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Java learning essay---trouble vector

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Jul 06, 2016 pm 01:30 PM

java vector study

I have more time recently, and I have time to work on Java. I personally feel that there are too many syntaxes to learn in this language, so I don’t want to learn them one by one. On a whim, I uploaded the source code of struct2, and it was as deep as the sea. It makes me dizzy, let’s start with the simplest one 1 public static void main(String[] args) { 2 3 Vector v = new Vector(4); 4 5 //

<span style="color: #008080;"> 1</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {
</span><span style="color: #008080;"> 2</span>         
<span style="color: #008080;"> 3</span>         Vector v = <span style="color: #0000ff;">new</span> Vector(4<span style="color: #000000;">);
</span><span style="color: #008080;"> 4</span> 
<span style="color: #008080;"> 5</span>         <span style="color: #008000;">//</span><span style="color: #008000;">向Vector中添加元素 静态数组+动态扩展
</span><span style="color: #008080;"> 6</span>         <span style="color: #008000;">//</span><span style="color: #008000;">使用add方法直接添加元素 </span>
<span style="color: #008080;"> 7</span>         v.add("Test0"<span style="color: #000000;">); 
</span><span style="color: #008080;"> 8</span>         v.add("Test1"<span style="color: #000000;">); 
</span><span style="color: #008080;"> 9</span>         v.add("Test0"<span style="color: #000000;">); 
</span><span style="color: #008080;">10</span>         v.add("Test2"<span style="color: #000000;">); 
</span><span style="color: #008080;">11</span>         v.add("Test2"<span style="color: #000000;">);
</span><span style="color: #008080;">12</span> 
<span style="color: #008080;">13</span>         <span style="color: #008000;">//</span><span style="color: #008000;">从Vector中删除元素 </span>
<span style="color: #008080;">14</span>         v.remove("Test0"); <span style="color: #008000;">//</span><span style="color: #008000;">删除指定内容的元素 </span>
<span style="color: #008080;">15</span>         v.remove(0); <span style="color: #008000;">//</span><span style="color: #008000;">按照索引号删除元素
</span><span style="color: #008080;">16</span> 
<span style="color: #008080;">17</span>         <span style="color: #008000;">//</span><span style="color: #008000;">获得Vector中已有元素的个数 </span>
<span style="color: #008080;">18</span>         <span style="color: #0000ff;">int</span> size =<span style="color: #000000;"> v.size(); 
</span><span style="color: #008080;">19</span>         System.out.PRintln("size:" +<span style="color: #000000;"> size);
</span><span style="color: #008080;">20</span> 
<span style="color: #008080;">21</span>         <span style="color: #008000;">//</span><span style="color: #008000;">遍历Vector中的元素 </span>
<span style="color: #008080;">22</span>         <span style="color: #0000ff;">for</span>(<span style="color: #0000ff;">int</span> i = 0;i < v.size();i++<span style="color: #000000;">){ 
</span><span style="color: #008080;">23</span> <span style="color: #000000;">        System.out.println(v.get(i)); 
</span><span style="color: #008080;">24</span> <span style="color: #000000;">        } 
</span><span style="color: #008080;">25</span> }

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The code is very simple. Anyone who has studied data structure knows that it is a simple matter of adding, modifying and checking. However, we need to take a deeper look at what is the difference between this thing and an array

The constructor is as follows, which means you can initialize a capacity number, you decide how much you want

<span style="color: #008080;"> 1</span>  <span style="color: #008000;">/**</span>
<span style="color: #008080;"> 2</span> <span style="color: #008000;">     * Constructs an empty vector with the specified initial capacity and
</span><span style="color: #008080;"> 3</span> <span style="color: #008000;">     * with its capacity increment equal to zero.
</span><span style="color: #008080;"> 4</span> <span style="color: #008000;">     *
</span><span style="color: #008080;"> 5</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@param</span><span style="color: #008000;">   initialCapacity   the initial capacity of the vector
</span><span style="color: #008080;"> 6</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@throws</span><span style="color: #008000;"> IllegalArgumentException if the specified initial capacity
</span><span style="color: #008080;"> 7</span> <span style="color: #008000;">     *         is negative
</span><span style="color: #008080;"> 8</span>      <span style="color: #008000;">*/</span>
<span style="color: #008080;"> 9</span>     <span style="color: #0000ff;">public</span> Vector(<span style="color: #0000ff;">int</span><span style="color: #000000;"> initialCapacity) {
</span><span style="color: #008080;">10</span>     <span style="color: #0000ff;">this</span>(initialCapacity, 0<span style="color: #000000;">);
</span><span style="color: #008080;">11</span>     }

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Let’s look at it next. Java’s constructor is really more powerful than PHP and supports calling with different parameters. If you change to PHP, you will get an error a long time ago

<span style="color: #008080;"> 1</span>     <span style="color: #008000;">/**</span>
<span style="color: #008080;"> 2</span> <span style="color: #008000;">     * Constructs an empty vector with the specified initial capacity and
</span><span style="color: #008080;"> 3</span> <span style="color: #008000;">     * capacity increment.
</span><span style="color: #008080;"> 4</span> <span style="color: #008000;">     *
</span><span style="color: #008080;"> 5</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@param</span><span style="color: #008000;">   initialCapacity     the initial capacity of the vector
</span><span style="color: #008080;"> 6</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@param</span><span style="color: #008000;">   capacityIncrement   the amount by which the capacity is
</span><span style="color: #008080;"> 7</span> <span style="color: #008000;">     *                              increased when the vector overflows
</span><span style="color: #008080;"> 8</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@throws</span><span style="color: #008000;"> IllegalArgumentException if the specified initial capacity
</span><span style="color: #008080;"> 9</span> <span style="color: #008000;">     *         is negative
</span><span style="color: #008080;">10</span>      <span style="color: #008000;">*/</span>
<span style="color: #008080;">11</span>     <span style="color: #0000ff;">public</span> Vector(<span style="color: #0000ff;">int</span> initialCapacity, <span style="color: #0000ff;">int</span><span style="color: #000000;"> capacityIncrement) {
</span><span style="color: #008080;">12</span>     <span style="color: #0000ff;">super</span><span style="color: #000000;">();
</span><span style="color: #008080;">13</span>         <span style="color: #0000ff;">if</span> (initialCapacity < 0<span style="color: #000000;">)
</span><span style="color: #008080;">14</span>             <span style="color: #0000ff;">throw</span> <span style="color: #0000ff;">new</span> IllegalArgumentException("Illegal Capacity: "+
<span style="color: #008080;">15</span> <span style="color: #000000;">                                               initialCapacity);
</span><span style="color: #008080;">16</span>     <span style="color: #0000ff;">this</span>.elementData = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Object[initialCapacity];
</span><span style="color: #008080;">17</span>     <span style="color: #0000ff;">this</span>.capacityIncrement =<span style="color: #000000;"> capacityIncrement;
</span><span style="color: #008080;">18</span>     }

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Isn’t the code very simple? It simply initializes an object array. Even a high school student like me can figure it out. I noticed the second parameter. This is to control how to increase the array after it is filled. It is understandable. Let’s use a strategy

Let’s take a look at how to add elements

<span style="color: #008080;"> 1</span>   <span style="color: #008000;">/**</span>
<span style="color: #008080;"> 2</span> <span style="color: #008000;">     * Appends the specified element to the end of this Vector.
</span><span style="color: #008080;"> 3</span> <span style="color: #008000;">     *
</span><span style="color: #008080;"> 4</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@param</span><span style="color: #008000;"> e element to be appended to this Vector
</span><span style="color: #008080;"> 5</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@return</span><span style="color: #008000;"> {</span><span style="color: #808080;">@code</span><span style="color: #008000;"> true} (as specified by {</span><span style="color: #808080;">@link</span><span style="color: #008000;"> Collection#add})
</span><span style="color: #008080;"> 6</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@since</span><span style="color: #008000;"> 1.2
</span><span style="color: #008080;"> 7</span>      <span style="color: #008000;">*/</span>
<span style="color: #008080;"> 8</span>     <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">synchronized</span> <span style="color: #0000ff;">boolean</span><span style="color: #000000;"> add(E e) {
</span><span style="color: #008080;"> 9</span>     modCount++<span style="color: #000000;">;
</span><span style="color: #008080;">10</span>     ensureCapacityHelper(elementCount + 1<span style="color: #000000;">);
</span><span style="color: #008080;">11</span>     elementData[elementCount++] =<span style="color: #000000;"> e;
</span><span style="color: #008080;">12</span>         <span style="color: #0000ff;">return</span> <span style="color: #0000ff;">true</span><span style="color: #000000;">;
</span><span style="color: #008080;">13</span>     }

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1	`<span style="font-size: 14px;">synchronized 这玩意就是多线程安全的时候用的，防止多个线程同事操作</span><br /><br /><span style="font-size: 14px;">关键是 ensureCapacityHelper 这个函数<br /><br /></span>`

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<span style="color: #008080;"> 1</span> <span style="color: #008000;">/**</span>
<span style="color: #008080;"> 2</span> <span style="color: #008000;">     * This implements the unsynchronized semantics of ensureCapacity.
</span><span style="color: #008080;"> 3</span> <span style="color: #008000;">     * Synchronized methods in this class can internally call this
</span><span style="color: #008080;"> 4</span> <span style="color: #008000;">     * method for ensuring capacity without incurring the cost of an
</span><span style="color: #008080;"> 5</span> <span style="color: #008000;">     * extra synchronization.
</span><span style="color: #008080;"> 6</span> <span style="color: #008000;">     *
</span><span style="color: #008080;"> 7</span> <span style="color: #008000;">     * </span><span style="color: #808080;">@see</span><span style="color: #008000;"> #ensureCapacity(int)
</span><span style="color: #008080;"> 8</span>      <span style="color: #008000;">*/</span>
<span style="color: #008080;"> 9</span>     <span style="color: #0000ff;">private</span> <span style="color: #0000ff;">void</span> ensureCapacityHelper(<span style="color: #0000ff;">int</span><span style="color: #000000;"> minCapacity) {
</span><span style="color: #008080;">10</span>     <span style="color: #0000ff;">int</span> oldCapacity =<span style="color: #000000;"> elementData.length;
</span><span style="color: #008080;">11</span>     <span style="color: #0000ff;">if</span> (minCapacity ><span style="color: #000000;"> oldCapacity) {
</span><span style="color: #008080;">12</span>         Object[] oldData =<span style="color: #000000;"> elementData;
</span><span style="color: #008080;">13</span>         <span style="color: #0000ff;">int</span> newCapacity = (capacityIncrement > 0) ?
<span style="color: #008080;">14</span>         (oldCapacity + capacityIncrement) : (oldCapacity * 2<span style="color: #000000;">);
</span><span style="color: #008080;">15</span>             <span style="color: #0000ff;">if</span> (newCapacity <<span style="color: #000000;"> minCapacity) {
</span><span style="color: #008080;">16</span>         newCapacity =<span style="color: #000000;"> minCapacity;
</span><span style="color: #008080;">17</span> <span style="color: #000000;">        }
</span><span style="color: #008080;">18</span>             elementData =<span style="color: #000000;"> Arrays.copyOf(elementData, newCapacity);
</span><span style="color: #008080;">19</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">20</span>     }

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1	`<span style="font-size: 14px;"><br>可以这么理解吧，上面这段代码就是看看数组满了没有，如果满了就动态的增加，还记得我们上面说的那个参数吗，就是可以理解为扩展因子，如果没有定义的话就double增加，就是这么简单，貌似跟c语言的动态数组好像啊<br><br>总结一下<br><br>上面我们学到的知识点<br><br></span>`

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1	`1. synchronized 同步用的，相当于一个锁吧`

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1	`<span><br>2. Arrays.copyOf 这函数是从一个数组复制到一个新数组里面，新数组容量可以自己定义<br><br>3. java 的构造函数可以支持多个，前提你每个构造函数的参数都不同<br><br>4. vector 这东西跟数组没什么区别，只不过它比静态数组可以自动扩展罢了<br>今天就到这里吧</span>`

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1	`<span><br><br></span>`

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1	`<span style="font-size: 14px;"><br><br></span>`

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