() is simpler here.

The replace() method replaces all occurrences of a character sequence with another.

4fill in blank

hard

Fill both blanks to create a new string with "Java" repeated 3 times.

Java

String repeated = "Java".[1](3);
String result = repeated.[2]();

Drag options to blanks, or click blank then click option'

Arepeat

BtoString

Ctrim

Dconcat

Attempts:

3 left

💡 Hint

Common Mistakes

Using concat(3) which is invalid.

Not using repeat() for repeating strings.

The repeat(3) method repeats the string 3 times, and toString() returns the string itself.

5fill in blank

hard

Fill all three blanks to create a new string with the first 4 characters of "immutable" in uppercase.

Java

String word = "immutable";
String part = word.[1](0, [2]);
String result = part.[3]();

Drag options to blanks, or click blank then click option'

Asubstring

B4

CtoUpperCase

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() instead of the number 4.

Not using substring correctly.

The substring(0, 4) extracts the first 4 characters, and toUpperCase() converts them to uppercase.

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