Java Collection Methods Useful for LeetCode Interviews

A curated list of high-impact Java Collection methods to solve LeetCode problems efficiently

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Introduction

Here are the Top Java Collection Methods that are super helpful for LeetCode-style problems, especially in coding interviews where performance and brevity matter:

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1. Map.getOrDefault(key, defaultValue)

• Returns the value for the specified key if present in the map; otherwise, returns the provided default value.

• Ideal for counting frequencies or caching intermediate results.

map.put(num, map.getOrDefault(num, 0) + 1);

Used in: Two Sum, Group Anagrams, Top K Frequent Elements etc

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2. Set.add() Return Value

• The add() method attempts to insert an element into a Set. It returns a boolean indicating whether the set was modified (i.e., whether the element was not already present). Detects duplicates in O(1) time.

if (!set.add(num)) {
    // Duplicate detected
}

Used in: Contains Duplicate, Valid Sudoku etc.