Substring Search Patterns in DSA Java - Build from Scratch

Substring Search Patterns

📖 Scenario: You are building a simple text search tool that helps users find if a small word or phrase exists inside a larger text. This is like looking for a word in a book or a message.

🎯 Goal: Build a Java program that checks if a given substring exists inside a main string using basic substring search.

📋 What You'll Learn

Create a main string variable with exact text

Create a substring variable with exact text to search

Use a loop to check each possible position for the substring inside the main string

Print whether the substring was found or not

💡 Why This Matters

🌍 Real World

Searching for words or phrases inside documents, messages, or web pages is a common task in many apps and tools.

💼 Career

Understanding substring search is a foundation for text processing, search engines, and data filtering tasks in software development.

1

Create the main string

Create a String variable called mainText and set it to the exact value "hello world, welcome to java programming".

DSA Java

// Your code here

Hint

Use String mainText = "your text"; to create the main string.

2

Create the substring to search

Add a String variable called searchText and set it to the exact value "java".

DSA Java

public class SubstringSearch {
    public static void main(String[] args) {
        String mainText = "hello world, welcome to java programming";
        // Your code here
    }
}

Hint

Use String searchText = "java"; to create the substring variable.

3

Search for the substring in the main string

Write a for loop with variable i from 0 to mainText.length() - searchText.length(). Inside the loop, use mainText.substring(i, i + searchText.length()).equals(searchText) to check if the substring matches. If it matches, set a boolean variable found to true and break the loop. Initialize found as false before the loop.

DSA Java

public class SubstringSearch {
    public static void main(String[] args) {
        String mainText = "hello world, welcome to java programming";
        String searchText = "java";
        // Your code here
    }
}

Hint

Use a for loop and substring method to compare parts of mainText with searchText.

4

Print the search result

Print "Found" if found is true, otherwise print "Not Found".

DSA Java

public class SubstringSearch {
    public static void main(String[] args) {
        String mainText = "hello world, welcome to java programming";
        String searchText = "java";
        boolean found = false;
        for (int i = 0; i <= mainText.length() - searchText.length(); i++) {
            if (mainText.substring(i, i + searchText.length()).equals(searchText)) {
                found = true;
                break;
            }
        }
        // Your code here
    }
}

Hint

Use System.out.println to print the result based on the found variable.

Practice

1. What does the indexOf method in Java's String class return when searching for a substring that exists in the string?

easy

A. The starting index of the first occurrence of the substring

B. The length of the substring

C. The number of times the substring appears

D. A boolean indicating if the substring exists

Solution

1. Step 1: Understand the purpose of indexOf

   The indexOf method searches for the first occurrence of a substring inside a string and returns its position.

2. Step 2: Identify the return value when substring exists

   If the substring is found, indexOf returns the starting index (0-based) of that substring in the string.

3. Final Answer:

   The starting index of the first occurrence of the substring -> Option A

4. Quick Check:

   indexOf returns index, not count or boolean [OK]

Hint: Remember: indexOf returns position, not count or boolean [OK]

Common Mistakes:

• Thinking indexOf returns count of occurrences
• Assuming it returns true/false
• Confusing indexOf with substring length

2. Which of the following is the correct syntax to find the first occurrence of substring "cat" in string str in Java?

easy

A. int pos = str.find("cat");

B. int pos = str.search("cat");

C. int pos = str.indexOf("cat");

D. int pos = str.contains("cat");

Solution

1. Step 1: Recall Java String methods for substring search

   Java's String class uses indexOf to find substring positions.

2. Step 2: Check method names and return types

   find and search are not valid String methods; contains returns boolean, not int.

3. Final Answer:

   int pos = str.indexOf("cat"); -> Option C

4. Quick Check:

   Use indexOf for substring position [OK]

Hint: Use indexOf for position, contains for boolean check [OK]

Common Mistakes:

• Using non-existent methods like find or search
• Using contains which returns boolean, not index
• Confusing method names from other languages

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

String text = "hello world";
int pos = text.indexOf("lo");
System.out.println(pos);

medium

A. -1

B. 3

C. 4

D. 5

Solution

1. Step 1: Locate substring "lo" in "hello world"

   Counting indices: h(0), e(1), l(2), l(3), o(4). The substring "lo" starts at index 3 (l at 3, o at 4).

2. Step 2: Understand indexOf return value

   indexOf returns the starting index of the substring, which is 3 here.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Substring "lo" starts at index 3 [OK]

Hint: Count string indices from 0 to find substring start [OK]

Common Mistakes:

• Counting from 1 instead of 0
• Returning index of second character in substring
• Assuming -1 means substring found

4. Identify the error in the following Java code snippet for substring search:

String s = "banana";
int index = s.indexOf('an');
System.out.println(index);

medium

A. Using single quotes for substring instead of double quotes

B. Missing semicolon after indexOf call

C. indexOf method does not exist for String

D. Printing index before assignment

Solution

1. Step 1: Check substring argument type in indexOf

   The indexOf method expects a String argument, but single quotes denote a char.

2. Step 2: Identify correct syntax for substring

   Substring literals must be in double quotes, so 'an' should be "an".

3. Final Answer:

   Using single quotes for substring instead of double quotes -> Option A

4. Quick Check:

   Substring literals need double quotes [OK]

Hint: Use double quotes for substrings, single for chars [OK]

Common Mistakes:

• Using single quotes for multi-character substrings
• Confusing char and String types
• Ignoring compiler error messages

5. Given a string text, which code snippet correctly finds all starting indices of substring "ab" in text using indexOf?

hard

A. int index = 0;
while (index < text.length()) {
index = text.indexOf("ab");
System.out.print(index + " ");
index++;
}

B. for (int i = 0; i < text.length(); i++) {
if (text.indexOf("ab") == i) System.out.print(i + " ");
}

C. int index = text.indexOf("ab");
while (index != -1) {
System.out.print(index + " ");
index = text.indexOf("ab");
}

D. int index = 0;
while ((index = text.indexOf("ab", index)) != -1) {
System.out.print(index + " ");
index++;
}

Solution

1. Step 1: Understand how to find multiple occurrences with indexOf

   Use indexOf(substring, fromIndex) to search from a moving start index to find all occurrences.

2. Step 2: Analyze each option's logic

   int index = 0; while ((index = text.indexOf("ab", index)) != -1) { System.out.print(index + " "); index++; } correctly updates index after each found occurrence to avoid infinite loop and find all positions. Others either do not update index properly or repeatedly search from start.

3. Final Answer:

   int index = 0; while ((index = text.indexOf("ab", index)) != -1) { System.out.print(index + " "); index++; } -> Option D

4. Quick Check:

   Use indexOf with start index and increment after each find [OK]

Hint: Increment start index after each found substring to continue search [OK]

Common Mistakes:

• Not updating start index causing infinite loop
• Always searching from start index 0
• Confusing indexOf usage with for-loop index