StringBuilder and StringBuffer in Java - Time & Space Complexity

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Time Complexity: StringBuilder and StringBuffer

O(n)

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Understanding Time Complexity

We want to understand how fast StringBuilder and StringBuffer work when building strings.

How does the time to add characters grow as the string gets longer?

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Scenario Under Consideration

Analyze the time complexity of the following code snippet.

StringBuilder sb = new StringBuilder();
for (int i = 0; i < n; i++) {
    sb.append('a');
}
String result = sb.toString();
    

This code builds a string by adding the letter 'a' n times using StringBuilder.

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Identify Repeating Operations

• Primary operation: The loop runs n times, each time appending one character.
• How many times: Exactly n times, once per loop iteration.

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How Execution Grows With Input

Each append adds one character, so the work grows as the string grows.

Input Size (n)	Approx. Operations
10	About 10 appends
100	About 100 appends
1000	About 1000 appends

Pattern observation: The time grows roughly in a straight line as n increases.

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Final Time Complexity

Time Complexity: O(n)

This means the time to build the string grows directly with the number of characters added.

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Common Mistake

[X] Wrong: "Appending characters takes the same time no matter how long the string is."

[OK] Correct: Sometimes the internal array needs to grow, which takes extra time, but overall it still grows linearly.

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Interview Connect

Understanding how string building scales helps you write efficient code and explain your choices clearly in interviews.

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Self-Check

"What if we used String concatenation (+) inside the loop instead of StringBuilder? How would the time complexity change?"