Common string methods in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the length of the string.

Java

String text = "Hello";
int length = text.[1]();

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

Asize

BgetLength

Clength

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using size() instead of length()

Trying to access length as a property without parentheses

The length() method returns the number of characters in a string.

2fill in blank

medium

Complete the code to convert the string to uppercase letters.

Java

String word = "java";
String upper = word.[1]();

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

AtoLowerCase

BtoUpperCase

Cuppercase

DmakeUpper

Attempts:

3 left

💡 Hint

Common Mistakes

Using toLowerCase() which converts to lowercase

Trying to use a method named uppercase which does not exist

The toUpperCase() method converts all letters in the string to uppercase.

3fill in blank

hard

Fix the error in the code to check if the string starts with "Hi".

Java

String greeting = "Hi there!";
boolean starts = greeting.[1]("Hi");

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

AstartsWith

BbeginsWith

CstartWith

Dstarts

Attempts:

3 left

💡 Hint

Common Mistakes

Using startWith instead of startsWith

Using beginsWith which is not a Java String method

The correct method to check the start of a string is startsWith().

4fill in blank

hard

Fill both blanks to extract the substring "cat" from the word "concatenate".

Java

String word = "concatenate";
String part = word.[1](3, [2]);

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

Asubstring

BsubString

C9

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using subString with capital S which is incorrect

Using wrong end index causing wrong substring

The substring(int start, int end) method extracts characters from start index up to but not including end index. Here, indexes 3 to 6 give "cat".

5fill in blank

hard

Fill all three blanks to create a new string with trimmed spaces and replaced spaces with underscores.

Java

String messy = "  hello world  ";
String clean = messy.[1]().[2](" ", [3]);

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

Atrim

Breplace

C"_"

Dstrip

Attempts:

3 left

💡 Hint

Common Mistakes

Using strip() instead of trim() which is similar but less common

Forgetting to put quotes around the underscore character

trim() removes spaces at the start and end. replace(" ", "_") changes spaces inside the string to underscores.

Practice

1. Which Java String method checks if a string contains a specific sequence of characters?

easy

A. equals()

B. contains()

C. substring()

D. replace()

Solution

1. Step 1: Understand the purpose of contains()

   The contains() method checks if the string includes a specific sequence of characters and returns true or false.

2. Step 2: Compare with other methods

   equals() checks full equality, substring() extracts part of a string, and replace() changes characters. None check for containment.

3. Final Answer:

   contains() -> Option B

4. Quick Check:

   Check substring presence = contains() [OK]

Hint: Use contains() to check if text is inside a string [OK]

Common Mistakes:

• Confusing contains() with equals()
• Using substring() to check presence
• Thinking replace() checks for text

2. Which of the following is the correct syntax to convert a string str to uppercase in Java?

easy

A. str.toUpperCase();

B. str.uppercase();

C. toUpperCase(str);

D. str.toUpper();

Solution

1. Step 1: Recall the method name for uppercase conversion

   The correct method name in Java String class is toUpperCase(), called on the string object.

2. Step 2: Check syntax correctness

   str.toUpperCase(); uses str.toUpperCase(); which is valid. Others use incorrect method names or syntax.

3. Final Answer:

   str.toUpperCase(); -> Option A

4. Quick Check:

   Uppercase method = toUpperCase() [OK]

Hint: Remember Java String methods use camelCase like toUpperCase() [OK]

Common Mistakes:

• Using uppercase() instead of toUpperCase()
• Calling method without object (toUpperCase(str))
• Using toUpper() which does not exist

3. What is the output of the following Java code?

String text = "Hello World";
int pos = text.indexOf("World");
System.out.println(pos);

medium

A. 6

B. 0

C. -1

D. 5

Solution

1. Step 1: Understand what indexOf() does

   The indexOf() method returns the starting index of the substring if found, or -1 if not found.

2. Step 2: Find the position of "World" in "Hello World"

   "Hello " has 6 characters (index 0 to 5), so "World" starts at index 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Substring start index = 6 [OK]

Hint: indexOf() returns first position of substring or -1 if missing [OK]

Common Mistakes:

• Confusing index with length
• Expecting index 0 for substring
• Thinking indexOf() returns boolean

4. Identify the error in this Java code snippet:

String s = "Java Programming";
String part = s.substring(5, 3);
System.out.println(part);

medium

A. substring() cannot be used on String objects

B. Missing semicolon after substring() call

C. substring() arguments are reversed; start index must be <= end index

D. The variable 'part' is not declared

Solution

1. Step 1: Check substring() method argument rules

   substring(start, end) requires start index less than or equal to end index; here 5 > 3, which is invalid.

2. Step 2: Confirm other syntax correctness

   Semicolons are present, substring() is valid on String, and 'part' is declared properly.

3. Final Answer:

   substring() arguments are reversed; start index must be <= end index -> Option C

4. Quick Check:

   substring(start, end) needs start ≤ end [OK]

Hint: Start index must not be greater than end index in substring() [OK]

Common Mistakes:

• Swapping start and end indexes
• Thinking substring() changes original string
• Ignoring exception thrown at runtime

5. Given the string input = "apple,banana,orange", which code snippet correctly replaces all commas with semicolons and converts the result to uppercase?

hard

A. input.replaceFirst(",", ";").toUpperCase();

B. input.toUpperCase().replaceFirst(",", ";");

C. input.replace(",", ";").toUpper();

D. input.replaceAll(",", ";").toUpperCase();

Solution

1. Step 1: Understand replace() vs replaceAll()

   Both replace() and replaceAll() can replace characters, but replaceAll() uses regex and is safer for multiple replacements.

2. Step 2: Check method chaining and correctness

   A and B use replaceFirst() which only replaces the first comma. C uses invalid toUpper().

3. Step 3: Choose best practice

   Using replaceAll() with chaining to toUpperCase() is correct and clear.

4. Final Answer:

   input.replaceAll(",", ";").toUpperCase(); -> Option D

5. Quick Check:

   Replace commas then uppercase = replaceAll + toUpperCase() [OK]

Hint: Chain replaceAll() then toUpperCase() for clean transformation [OK]

Common Mistakes:

• Using toUpper() instead of toUpperCase()
• Using replaceFirst() which only replaces the first occurrence
• Confusing replace() and replaceAll() usage