Common string methods in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the length() method do in a Java String?

It returns the number of characters in the string, including spaces and special characters.

beginner

How does the toUpperCase() method affect a string?

It creates a new string where all letters are converted to uppercase. The original string stays the same.

intermediate

What is the purpose of the substring(int beginIndex, int endIndex) method?

It extracts a part of the string starting at beginIndex and ending just before endIndex, returning that part as a new string.

beginner

Explain what equals(String anotherString) does.

It checks if two strings have exactly the same characters in the same order and returns true if they do, otherwise false.

beginner

What does the trim() method do to a string?

It removes any whitespace at the start and end of the string but does not change spaces inside the string.

Which method returns the number of characters in a Java string?

A size()

B length()

C count()

D getLength()

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

What does toLowerCase() do to a string?

A Converts all letters to lowercase

B Removes spaces

C Checks if string is lowercase

D Changes the original string

toLowerCase() returns a new string with all letters in lowercase.

How do you get a part of a string from index 2 to 5 (exclusive) in Java?

A substring(2, 5)

B slice(2, 5)

C substr(2, 5)

D part(2, 5)

The substring(2, 5) method extracts characters from index 2 up to but not including 5.

Which method compares two strings for exact equality?

A compare()

B match()

C equals()

D == operator

equals() checks if two strings have the same characters in the same order.

What does the trim() method remove from a string?

A All spaces inside the string

B All whitespace characters everywhere

C Tabs inside the string only

D Spaces at the start and end only

trim() removes whitespace only at the beginning and end of the string.

Describe five common string methods in Java and what each one does.

Think about how you check size, change case, cut parts, compare, and clean spaces in strings.

Explain how you would extract a substring and compare two strings for equality in Java.

Focus on methods that cut strings and check if two strings are the same.

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