java从集合中快速取某个元素

<svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> <path stroke-linecap="round" d="M5,0 0,2.5 5,5z" id="raphael-marker-block" style="-webkit-tap-highlight-color: rgba(0, 0, 0, 0);"></path>
</svg>

java从集合中快速取某个元素

对于元素的泛型为对象时，提供了五种方式从集合中取元素，每种方式都有各自的优缺点，都有各自的应用场景，可以依据自己的业务场景选择合适的方案，笔者更倾向于第四、第五种。

方式一：通过集合自带的工具类，get下标的方式获取。推荐指数：⭐⭐
优点：速度快
缺点：适用范围小（必须明确元素所属的下标，不然就取不到想要的元素）

方式二：普通遍历取值，通过for循环或增强for循环取值，取到对应的元素，break跳出循环。推荐指数：⭐⭐⭐
优点：取值准确，适用范围广
缺点：集合基数大的时候效率不高、消耗资源（从10w个集合元素中拿第9w个元素）

方式三：迭代器取值，通过迭代器取到相应的元素，break跳出循环。推荐指数：⭐⭐⭐
优缺点同方式二

方式四：通过stream流的filter方法筛选取值。推荐指数：⭐⭐⭐⭐⭐
优点：用法简单，取值准确，适用范围广
缺点：如果匹配不准确，可能会取不到值或异常（根据匹配规则取不到值或取到多个值）

方式五：集合转Map（key->主键，val->对象），从Map中通过key取value值。推荐指数：⭐⭐⭐⭐⭐
优点：取值准确，适用范围广，可复用性强（转map之后可以根据主键ID快速取任何一个元素）
缺点：集合数据量大的时候，转Map会比较耗时耗资源

接下来是代码的实现

场景：从Task的集合中，取主键ID为8的Task对象。示例代码如下↓

Task对象

@Data
@ToString
public class Task {
    private Integer taskId;
    private String taskUid;
    private String taskType;
    private String createTime;
}

代码实现

public class ListGetObjDemo {
<span class="token comment">//模拟Task集合数据源（json字符串格式）</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> BASE_DATA <span class="token operator">=</span> <span class="token string">"["</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':1,'taskUid':'fe7c0d4b-7000-4162-b973-79b1e2fbe32c','taskType':'9008','createTime':'2022-07-08 16:23:13'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':2,'taskUid':'1e17b929-f1cc-4c73-8fc8-be296d445d1d','taskType':'9008','createTime':'2022-07-08 16:23:12'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':3,'taskUid':'f51b4fbe-82b1-4e50-b879-10c7e1456d21','taskType':'9008','createTime':'2022-07-08 16:23:12'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':4,'taskUid':'5d71d61e-fa6f-41d2-8286-1ae2387ea480','taskType':'9008','createTime':'2022-07-08 16:23:14'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':5,'taskUid':'d5fc236f-8256-4e2b-8540-b22b1d9d4ad7','taskType':'9008','createTime':'2022-07-08 16:23:14'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':6,'taskUid':'e10053a8-6785-4da1-83ea-54ef2b325c90','taskType':'9008','createTime':'2022-07-08 16:23:14'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':7,'taskUid':'852142d8-c990-4fd7-af1b-826acfea912d','taskType':'9009','createTime':'2022-07-08 16:23:13'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':8,'taskUid':'243a70f6-a77e-4a4a-9727-7674605a786f','taskType':'9008','createTime':'2022-07-08 16:23:14'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':9,'taskUid':'c33356d9-0362-4af7-9030-4037d631a036','taskType':'9010','createTime':'2022-07-08 16:23:13'},"</span> <span class="token operator">+</span>
        <span class="token string">"{'taskId':10,'taskUid':'5ea3a5ac-89ee-4a04-af11-4bf29524396e','taskType':'9008','createTime':'2022-07-08 16:23:14'}"</span> <span class="token operator">+</span>
        <span class="token string">"]"</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * main方法入口
 */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token comment">//用fastjson把json字符串转为集合</span>
    <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> tasks <span class="token operator">=</span> JSON<span class="token punctuation">.</span><span class="token function">parseArray</span><span class="token punctuation">(</span>BASE_DATA<span class="token punctuation">,</span> <span class="token class-name">Task</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//分别调用五个方法，得到taskId为8的对象信息</span>
    <span class="token function">method1</span><span class="token punctuation">(</span>tasks<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">method2</span><span class="token punctuation">(</span>tasks<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">method3</span><span class="token punctuation">(</span>tasks<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">method4</span><span class="token punctuation">(</span>tasks<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">method5</span><span class="token punctuation">(</span>tasks<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
 * 方式一：集合下标取值
 */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">method1</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> list<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
    <span class="token class-name">Task</span> task <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"方式一结果："</span><span class="token operator">+</span>task<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
 * 方式二：普通遍历取值
 */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">method2</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> list<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
    <span class="token class-name">Task</span> task <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Task</span> t <span class="token operator">:</span> list<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>t<span class="token punctuation">.</span><span class="token function">getTaskId</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">8</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            task <span class="token operator">=</span> t<span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"方式二结果："</span><span class="token operator">+</span>task<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
 * 方式三：迭代器取值
 */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">method3</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> list<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
    <span class="token class-name">Task</span> task <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> iterator <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">while</span> <span class="token punctuation">(</span>iterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
        <span class="token class-name">Task</span> next <span class="token operator">=</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>next<span class="token punctuation">.</span><span class="token function">getTaskId</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">8</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
            task <span class="token operator">=</span> next<span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"方式三结果："</span><span class="token operator">+</span>task<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
 * 方式四：stream流filter筛选取值
 */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">method4</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> list<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
    <span class="token class-name">Task</span> task <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>i <span class="token operator">-&gt;</span> i<span class="token punctuation">.</span><span class="token function">getTaskId</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">findFirst</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"方式四结果："</span><span class="token operator">+</span>task<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
 * 方式五：集合转Map，从Map中通过key取value值
 */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">method5</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> list<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
    <span class="token class-name">Map</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">Task</span><span class="token punctuation">&gt;</span></span> taskMap <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toMap</span><span class="token punctuation">(</span>key <span class="token operator">-&gt;</span> key<span class="token punctuation">.</span><span class="token function">getTaskId</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> val <span class="token operator">-&gt;</span> val<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Task</span> task <span class="token operator">=</span> taskMap<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"方式五结果："</span><span class="token operator">+</span>task<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

运行结果