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

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> }

Copy after login

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>     }

Copy after login

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>     }

Copy after login

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>     }

Copy after login

<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>

Copy after login

<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>     }

Copy after login

<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>

Copy after login

1. synchronized  同步用的，相当于一个锁吧

Copy after login

<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>

Copy after login