自己手写一个Java的ArrayList

思想

• 基本要求：需要使用泛型，很多类可以形成列表。可迭代的
• 成员变量：默认容量，数量，一个泛型数组
• 成员方法：构造方法、clear、size、isEmpty、get、set、add、remove、ensureCapacity

代码

class MyArrayList<AnyType> implements Iterable<AnyType> {

    private static final int DEFAULT_CAPACITY = 10;
    private int theSize;
    private AnyType[] theItems;

    public MyArrayList() {
        doClear();
    }

    private void clear() {
        doClear();
    }

    private void doClear() {
        theSize = 0;
        ensureCapacity(DEFAULT_CAPACITY);
    }

    public int size() {
        return theSize;
    }

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

    public void trimToSize() {
        ensureCapacity(size());
    }

    public AnyType get(int idx) {
        if (idx < 0 || idx >= size()) {
            throw new ArrayIndexOutOfBoundsException();
        }
        return theItems[idx];
    }

    public AnyType set(int idx, AnyType newVal) {
        if (idx < 0 || idx >= size()) {
            throw new ArrayIndexOutOfBoundsException();
        }
        AnyType old = theItems[idx];
        theItems[idx] = newVal;
        return old;
    }

    public void ensureCapacity(int newCapacity) {
        if (newCapacity < theSize) {
            return;
        }

        AnyType[] old = theItems;
        theItems = (AnyType[]) new Object[newCapacity];
        for (int i = 0; i < size(); i++) {
            theItems[i] = old[i];
        }
    }

    public void add(int idx, AnyType x) {
        if (theItems.length == size()) {
            ensureCapacity(size() * 2 + 1);
        }
        for (int i = theSize; i > idx; i--) {
            theItems[i] = theItems[i - 1];
        }
        theItems[idx] = x;
        theSize++;
    }

    public void add(AnyType x) {
        if (theItems.length == size()) {
            ensureCapacity(size() * 2 + 1);
        }

        theItems[theSize] = x;
        theSize++;
    }

    public AnyType remove(int idx) {
        AnyType removedItem = theItems[idx];
        for (int i = idx; i < size() - 1; i++) {
            theItems[i] = theItems[i + 1];
        }
        theSize--;
        return removedItem;
    }


    @Override

    public Iterator<AnyType> iterator() {
        return new ArrayListIterator();
    }

    // 加一个迭代器内部类，表的iterator()要返回一个迭代器
    class ArrayListIterator implements java.util.Iterator<AnyType> {

        private int current = 0;
	
	//下面实现迭代器基本的三个方法
        @Override
        public boolean hasNext() {
            return current < size();
        }

        @Override
        public AnyType next() {
            if (!hasNext()) {
                throw new java.util.NoSuchElementException();
            }
            return theItems[current++];
        }

        @Override
        public void remove() {
            MyArrayList.this.remove(--current);
        }

    }

}