动态数组实现

import java.util.Arrays;
import java.util.Objects;

/**
 * @author john
 * @date 2020/11/27 - 8:43
 */
@SuppressWarnings("unchecked")
public class ArrayList<E> {
    // 元素的数量
    private int size;
    // 所有的元素
    private E[] elements;

    private static final int DEFAULT_CAPACITY = 10;
    private static final int ELEMENT_NOT_FOUND = -1;

    public ArrayList(int capacity) {
        capacity = Math.max(capacity, DEFAULT_CAPACITY);
        elements = (E[]) new Object[capacity];
    }

    public ArrayList() {
        this(DEFAULT_CAPACITY);
    }

    /**
     * 清除所有元素
     */
    public void clear() {
        for (int i = 0; i < size; i++) {
            elements[i] = null;
        }
        size = 0;
    }

    /**
     * 获取当前数组元素的数量
     *
     * @return
     */
    public int size() {
        return size;
    }

    /**
     * 判断数组是否为空
     *
     * @return
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 是否包含某个元素
     *
     * @param element
     * @return
     */
    public boolean contains(E element) {
        return indexOf(element) != ELEMENT_NOT_FOUND;
    }

    /**
     * 查看元素索引
     *
     * @param element
     * @return
     */
    public int indexOf(E element) {
        if (element == null) {
            for (int i = 0; i < size; i++) {
                if (elements[i] == null) return i;
            }
        } else {
            for (int i = 0; i < size; i++) {
                if (element.equals(elements[i])) return i;
            }
        }

        return ELEMENT_NOT_FOUND;
    }

    /**
     * 添加元素到尾部
     *
     * @param element
     */
    public void add(E element) {
        add(size, element);
    }

    /**
     * 获取index位置的元素
     *
     * @param index
     * @return
     */
    public E get(int index) {
        rangeCheck(index);
        return elements[index];
    }

    /**
     * 设置index位置的元素
     *
     * @param index
     * @param element
     * @return
     */
    public E set(int index, E element) {
        rangeCheck(index);
        E old = elements[index];
        elements[index] = element;
        return old;
    }

    /**
     * 在index位置插入一个元素
     *
     * @param index
     * @param element
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        ensureCapacity(size + 1);
        for (int i = size; i > index; i--) {
            elements[i] = elements[i - 1];
        }
        elements[index] = element;
        size++;
    }

    /**
     * 删除指定index位置的元素
     *
     * @param index
     * @return
     */
    public E remove(int index) {
        rangeCheck(index);
        E old = elements[index];
        for (int i = index + 1; i < size; i++) {
            elements[i - 1] = elements[i];
        }
        elements[--size] = null;
        return old;
    }

    /**
     * 保证要有capacity的容量
     *
     * @param capacity
     */
    private void ensureCapacity(int capacity) {
        int oldCapacity = elements.length;
        if (oldCapacity >= capacity) return;

        // 新容量为旧容量的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        E[] newElements = (E[]) new Object[newCapacity];
        for (int i = 0; i < size; i++) {
            newElements[i] = elements[i];
        }
        elements = newElements;
        System.out.println(oldCapacity + "扩容为" + newCapacity);
    }

    // 抛索引越界异常
    private void outOfBound(int index) {
        throw new IndexOutOfBoundsException("Index:" + index + ", Size:" + size);
    }

    private void rangeCheck(int index) {
        if (index < 0 || index >= size) {
            outOfBound(index);
        }
    }

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size) {
            outOfBound(index);
        }
    }

    @Override
    public String toString() {
        StringBuilder string = new StringBuilder();
        string.append("size=").append(size).append(", [");
        for (int i = 0; i < size; i++) {
            if (i != 0) {
                string.append(", ");
            }
            string.append(elements[i]);
        }
        string.append("]");
        return string.toString();
    }
}

测试类

/**
 * @author john
 * @date 2020/11/27 - 9:27
 */
public class Person {
    private int age;
    private String name;

    public Person(int age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person [age=" + age + ", name=" + name + "]";
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();

        System.out.println("Person - finalize");
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) return false;
        if (obj instanceof Person) {
            Person person = (Person) obj;
            return this.age == person.age;
        }
        return false;
    }
}

public class Main {
    public static void main(String[] args) {
        // int -> Integer

        // 所有的类，最终都继承java.lang.Object

        // new是向堆空间申请内存
        ArrayList<Person> persons = new ArrayList<>();
        persons.add(new Person(10, "Jack"));
        persons.add(new Person(12, "James"));
        persons.add(new Person(15, "Rose"));
        persons.clear();
        persons.add(new Person(22, "abc"));

        System.out.println(persons);
        ArrayList<Integer> ints = new ArrayList<>();
        ints.add(10);
        ints.add(10);
        ints.add(22);
        ints.add(33);
        System.out.println(ints);
        System.out.println(ints.indexOf(33));
        ints.remove(0);
        System.out.println(ints);
        ints.set(0,11);
        System.out.println(ints);

    }
}