java 内存计算

一、计算java对象在内存中占用的大小

来源：https://www.coursera.org/learn/algorithms-part1/supplement/mpK20/lecture-slides

二、实例说明：

以下为双向列队Deque.java

import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdRandom;

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Deque<Item> implements Iterable<Item> {

    private int size;
    private final Node sentinel;

    private class Node {
        Node next;
        Node prev;
        Item item;

        Node() {
        }

        Node(Item item, Node prev, Node next) {
            this.item = item;
            this.prev = prev;
            this.next = next;
        }
    }

    public Deque() {
        sentinel = new Node();
        sentinel.next = sentinel;
        sentinel.prev = sentinel;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    public int size() {
        return size;
    }

    public void addFirst(Item item) {
        if (item == null) throw new IllegalArgumentException("Cannot add null");
        Node newNode = new Node(item, sentinel, sentinel.next);
        sentinel.next.prev = newNode;
        sentinel.next = newNode;
        size++;
    }

    public void addLast(Item item) {
        if (item == null) throw new IllegalArgumentException("Cannot add null");
        Node newNode = new Node(item, sentinel.prev, sentinel);
        sentinel.prev.next = newNode;
        sentinel.prev = newNode;
        size++;
    }

    public Item removeFirst() {
        if (size == 0) throw new NoSuchElementException("Stack underflow");
        Item oldItem = sentinel.next.item;
        sentinel.next.next.prev = sentinel;
        sentinel.next = sentinel.next.next;
        size--;
        return oldItem;
    }

    public Item removeLast() {
        if (size == 0) throw new NoSuchElementException("Stack underflow");
        Item oldItem = sentinel.prev.item;
        sentinel.prev.prev.next = sentinel;
        sentinel.prev = sentinel.prev.prev;
        size--;
        return oldItem;
    }

    public Iterator<Item> iterator() {
        return new DequeIterator();
    }

    private class DequeIterator implements Iterator<Item> {
        private Node first;
        private int count;

        private DequeIterator() {
            first = sentinel;
            count = size();
        }

        @Override
        public boolean hasNext() {
            return !(count == 0);
        }

        @Override
        public Item next() {
            if (count == 0) throw new NoSuchElementException("Stack underflow");
            Item item = first.next.item;
            first = first.next;
            count--;
            return item;
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException("the operation of remove is not supported");
        }
    }

• 当使用一Deque对象增加n个Item后，该Deque对象所占用的内存大小为：48n + 80 字节
  • Object overhead: 16
  • 内部类Node的大小: 48
    • Object overhead：16
    • 变量next，prev，Item: 3 * 8 = 24
    • 外部类指针: 8
    • padding：0
  • 变量size: 4
  • 变量sentinel: 8
  • sentinel变量指向的Node对象: 48 * 1 = 48
  • padding: 4