jdk API數(shù)組拷貝

Arrays.copyOf(objects, size);
System.arraycopy(src, srcPos, dest, destPos, length);
src：源數(shù)組
srcPos：原數(shù)組起始位置
dest：目標(biāo)數(shù)組
destPos：目標(biāo)數(shù)組起始位置
length：復(fù)制長(zhǎng)度

注意點(diǎn)

1.ArrayList底層采用該數(shù)組實(shí)現(xiàn)，數(shù)組名稱為elementData
2.ArrayList默認(rèn)大小為10斤讥；
3.如果ArrayList構(gòu)造器中指定初始化大小為1;初次位移擴(kuò)容始終為1，新容量大小以最小容量為主

        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;

4.刪除原理：使用System.arraycopy方法，往前移動(dòng)數(shù)組，將最后一個(gè)元素設(shè)置為空
5.Arrays.asList返回的不是ArrayList减余，而是Arrays的私有靜態(tài)內(nèi)部類午笛，它與ArrayList區(qū)別是沒(méi)有實(shí)現(xiàn)Cloneable接口，其它一致鹦肿，它沒(méi)有實(shí)現(xiàn)添加方法，所以不能添加元素

Vector與ArrayList

1.vector線程安全辅柴，方法使用synchronize修飾箩溃，效率比ArrayList差
2.Vector和ArrayList都采用連續(xù)的存儲(chǔ)空間，當(dāng)存儲(chǔ)空間不足的時(shí)候碌嘀，vector默認(rèn)增加為原來(lái)的一倍涣旨，ArrayList默認(rèn)正價(jià)為原來(lái)的50%
3.Vector可以設(shè)置capacityIncrement(容量增長(zhǎng)參數(shù))，而ArrayList不可以設(shè)置

  public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

  //vector 擴(kuò)容方式（注意:擴(kuò)容參數(shù)）
  private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

手寫ArrayList代碼

  public class ExtArrayList<E> implements ExtList<E>{
    
    //ArrayList底層采用數(shù)組存放
    private Object[] elementData;
    //默認(rèn)數(shù)組容量
    private static final int DEFAULT_CAPACITY = 10;
    //記錄實(shí)際ArrayList大小
    private int size;
    
    
    //默認(rèn)初始化容量為10
    public ExtArrayList() {
        this(DEFAULT_CAPACITY);
    }
    
    //ArryList指定數(shù)組初始的容量
    public ExtArrayList(int initialCapacity) {
        if (initialCapacity < 0) {
            throw new IllegalArgumentException("初始化容量不能小于0");
        }
        elementData = new Object[initialCapacity];
    }
    
    public void add(E e) {
        //1.判斷實(shí)際存放的數(shù)據(jù)容量是否大于elementData
        ensureCapacityInternal(size + 1);
        //2.使用下標(biāo)進(jìn)行賦值
        elementData[size++] = e; 
    }
    
    public void add(int index, E e) {
        rangeCheck(index);
        ensureCapacityInternal(size + 1);
        System.arraycopy(elementData, index, elementData, index + 1, size - index);
        elementData[index] = e;
        size ++; 
    }
    
    // int minCapacity 當(dāng)前 size + 1
    private void ensureCapacityInternal(int minCapacity) {
        if (size == elementData.length) {
//          //新數(shù)組容量大小
//          int newCapacity = 2 * size;
//          Object[] newObjects = new Object[newCapacity];
            //新舊數(shù)組復(fù)制
//          for (int i = 0; i < elementData.length; i++) {
//              newObjects[i] = elementData[i];
//          }
//          elementData = newObjects;
            //新舊數(shù)組復(fù)制
            int oldCapacity = elementData.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1);
            //最小擴(kuò)容容量股冗；防止初始容量大小為1
            if (newCapacity - minCapacity < 0) {
                newCapacity = minCapacity;
            }
            elementData = Arrays.copyOf(elementData, newCapacity);
        }
    }
    
    //使用下標(biāo)獲取數(shù)組元素
    public Object get(int index) {
        return elementData[index];
    }
    
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }
    
    public Object remove(int index) {
        rangeCheck(index);
        //1.使用下標(biāo)查詢?cè)撝凳欠翊嬖?        Object object = get(index);
        //計(jì)算刪除元素后面的長(zhǎng)度
        int numMoved = size - index - 1;
        if (numMoved > 0) {
            //2.刪除原理分析
            System.arraycopy(elementData, index + 1, elementData, index, numMoved);
        }
        //將最后一個(gè)元素變?yōu)榭?        elementData[--size] = null;
        return object;
    }
    
    // 刪除相同元素 刪除第一個(gè)
    public boolean remove(E e) {
        for (int i = 0; i < elementData.length; i++) {
            Object value = elementData[i];
            if (value.equals(e)) {
                remove(i);
                return true;
            }
        }
        return false;
    }
    
    //判斷下標(biāo)是否越界
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException("越界啦!");
    }
    
    public int getSize() {
        return size;
    }

}