Check Substring in Java

intermediate· Strings · String

Problem

A substring check determines whether one string appears anywhere inside another, as a contiguous sequence of characters.

Given two strings, determine whether the second is a substring of the first.

Input

"Hello World", "World"

Output

Contains substring: true

Java Program

Java

public class SubstringCheck {
    public static void main(String[] args) {
        String str = "Hello World";
        String sub = "World";
        boolean found = false;

        // Slide a window of sub's length across str, comparing at each position
        for (int i = 0; i <= str.length() - sub.length(); i++) {
            if (str.substring(i, i + sub.length()).equals(sub)) {
                found = true;
                break; // match found, no need to keep scanning
            }
        }

        System.out.println("Contains substring: " + found);
    }
}

▶ Run Code

Output

Contains substring: true

Core Logic

Sliding a window the length of the target string across every starting position, and comparing at each one, finds a match the same way String.contains() would internally.

How It Works

1The loop tries every starting index i from 0 up to str.length() - sub.length(), the last position wide enough to fit sub.
2At each position, str.substring(i, i + sub.length()) extracts a window the same length as sub.
3.equals(sub) compares that window against the target string.
4The first matching window sets found to true and break exits immediately.

For "Hello World" and "World", the window at index 6 — "World" — matches, so found becomes true.

💡

Key Point: This is the brute-force algorithm that a method like contains() conceptually performs — trading the convenience of a built-in call for seeing exactly how the search works.

Complexity

Time Complexity: O(n × m)Space Complexity: O(m)

Why: Each of the up to n candidate positions requires allocating and comparing a new substring of length m, so the work multiplies across positions and comparison length.

Key Concepts

sliding windowString.substring()for loop

Approach 2: Using contains()

Java

public class SubstringCheckBuiltin {
    public static void main(String[] args) {
        String str = "Hello World";
        String sub = "World";
        // contains() already implements an efficient substring search internally
        boolean found = str.contains(sub);
        System.out.println("Contains substring: " + found);
    }
}

▶ Run Code

Output

Contains substring: true

Core Logic

In real code, there's no reason to write the sliding window yourself — contains() already implements this search internally.

How It Works

1str.contains(sub) takes the target string directly and returns a boolean.
2Internally, it performs the same kind of position-by-position search as the manual version, just already written and tested.
3No explicit loop, substring extraction, or comparison is needed in your own code.

"Hello World".contains("World") returns true in a single call.

💡

Key Point: This is the version to actually use — the manual sliding-window loop exists only to show what contains() is conceptually doing under the hood.

Complexity

Time Complexity: O(n × m)Space Complexity: O(1)

Why: contains() still performs the same kind of scan internally, but that scan happens inside the JDK — no manual substring allocation happens in your own code.

Key Concepts

String.contains()

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