Common string methods in Java - Step-by-Step Execution

Concept Flow - Common string methods

Start with a String

↓

Call method on String

↓

Method processes String

↓

Returns result (new String, int, boolean, etc.)

↓

Use or print result

↓

End

This flow shows how you start with a string, call a method on it, get a result, and then use that result.

Execution Sample

Java

String text = "Hello World";
int length = text.length();
String upper = text.toUpperCase();
boolean contains = text.contains("lo");
System.out.println(length);
System.out.println(upper);

This code gets the length of a string, converts it to uppercase, checks if it contains "lo", and prints results.

Execution Table

Step	Action	Expression	Result	Output
1	Assign string	text = "Hello World"	"Hello World"
2	Get length	text.length()	11
3	Convert to uppercase	text.toUpperCase()	"HELLO WORLD"
4	Check contains substring	text.contains("lo")	true
5	Print length	System.out.println(length)		11
6	Print uppercase	System.out.println(upper)		HELLO WORLD

💡 All methods executed; program ends after printing results.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
text	null	"Hello World"	"Hello World"	"Hello World"	"Hello World"	"Hello World"
length	undefined	undefined	11	11	11	11
upper	undefined	undefined	undefined	"HELLO WORLD"	"HELLO WORLD"	"HELLO WORLD"
contains	undefined	undefined	undefined	true	true	true

Key Moments - 3 Insights

Why does calling toUpperCase() not change the original string?

Because strings in Java are immutable, toUpperCase() returns a new string (see step 3 in execution_table), original 'text' stays the same.

What does length() return exactly?

length() returns the number of characters in the string, including spaces (step 2 shows length is 11 for "Hello World").

How does contains() work with substrings?

contains() checks if the exact sequence of characters exists in the string (step 4 shows it returns true for "lo" in "Hello World").

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'length' after step 2?

A 11

B 10

C undefined

D 12

💡 Hint

Check the 'Result' column for step 2 in execution_table.

At which step does the string become uppercase?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for toUpperCase() method call in execution_table.

If we changed the substring in contains() to "xyz", what would the result be at step 4?

A true

B false

C null

D Exception

💡 Hint

contains() returns true only if substring exists; check step 4 result for "lo".

Concept Snapshot

Common String Methods in Java:
- length() returns number of characters
- toUpperCase() returns uppercase copy
- contains(sub) checks if substring exists
Strings are immutable; methods return new values
Use methods by calling on string variable like text.method()

Full Transcript

This visual execution shows how common string methods work in Java. We start with a string variable 'text' holding "Hello World". We call length() to get the number of characters, which is 11. Then we call toUpperCase() which returns a new string "HELLO WORLD" without changing the original. Next, contains("lo") checks if "lo" is inside the string and returns true. Finally, we print the length and uppercase string. Variables track these values step-by-step. Key points include that strings do not change when calling methods like toUpperCase(), length counts all characters including spaces, and contains checks for exact substrings. The quiz questions help confirm understanding of these steps.

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