模拟ArrayList、LinkedList底层实现

1、MyList接口：

package shixian;

public interface MyList<E> extends Iterable<E> {
    /**添加一个新元素在list末尾*/
	public void add(E e);
	/**添加一个新元素在指定的索引处*/
	public void add(int index,E e);
	/**clear the list*/
	public void clear();
	/**是否包含某个指定元素*/
	public boolean contains(E e);
	/**返回指定的索引元素*/
	public E get(int index);
	/**获取第一次匹配时的索引，否则return -1*/
	public int indexOf(E e);
	/**判断list是否为空*/
	public boolean isEmpty();
	/**返回最后匹配元素索引，未找到返回-1*/
	public int lastIndexOf(E e);
	/**移除list中第一次出现的匹配元素，成功返回true*/
	public boolean remove(E e);
	/**移除指定位置元素，移除成功返回移除的元素*/
	public E remove(int index);
	/**设置指定位置元素,返回旧元素*/
	public E set(int index,E e);
	/**返回list大小*/
	public int size();
}

2、抽象接口MyAbstractList实现部分MyList接口方法：

package shixian;

public abstract class MyAbstractList<E> implements MyList<E>{
    protected int size = 0; //元素个数
    
    protected MyAbstractList() {
    	
    }
    
    protected MyAbstractList(E[] objects) {
		for(int i = 0; i < objects.length; i++)
			add(objects[i]);
	}
    
    /**在list末尾添加新元素*/
    @Override
    public void add(E e) {
    	add(size, e);
    }
    
    /**list是否不包含元素*/
    @Override
    public boolean isEmpty() {
    	return size == 0;
    }
    
    /**删除首次匹配元素*/
    public boolean remove(E e) {
      if(indexOf(e) >= 0) {
    	  remove(indexOf(e));
    	  return true;
      }
      else 
    	  return false;
    }
    
    /**返回元素个数*/
	@Override
	public int size() {
		return size;
	}    
}

3、MyArrayList继承抽象接口：

package shixian;

import java.util.Iterator;
//底层通过数组实现
public class MyArrayList<E> extends MyAbstractList<E>{
    public static final int INITIAL_CAPACITY = 16;  //初始大小
   
	private E[] data = (E[]) new Object[INITIAL_CAPACITY];
	
    public MyArrayList() {
		// TODO Auto-generated constructor stub
    }
    
    public MyArrayList(E[] objects) {
    	for(int i = 0; i < objects.length; i++) {
    		add(objects[i]);
    	}
    }
   
   /**指定位置添加元素*/
    @Override
	public void add(int index, E e) {
		ensureCapacity();
		
		for(int i = size - 1; i >= index; i--)
          data[i+1] = data[i];
		
		data[index] = e;
		size++;
	}

    /**如果数组存满元素， 则另外创建大小为原数组两倍+1的数组*/
    private void ensureCapacity() {
    	if(size >= data.length) {
    		E[] newData = (E[]) new Object[size * 2 + 1];
    		System.arraycopy(data, 0, newData, 0, size);
    		data = newData;
    	}
    }
    
    /**clear the list*/
	@Override
	public void clear() {
		data = (E[]) new Object[INITIAL_CAPACITY];
		size = 0;
	}

	@Override
	public boolean contains(E e) {
		for(int i = 0; i < size; i++)
			if(e.equals(data[i])) return true;
		return false;
	}

	@Override
	public E get(int index) {
		checkIndex(index);
 		return data[index];
	}

	private void checkIndex(int index) {
		if(index < 0 || index >= size)
			throw new IndexOutOfBoundsException
			 ("index " + index + " out of bounds");
	}
	
	@Override
	public int indexOf(E e) {
		for(int i = 0; i < size; i++)
			if(e.equals(data[i])) return i;
		return -1;
	}

	@Override
	public int lastIndexOf(E e) {
		for(int i = size - 1; i >= 0; i--)
			if(e.equals(data[i])) return i;
		return -1;
	}

	@Override
	public E remove(int index) {
		checkIndex(index);
		
		E e = data[index];
		
		for(int j = index; j < size -1; j++)
			data[j] = data[j + 1];
		data[size -1] = null;
		
		size--;
		return e;
	}

	@Override
	public E set(int index, E e) {
		checkIndex(index);
		E old = data[index];
		data[index] = e;
		return old;
	}

	@Override
	public String toString() {
		StringBuilder result = new StringBuilder("[");
		
		for(int i = 0; i < size; i++) {
			result.append(data[i]);
			if(i < size - 1) result.append(", ");
		}
		
		return result.toString() + "]";
	}
	
	/**改变当前capacity大小为当前size大小*/
	public void trimToSize() {
		if(size != data.length) {
			E[] newData = (E[]) new Object[size];
			System.arraycopy(data, 0, newData, 0, size);
			data  = newData;
		}
	}
	
	/**返回迭代器*/
	@Override
	public Iterator<E> iterator() {
		return new ArrayListIterator();
	}

	private class ArrayListIterator implements java.util.Iterator<E>{
        private int current = 0;//current index
        
		@Override
		public boolean hasNext() {
			return current < size;
		}

		@Override
		public E next() {
			return data[current++];
		}

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

4、MyArrayList测试：

package shixian;

public class TestMyArrayList {

	public static void main(String[] args) {
	   MyList<String> list = new MyArrayList<>();
	   list.add("America");
	   System.out.println("(1) " + list);

	   list.add(0,"Canada");
	   System.out.println("(2) " + list);
	   
	   list.add("Russia");
	   System.out.println("(3) " + list);
	   
	   list.add("France");
	   System.out.println("(4) " + list);
	   
	   list.add(2,"Germany");
	   System.out.println("(5) " + list);

	   list.add(5,"Norway");
	   System.out.println("(6) " + list);	   
	   
	   list.remove("Canada");
	   System.out.println("(7) " + list);
	   
	   list.remove(2);
	   System.out.println("(8) " + list);
	   
	   list.remove(list.size() - 1);
	   System.out.println("(9) " + list);
	   
	   for (String s : list) {
		   System.out.print(s.toUpperCase() + " ");
		
	}
	}

}

/**
 * 
(1) [America]
(2) [Canada, America]
(3) [Canada, America, Russia]
(4) [Canada, America, Russia, France]
(5) [Canada, America, Germany, Russia, France]
(6) [Canada, America, Germany, Russia, France, Norway]
(7) [America, Germany, Russia, France, Norway]
(8) [America, Germany, France, Norway]
(9) [America, Germany, France]
AMERICA GERMANY FRANCE 
 * */

----------------------------------------------------------------------------------------

LinkedList底层实现：

package shixian;

import java.util.Iterator;

public class MyLinkedList<E> extends MyAbstractList<E>{
    private Node<E> head,tail;
    
    /**无参构造函数*/
    public MyLinkedList() {
		// TODO Auto-generated constructor stub
	}
    
    /**带参构造函数*/
    public MyLinkedList(E[] objects) {
    	super(objects);
    }
    
    /**获取第一个元素*/
    public E getFirst() {
    	if(size == 0) {
    		return null;
    	}else {
    		return head.element;
    	}
    }
    
    /**获取最后一个元素*/
    public E getLast() {
    	if(size == 0) {
    		return null;
    	} else {
			return tail.element;
		}
    }
    
    /**添加第一个元素*/
    public void addFirst(E e) {
    	Node<E> newNode = new Node<>(e);
    	newNode.next = head;
    	head = newNode;
    	size++;
    	
    	if(tail == null) {
    		tail = head;
    	}
    }
    
    /**末尾添加元素*/
    public void addLast(E e) {
    	Node<E> newNode = new Node<>(e);
    	
    	if(tail == null) {
    		head = tail = newNode;
    	}
    	else {
    		tail.next = newNode;
    		tail = tail.next;
    	}
    	
    	size++;
    }
    
    /**移除第一个元素，并返回移除元素*/
    public E removeFirst() {
    	if(size == 0) return null;
    	else {
    		Node<E> temp = head;
    		head = head.next;
    		size--;
    		if(head == null) tail = null;
    		return temp.element;
    	}
    }
    
    /**移除最后一个元素，并返回移除元素*/
    public E removeLast() {
    	if(size == 0) return null;
    	else if(size == 1) {
    		Node<E> temp = head;
    		head = tail = null;
    		size = 0;
    		return temp.element;
    	}
    	else {
    		Node<E> current = head;
    		for(int i = 0; i < size - 2; i++)
    			current = current.next;
    		
    		Node<E> temp = tail;
    		tail = current;
    		tail.next = null;
    		size--;
    		return temp.element;
    	}
    }
    
    /**指定位置添加元素*/
	@Override
	public void add(int index, E e) {
		if(index == 0) addFirst(e);
		else if(index >= size) addLast(e);
		else {
			Node<E> current = head;
			for(int i = 1; i < index; i++)
				current = current.next;
			Node<E> temp = current.next;
			current.next = new Node<>(e);
			(current.next).next = temp;
			size++;
		}
	}

	/**clear the list*/
	@Override
	public void clear() {
		size = 0;
		head = tail = null;
	}

	/**重写toString方法*/
	@Override
	public String toString() {
		StringBuilder result = new StringBuilder("[");
		
		Node<E> current = head;
		for(int i = 0; i < size; i++) {
			result.append(current.element);
			current = current.next;
			if(current != null) {
				result.append(", ");
			}else {
				result.append("]");
			}
		}
		return result.toString();
	}
	
	
	
	@Override
	public boolean contains(E e) {
		if(size == 0) return false;
		else {
			Node<E> current = head;
			for(int i = 0; i < size; i++) {
				if(current.element.equals(e)) {
					return true;
				}
				current = current.next;
			}
		}
		return false;
	}

	@Override
	public E get(int index) {
		if(index < 0 || index >= size) return null;
		else {
			Node<E> current = head;
			for(int i = 0; i < size ; i++) {
				if(index == i) {
					return current.element;
				}
				current = current.next;
			}
		}
		return null;
	}

	/**返回第一个匹配元素下标*/
	@Override
	public int indexOf(E e) {
		Node<E> current = head;
		for(int i = 0; i < size; i++) {
			if(current.element.equals(e)) {
				return i;
			}
			current = current.next;
		}
		return -1;
	}

	/**返回最后匹配元素下标*/
	@Override
	public int lastIndexOf(E e) {
		Node<E> current = head;
		int temp = -1;
		for(int i = 0; i < size; i++) {
			if(current.element.equals(e)) {
				temp = i;
			}
			current = current.next;
		}
		return temp;
	}

	/**移除指定位置元素，并返回删除元素*/
	@Override
	public E remove(int index) {
		if(index < 0 || index >= size) return null;
		else if(index == 0) return removeFirst();
		else if(index == size -1) return removeLast();
		else {
			Node<E> previous = head;
			for(int i = 1; i < index; i++)
				previous = previous.next;
			
			Node<E> current = previous.next;
			previous.next = current.next;
			size--;
			return current.element;
		}
	}

	/**设置指定下标元素，并返回旧元素*/
	@Override
	public E set(int index, E e) {
		Node<E> current = head;
		E temp = null;
		for(int i = 0; i < size;i++) {
			if(i == index) {
				temp = current.element;
				current.element = e;
			}
			current = current.next;
		}
		return temp;
	}

	@Override
	public Iterator<E> iterator() {
		return new LinkedListIterator();
	}

	/**实现迭代器，遍历list容器*/
	private class LinkedListIterator implements java.util.Iterator<E>{
        private Node<E> current = head;
        
		@Override
		public boolean hasNext() {
			return (current != null);
		}

		@Override
		public E next() {
			E e = current.element;
			current = current.next;
			return e;
		}

		@Override
		public void remove() {
			MyLinkedList.this.remove(current.element);
		}
		
	}
	
	/**
	 * 该类只用于LinkList中，故为private
	 * 该类不需要任何LinkList的实例访问，故为static
	 * */
	private static class Node<E> {
	    E element;
	    Node<E> next;
	    
	    public Node(E e) {
	    	element = e;
	    }		
	}
}

测试：

package shixian;

public class TestMyLinkedList {

	public static void main(String[] args) {
		MyLinkedList<String> list = new MyLinkedList<>();
		
		list.add("America");
		System.out.println("(1) "+list);

		list.add(0,"Canada");
		System.out.println("(2) "+list);
		
		list.add("Russia");
		System.out.println("(3) "+list);
		
		list.addLast("France");
		System.out.println("(4) "+list);
		
		list.add(2,"Germany");
		System.out.println("(5) "+list);
		
		list.add(5,"Norway");
		System.out.println("(6) "+list);
		
		list.addFirst("Poland");
		System.out.println("(7) "+list);
		
		list.remove(0);
		System.out.println("(8) "+list);
		
		list.remove(2);
		System.out.println("(9) "+list);
		
		list.remove(list.size() - 1);
		System.out.print("(10) "+list + "\n(11) ");
		
		for (String s : list) {
			System.out.print(s.toUpperCase() + " ");
		}
		
		System.out.println();
		System.out.println("(12) is contains Russia ? " + list.contains("Russia"));
		System.out.println("(13) get(0) : "+list.get(0));
		System.out.println("(14) lastIndexOf : "+list.lastIndexOf("Russia"));
	}

}


/**
(1) [America]
(2) [Canada, America]
(3) [Canada, America, Russia]
(4) [Canada, America, Russia, France]
(5) [Canada, America, Germany, Russia, France]
(6) [Canada, America, Germany, Russia, France, Norway]
(7) [Poland, Canada, America, Germany, Russia, France, Norway]
(8) [Canada, America, Germany, Russia, France, Norway]
(9) [Canada, America, Russia, France, Norway]
(10) [Canada, America, Russia, France]
(11) CANADA AMERICA RUSSIA FRANCE 
(12) is contains Russia ? true
(13) get(0) : Canada
(14) lastIndexOf : 2
 * */

ArrayList与LinkedList效率比较：

1、get(index : int)  :  ArrayList   （O(1)），LinkedList （O(n)）

2、addFirst(e : E)  ： ArrayList    （O(n)），LinkedList （O(1)）

3、removeFirst () ：ArrayList  （O(n)） ，LinkedList （O(1)）

其他方法效率都一样

MyArrayList 的开销比MyLinkedList 小，但是，如果要需要在线性表的开始位置插入和删除元素，那么MyLinkedList的效率会高一些。