java learning essay---making trouble with vector, java essay---vector_PHP tutorial

java learning essay---trick vector, java essay---vector

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 feels like the deep sea. It makes me dizzy, let’s start with the simplest one

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

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

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

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

<span>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>关键是 ensureCapacityHelper  这个函数<br /><br /></span>

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

<span><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>

1. synchronized  同步用的，相当于一个锁吧

<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>

<span><br /><br /></span>

<span><br /><br /></span>

http://www.bkjia.com/PHPjc/1081561.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/1081561.htmlTechArticlejava learning essay---trick vector, java essay---vector I have more time recently, I have time to do it When I download Java, I personally feel that there are too many grammars to learn in this language. I don’t want to learn them one by one, just on a whim...