自己實現ArrayList,最簡單的java的GclArrayList

自己實現ArrayList,最簡單的java的GclArrayList

說明：只實現了簡單的增刪改查，爲了新手讀源碼方便。

代碼如下：

package jdk;

import java.util.AbstractList;
import java.util.Arrays;
import java.util.ConcurrentModificationException;

public class GclArrayList<E> extends AbstractList<E> {

    //默認數組容量10
    private static final int default_capacity = 10;
    //空元素數據數組
    private static final Object[] empty_elementdata = {};
    //默認空元素數據數組
    private static final Object[] defaultcapacity_empty_elementdata = {};
    //元素數據數組，實際操作的數組
    private transient Object[] elementData;

    //允許的最大數組容量
    private static final int max_array_size = Integer.MAX_VALUE - 8;

    private int size;

    public GclArrayList(){
        this.elementData = defaultcapacity_empty_elementdata;
    }

    //巨大容量，超過了最大數組長度max_array_size,返回Int最大值21億多
    private static int hugeCapacity(int minCapacity){
        //int值溢出，則會成爲負數，拋出內容溢出錯誤。
        if (minCapacity < 0){
            throw new OutOfMemoryError();
        }
        return (minCapacity > max_array_size) ? Integer.MAX_VALUE : max_array_size;
    }

    //擴容，很多方法的前置方法
    private void grow(int minCapacity){
        //當前元素數組長度
        int oldCapacity = elementData.length;
        //加一半容量
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if(newCapacity - minCapacity < 0){
            newCapacity = minCapacity;
        }
        if (newCapacity - max_array_size > 0){
            newCapacity = hugeCapacity(minCapacity);
        }
        //將擴容後的數組賦給元素數組
        elementData = Arrays.copyOf(elementData,newCapacity);
    }

    //確保外部容量能放下新元素
    private void ensureExplicitCapacity(int minCapacity){
        //擴容次數
        modCount ++;
        if (minCapacity - elementData.length > 0){
            grow(minCapacity);
        }
    }

    //確保內部容量足夠
    private void ensureCapacityInternal(int minCapacity){
        if (elementData == defaultcapacity_empty_elementdata){
            minCapacity = Math.max(default_capacity,minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }
    //增加元素
    public boolean add(E e){
        ensureCapacityInternal(size + 1);
        elementData[size++] = e;
        return true;
    }

    //越界信息
    private String outOfBoundsMsg(int index){
        return "Index: "+index+",Size: "+size;
    }

    //範圍檢查,訪問數組下標必須不大於數組中的元素個數
    private void rangeCheck(int index){
        if (index >= size){
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
    }

    E elementData(int index) {
        return (E) elementData[index];
    }


    //刪除元素
    public E remove(int index){
        //範圍檢查
        rangeCheck(index);
        //修改次數
        modCount++;
        E oldValue = elementData(index);
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                    numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    //修改元素
    public E set(int index,E element){
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    //查詢元素
    @Override
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    //同步檢查修改
    private void checkForComdification(){
        if (GclArrayList.this.modCount != this.modCount){
            throw new ConcurrentModificationException();
        }
    }

    @Override
    public int size() {
        checkForComdification();
        return this.size;
    }

}

測試代碼：

package jdk;

public class ArrayListMain {

    public static void main(String[] args) {

        GclArrayList gclArrayList = new GclArrayList();
        gclArrayList.add("1asdfdsfds");
        gclArrayList.add(2);
        gclArrayList.add("sfdsdfdsfsfff");
        System.out.println(gclArrayList.toString());
        System.out.println(gclArrayList.size());
        System.out.println(gclArrayList.get(2));
        gclArrayList.remove(1);
        System.out.println(gclArrayList.toString());
        gclArrayList.set(1,"改變之後的");
        System.out.println(gclArrayList.toString());
    }

}