java学习_5_23

Posted 2021-12-02 ustc-anmin

Collection接口中定义的方法如下，所有继承自Collection接口的接口(List,Set)的实现类均实现了这些方法。

List容器是有序、可重复的，常用的实现类：ArrayList,LinkedList,Vector（线程安全的）

带索引的add,remove操作。

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自己实现的MyArrayList:

package test.stringTest;

public class MyArrayList<E> {
    private Object[] elementData;
    // 元素个数
    private int size;
    // 数组默认长度
    private final static int DEFAULT_CAPACITY = 10;

    public MyArrayList() {
        elementData = new Object[DEFAULT_CAPACITY];
    }

    public MyArrayList(int length) {
        if (length <= 0)
            throw new RuntimeException("容器容量值需为正整数:" + length);
        elementData = new Object[length];
    }

    public void add(E element) {
        if (size == elementData.length)
            grow();
        elementData[size++] = element;
    }

    @SuppressWarnings("unchecked")
    public E get(int index) {
        if (indexInRange(index))
            return (E) elementData[index];
        else
            throw new RuntimeException("索引越界:" + index);
    }

    public void remove(E element) {
        int index = -1;
        for (int i = 0; i < size; i++) {
            if (element.equals(elementData[i])) {
                index = i;
                break;
            }
        }
        if (index != -1) {
            remove(index);
        }
    }

    public void remove(int index) {
        if (!indexInRange(index)) {
            throw new RuntimeException("索引越界:" + index);
        }
        int numMoved = elementData.length - index - 1;
        if (numMoved > 0) {
            System.arraycopy(elementData, index + 1, elementData, index, numMoved);
        } else {
            elementData[index] = null;
        }
        size--;
    }

    private boolean indexInRange(int index) {
        return index < size && index >= 0 ? true : false;
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return size == 0 ? true : false;
    }

    // 数组扩容
    private void grow() {
        Object[] newArray = new Object[elementData.length + (elementData.length >> 1)];
        System.arraycopy(elementData, 0, newArray, 0, elementData.length);
        elementData = newArray;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        for (int i = 0; i < size; i++) {
            sb.append(elementData[i].toString() + ",");
        }
        sb.setCharAt(sb.length() - 1, ‘]‘);
        return sb.toString();
    }

    public static void main(String[] args) {
        MyArrayList<String> list = new MyArrayList<>(20);
        int i = 0;
        while (i <= 50) {
            list.add("aaa" + i);
            i++;
        }
        System.out.println(list);
        System.out.println(list.get(31));
        System.out.println(list.size());
        list.remove(21);
        list.remove("aaa4");
        System.out.println(list);
        System.out.println(list.size);
        System.out.println(list.isEmpty());
    }
}