FastList源碼分析

FastList 是 HikariCP數據庫連接池中使用的自定義的list。其實也沒什麼特別的，主要就是去掉了range check取數據更快一些。

（1）類定義

public final class FastList<T> extends ArrayList<T>

繼承自ArrayList，

（2）成員變量

   private final Class<?> clazz;
   private T[] elementData;
   private int size;

clazz ：list中存儲的數據類型class。

elementData：存儲數據的源數組。

size：列表目前大小

（3）構造函數

   @SuppressWarnings("unchecked")
   public FastList(Class<?> clazz)
   {
      this.elementData = (T[]) Array.newInstance(clazz, 32);
      this.clazz = clazz;
   }

   @SuppressWarnings("unchecked")
   public FastList(Class<?> clazz, int capacity)
   {
      this.elementData = (T[]) Array.newInstance(clazz, capacity);
      this.clazz = clazz;
   }

傳遞數據類型和初始的數組容量，默認容量大小爲32，，不同於size。

（4）add添加一個元素

@Override
   public boolean add(T element)
   {
      try {
         elementData[size++] = element;
      }
      catch (ArrayIndexOutOfBoundsException e) {
         // overflow-conscious code
         final int oldCapacity = elementData.length;
         final int newCapacity = oldCapacity << 1;
         @SuppressWarnings("unchecked")
         final T[] newElementData = (T[]) Array.newInstance(clazz, newCapacity);
         System.arraycopy(elementData, 0, newElementData, 0, oldCapacity);
         newElementData[size - 1] = element;
         elementData = newElementData;
      }

      return true;
   }

重寫了add方法。

往數組裏面插入元素，size增加1，如果是數組越界，擴容爲原來容量的2倍。最後將擴容後的數組賦值給源數組。

（5）get獲取指定位置的元素

   @Override
   public T get(int index)
   {
      return elementData[index];
   }

重寫了get方法。

直接獲取數組的index下標，越界直接拋出異常。

（6）移除最後一個元素

   public T removeLast()
   {
      T element = elementData[--size];
      elementData[size] = null;
      return element;
   }

將size -1 的位置元素取出來，然後置空，size 減1；

（7）移除指定元素

   @Override
   public boolean remove(Object element)
   {
      for (int index = size - 1; index >= 0; index--) {
         if (element == elementData[index]) {
            final int numMoved = size - index - 1;
            if (numMoved > 0) {
               System.arraycopy(elementData, index + 1, elementData, index, numMoved);
            }
            elementData[--size] = null;
            return true;
         }
      }

      return false;
   }

從數組的最後面往前面找，如果碰到第一個相同的元素的時候就直接移除，並且copy原來的數組，size減1.注意的是判斷相等是用的最基礎的 ==。

（8）清除所有的元素

@Override
   public void clear()
   {
      for (int i = 0; i < size; i++) {
         elementData[i] = null;
      }

      size = 0;
   }

從頭到尾（size）設置爲空，size = 0；

（9）返回大小，判斷是否爲空，不多說

 @Override
   public int size()
   {
      return size;
   }

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

（10）最後還是源碼直接粘貼一下吧

/*
 * Copyright (C) 2013, 2014 Brett Wooldridge
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.zaxxer.hikari.util;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

/**
 * Fast list without range checking.
 *
 * @author Brett Wooldridge
 */
public final class FastList<T> extends ArrayList<T>
{
   private static final long serialVersionUID = -4598088075242913858L;

   private final Class<?> clazz;
   private T[] elementData;
   private int size;

   /**
    * Construct a FastList with a default size of 32.
    * @param clazz the Class stored in the collection
    */
   @SuppressWarnings("unchecked")
   public FastList(Class<?> clazz)
   {
      this.elementData = (T[]) Array.newInstance(clazz, 32);
      this.clazz = clazz;
   }

   /**
    * Construct a FastList with a specified size.
    * @param clazz the Class stored in the collection
    * @param capacity the initial size of the FastList
    */
   @SuppressWarnings("unchecked")
   public FastList(Class<?> clazz, int capacity)
   {
      this.elementData = (T[]) Array.newInstance(clazz, capacity);
      this.clazz = clazz;
   }

   /**
    * Add an element to the tail of the FastList.
    *
    * @param element the element to add
    */
   @Override
   public boolean add(T element)
   {
      try {
         elementData[size++] = element;
      }
      catch (ArrayIndexOutOfBoundsException e) {
         // overflow-conscious code
         final int oldCapacity = elementData.length;
         final int newCapacity = oldCapacity << 1;
         @SuppressWarnings("unchecked")
         final T[] newElementData = (T[]) Array.newInstance(clazz, newCapacity);
         System.arraycopy(elementData, 0, newElementData, 0, oldCapacity);
         newElementData[size - 1] = element;
         elementData = newElementData;
      }

      return true;
   }

   /**
    * Get the element at the specified index.
    *
    * @param index the index of the element to get
    * @return the element, or ArrayIndexOutOfBounds is thrown if the index is invalid
    */
   @Override
   public T get(int index)
   {
      return elementData[index];
   }

   /**
    * Remove the last element from the list.  No bound check is performed, so if this
    * method is called on an empty list and ArrayIndexOutOfBounds exception will be
    * thrown.
    *
    * @return the last element of the list
    */
   public T removeLast()
   {
      T element = elementData[--size];
      elementData[size] = null;
      return element;
   }

   /**
    * This remove method is most efficient when the element being removed
    * is the last element.  Equality is identity based, not equals() based.
    * Only the first matching element is removed.
    *
    * @param element the element to remove
    */
   @Override
   public boolean remove(Object element)
   {
      for (int index = size - 1; index >= 0; index--) {
         if (element == elementData[index]) {
            final int numMoved = size - index - 1;
            if (numMoved > 0) {
               System.arraycopy(elementData, index + 1, elementData, index, numMoved);
            }
            elementData[--size] = null;
            return true;
         }
      }

      return false;
   }

   /**
    * Clear the FastList.
    */
   @Override
   public void clear()
   {
      for (int i = 0; i < size; i++) {
         elementData[i] = null;
      }

      size = 0;
   }

   /**
    * Get the current number of elements in the FastList.
    *
    * @return the number of current elements
    */
   @Override
   public int size()
   {
      return size;
   }

   /** {@inheritDoc} */
   @Override
   public boolean isEmpty()
   {
      return size == 0;
   }

   /** {@inheritDoc} */
   @Override
   public T set(int index, T element)
   {
      T old = elementData[index];
      elementData[index] = element;
      return old;
   }

   /** {@inheritDoc} */
   @Override
   public T remove(int index)
   {
      if (size == 0) {
         return null;
      }

      final T old = elementData[index];

      final int numMoved = size - index - 1;
      if (numMoved > 0) {
         System.arraycopy(elementData, index + 1, elementData, index, numMoved);
      }

      elementData[--size] = null;

      return old;
   }

   /** {@inheritDoc} */
   @Override
   public boolean contains(Object o)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public Iterator<T> iterator()
   {
      return new Iterator<T>() {
         private int index;

         @Override
         public boolean hasNext()
         {
            return index < size;
         }

         @Override
         public T next()
         {
            if (index < size) {
               return elementData[index++];
            }

            throw new NoSuchElementException("No more elements in FastList"); 
         }
      };
   }

   /** {@inheritDoc} */
   @Override
   public Object[] toArray()
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public <E> E[] toArray(E[] a)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public boolean containsAll(Collection<?> c)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public boolean addAll(Collection<? extends T> c)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public boolean addAll(int index, Collection<? extends T> c)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public boolean removeAll(Collection<?> c)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public boolean retainAll(Collection<?> c)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public void add(int index, T element)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public int indexOf(Object o)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public int lastIndexOf(Object o)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public ListIterator<T> listIterator()
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public ListIterator<T> listIterator(int index)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public List<T> subList(int fromIndex, int toIndex)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public void trimToSize()
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public void ensureCapacity(int minCapacity)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   public Object clone()
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   @Override
   protected void removeRange(int fromIndex, int toIndex)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   public void forEach(Consumer<? super T> action)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   public Spliterator<T> spliterator()
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   public boolean removeIf(Predicate<? super T> filter)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   public void replaceAll(UnaryOperator<T> operator)
   {
      throw new UnsupportedOperationException();
   }

   /** {@inheritDoc} */
   public void sort(Comparator<? super T> c)
   {
      throw new UnsupportedOperationException();
   }
}