int[] a = new int[4];//推介使用int[] 這種方式初始化

int c[] = {23,43,56,78};//長度：4，索引范圍：[0,3]

List<Integer>?list?=?new?List<Integer>();//錯誤

package javatest;
 
import java.util.ArrayList;
import java.util.List;
 
/**
 * @ClassName Jtest
 * @Description TODO
 * @Author lingxiangxiang
 * @Date 4:54 PM
 * @Version 1.0
 **/
public class Jtest {
 
    public static int length = 1048576; //10的20次冪
    public static List<Integer> list1 = new ArrayList<>();
    public static List<Integer> list2 = new ArrayList<>(length);
 
    public static void addList(int sign) {
        long start = System.currentTimeMillis();
        for (int i = 0; i < length; i++) {
            if (sign == 0) {
                list1.add(sign);
            } else {
                list2.add(sign);
            }
        }
        long end = System.currentTimeMillis();
        System.out.println(sign + " exec time is: " + (end - start));
    }
 
    public static void main(String[] args) {
        addList(0);
        addList(1);
    }
}

  private static final int DEFAULT_CAPACITY = 10;
  // ArrayList的默認(rèn)長度是多少
    private static final Object[] EMPTY_ELEMENTDATA = {};
  // ArrayList的默認(rèn)空元素鏈表
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
  // ArrayList存放的數(shù)據(jù)
    transient Object[] elementData; // non-private to simplify nested class access
  // ArrayList的長度
    private int size;

// 構(gòu)造一個初始化容量為10的空列表
public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
// 初始化一個指定大小容量的列表
public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }
// 構(gòu)造一個包含指定集合的元素列表, 按照它們由集合迭代器返回的順序
public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

// 增加元素的方法
public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
 
 //判斷當(dāng)前數(shù)組是否是默認(rèn)構(gòu)造方法生成的空數(shù)組，如果是的話minCapacity=10反之則根據(jù)原來的值傳入下一個方法去完成下一步的擴容判斷
private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
        }
 
//minCapacitt表示修改后的數(shù)組容量，minCapacity = size + 1
 private void ensureCapacityInternal(int minCapacity) {
        //判斷看看是否需要擴容
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

//判斷當(dāng)前ArrayList是否需要進行擴容
private void ensureExplicitCapacity(int minCapacity) {
  modCount++;
 
  // overflow-conscious code
  // int[] a = new int[5]; 數(shù)組創(chuàng)建的時候是多大, a.length就等于5
  if (minCapacity - elementData.length > 0)
    grow(minCapacity);
}

private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }