Dynamic Growth and Internal Implementation of Java's ArrayList

답변:

In Java, an ArrayList is a resizable array implementation of the List interface. It can grow dynamically as elements are added to it. The internal mechanism of how an ArrayList grows involves a few key concepts, including an underlying array, a size counter, and a resizing strategy.

Internal Implementation of ArrayList

Underlying Array:
- An ArrayList uses an array to store its elements. When you create an ArrayList, it initializes an array of a certain capacity (default is 10).
Size Counter:
- It maintains a size counter that keeps track of the number of elements currently in the list.
Dynamic Resizing:
- When you add an element to the ArrayList and the current size equals the capacity of the underlying array, the ArrayList needs to grow. This is done by creating a new array with a larger capacity (usually 1.5 to 2 times the current capacity), copying the elements from the old array to the new array, and then adding the new element.

Example of ArrayList Growth

Here’s a simplified version of how the ArrayList might be implemented internally:

import java.util.Arrays;

public class MyArrayList<E> {
    private Object[] elementData; // Array to store elements
    private int size; // Current number of elements

    // Default constructor
    public MyArrayList() {
        this.elementData = new Object[10]; // Initial capacity
        this.size = 0;
    }

    // Method to add an element
    public void add(E element) {
        ensureCapacity(size + 1); // Ensure there's enough space
        elementData[size++] = element; // Add element and increment size
    }

    // Method to ensure capacity
    private void ensureCapacity(int minCapacity) {
        if (minCapacity - elementData.length > 0) {
            // Calculate new capacity (1.5 times the current capacity)
            int newCapacity = elementData.length + (elementData.length >> 1);
            // If new capacity is less than minCapacity, use minCapacity
            if (newCapacity < minCapacity) {
                newCapacity = minCapacity;
            }
            // Resize the array
            elementData = Arrays.copyOf(elementData, newCapacity);
        }
    }

    // Method to get an element
    public E get(int index) {
        if (index >= size || index < 0) {
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
        }
        return (E) elementData[index];
    }

    // Method to get the current size
    public int size() {
        return size;
    }
}

Explanation of the Code

Array Initialization:
- The elementData array is initialized with a default capacity of 10.
Adding Elements:
- The add method first calls ensureCapacity to check if there is enough space to add a new element. If not, it resizes the array.
Ensuring Capacity:
- The ensureCapacity method checks if the current array can accommodate the new element. If not, it calculates a new capacity (1.5 times the current capacity) and uses Arrays.copyOf to create a new array and copy the existing elements into it.
Getting Elements:
- The get method retrieves an element at a specified index, checking for bounds to avoid IndexOutOfBoundsException.
Size Method:
- The size method returns the current number of elements in the ArrayList.

Performance Considerations

Amortized Time Complexity: The add operation has an amortized time complexity of O(1). Although resizing the array takes O(n) time, it happens infrequently, so the average time per addition remains constant.
Memory Overhead: The ArrayList may use more memory than necessary due to the resizing strategy, especially if many elements are added and removed.

Conclusion