What Is Underlying Data Structure in Java?


Angela Bailey

In Java, the underlying data structure refers to the way data is organized and stored in memory. It plays a crucial role in determining the efficiency and performance of various operations performed on the data. Understanding the underlying data structure is essential for writing efficient code and optimizing program execution.


Arrays are one of the most basic and widely used data structures in Java. They provide a contiguous block of memory to store elements of the same type. Elements in an array can be accessed using their index, which starts from 0.


int[] numbers = new int[5];
numbers[0] = 1;
numbers[1] = 2;
numbers[2] = 3;
numbers[3] = 4;
numbers[4] = 5;

Arrays offer constant-time access to elements, making them efficient for random access. However, their size is fixed once they are created, and inserting or deleting elements can be costly as it requires shifting existing elements.


ArrayList is a dynamic array implementation provided by Java’s standard library. It overcomes the limitations of fixed-sized arrays by automatically resizing itself as elements are added or removed.


import java.util.ArrayList;

ArrayList<String> fruits = new ArrayList<>();

The ArrayList class provides methods for adding, removing, and accessing elements at specific positions. However, due to its dynamic nature, ArrayList may require resizing when adding or removing elements at arbitrary positions, resulting in a performance overhead.


LinkedList is another commonly used data structure in Java. Unlike arrays and ArrayLists, LinkedList is implemented as a sequence of nodes, where each node contains a reference to the next node.LinkedList;

LinkedList<Integer> numbers = new LinkedList<>();

LinkedList provides efficient insertion and deletion operations at both ends of the list. However, accessing elements at arbitrary positions requires traversing the list from the beginning or end, resulting in slower access times compared to arrays or ArrayLists.


HashMap is a data structure that stores key-value pairs. It uses an underlying array-based structure to provide fast access to values based on their keys.HashMap;

HashMap<String, Integer> ages = new HashMap<>();
ages.put(“Alice”, 25);
ages.put(“Bob”, 30);
ages.put(“Charlie”, 35);

int ageOfBob = ages.get(“Bob”);

HashMap offers constant-time average case complexity for insertion, retrieval, and deletion operations. However, the actual performance may degrade if there are many hash collisions.

In Conclusion

In Java, understanding the underlying data structures such as arrays, ArrayLists, LinkedLists, and HashMaps is crucial for writing efficient code. Each data structure has its own strengths and weaknesses in terms of performance characteristics. Choosing the right data structure based on your specific requirements can significantly impact the efficiency and performance of your Java programs.

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