Java 8 中的 Optional

妖狐艹你老母 2023-09-27 11:00 117阅读 0赞

作为 Java 开发人员，您是否遇到过[NullPointerException ？][NullPointerException]如果您是经验丰富的 Java 开发人员，您可能会在某个时间点遇到 NullPointerException。  
您可能同意 NullPointerException 对新手或专家[核心 Java][Java]开发人员来说是痛苦的。如果您想避免 NullPointerException，您必须放置大量防御性代码。  
让我们借助一个例子来看看。

创建一个名为“JavaFindEmployeeMain.java”的主类

<table style="margin-left:-26px;"> 
 <tbody> 
 <tr> 
 <td style="background-color:#eeeeee;vertical-align:top;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:right;"> 
 1 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 2 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 3 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 4 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 5 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 6 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 7 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 8 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 9 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 10 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 11 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 12 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 13 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 14 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 15 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 16 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 17 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 18 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 19 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 20 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 21 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 22 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 23 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 24 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 25 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 26 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 27 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 28 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 29 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 30 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 31 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 32 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 33 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 34 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 35 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 36 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 37 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 38 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 39 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 40 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 41 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 42 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 43 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 44 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 45 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 46 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 47 
 </div> 
 </div> </td> 
 <td style="vertical-align:top;width:695.984px;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 package org.arpit.java2blog; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 import java.util.ArrayList; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 import java.util.List; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 public class JavaFindEmployeeMain { 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;public static void main(String[] args) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;List&lt;Employee&gt; employeeList = createEmployeeList(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee employee = findEmployee(employeeList,"Adam"); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("Employee name: "+employee.getName()); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;public static Employee findEmployee(List&lt;Employee&gt; employeeList,String name) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;for(Employee e:employeeList) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(e.getName().equalsIgnoreCase(name)) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return e; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return null; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;public static List&lt;Employee&gt; createEmployeeList() 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;List&lt;Employee&gt; employeeList=new ArrayList&lt;&gt;(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e1=new Employee("John",21); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e2=new Employee("Martin",22); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e3=new Employee("Mary",31); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e4=new Employee("Stephan",18); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e5=new Employee("Gary",26); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e1); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e2); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e3); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e4); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e5); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return employeeList; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 } 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

当你运行上面的程序时，你会得到下面的输出：

org.arpit.java2blog.JavaOptionalMain.main(JavaOptionalMain.java:12)的线程“主”java.lang.NullPointerException中的异常

如您所见，employeeList 中不存在“Adam”，这就是我们在这里得到 NullPointerException 的原因。

你看到这里的问题了吗？当我们试图在列表中查找员工时，我们忘记检查 null。当您调用库函数并且它返回 null 并且您忘记检查它时，这种情况更常见。

--------------------

### **Java 8 可选** ###

你可以使用 Optional 来解决这个问题。您可以如下更改“JavaOptionalMain”。

<table style="margin-left:-26px;"> 
 <tbody> 
 <tr> 
 <td style="background-color:#eeeeee;vertical-align:top;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:right;"> 
 1 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 2 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 3 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 4 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 5 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 6 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 7 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 8 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 9 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 10 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 11 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 12 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 13 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 14 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 15 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 16 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 17 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 18 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 19 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 20 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 21 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 22 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 23 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 24 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 25 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 26 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 27 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 28 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 29 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 30 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 31 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 32 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 33 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 34 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 35 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 36 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 37 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 38 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 39 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 40 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 41 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 42 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 43 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 44 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 45 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 46 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 47 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 48 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 49 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 50 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 51 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 52 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 53 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 54 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 55 
 </div> 
 </div> </td> 
 <td style="vertical-align:top;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 package org.arpit.java2blog; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 import java.util.ArrayList; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 import java.util.List; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 import java.util.Optional; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 public class JavaOptionalMain { 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;public static void main(String[] args) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;List&lt;Employee&gt; employeeList = createEmployeeList(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Optional&lt;Employee&gt; employeeOpt = findEmployee(employeeList,"Adam"); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(employeeOpt.isPresent()) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee employee = employeeOpt.get(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("Employee name: "+employee.getName()); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;else 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("There is no employee with name Adam"); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;public static Optional&lt;Employee&gt; findEmployee(List&lt;Employee&gt; employeeList,String name) 
 </div> 
 <div style="margin-left:0;text-align:left;"></div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;for(Employee e:employeeList) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(e.getName().equalsIgnoreCase(name)) 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return Optional.of(e); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return Optional.empty(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;public static List&lt;Employee&gt; createEmployeeList() 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;{ 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;List&lt;Employee&gt; employeeList=new ArrayList&lt;&gt;(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e1=new Employee("John",21); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e2=new Employee("Martin",22); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e3=new Employee("Mary",31); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e4=new Employee("Stephan",18); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Employee e5=new Employee("Gary",26); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e1); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e2); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e3); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e4); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;employeeList.add(e5); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return employeeList; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp;&nbsp;&nbsp;&nbsp;} 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 } 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

当你运行上面的程序时，你会得到下面的输出：

没有名为 Adam 的员工

您可能认为您也可以在 JavaFindEmployeeMain 中处理 null，但是当您从方法返回 Optional 时，这意味着可以从方法中预期缺少值。

--------------------

### **创建可选的方法** ###

有多种创建 Optional 的方法。

#### **空 可选** ####

25.6M

790

如何在 Ubuntu 18.04 上安装 Unity \[完整过程\]

您可以使用静态工厂方法“empty”创建空的可选对象

<table style="margin-left:-19px;"> 
 <tbody> 
 <tr> 
 <td style="background-color:#eeeeee;vertical-align:top;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:right;"> 
 1 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 2 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 3 
 </div> 
 </div> </td> 
 <td style="vertical-align:top;width:696.188px;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 Optional&lt;Employee&gt; optCar = Optional.empty(); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

#### **非空值可选** ####

您可以使用静态工厂方法“of”从非空值创建可选

<table style="margin-left:-19px;"> 
 <tbody> 
 <tr> 
 <td style="background-color:#eeeeee;vertical-align:top;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:right;"> 
 1 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 2 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 3 
 </div> 
 </div> </td> 
 <td style="vertical-align:top;width:696.188px;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 Optional&lt;Employee&gt; optCar = Optional.of(employee); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

如果员工为空，则上述方法将抛出 NullPointerException。

#### **可选从 null 或非 null 值** ####

您可以使用静态工厂方法“ofNullable”从 null 或非 null 值创建 Optional

<table style="margin-left:-19px;"> 
 <tbody> 
 <tr> 
 <td style="background-color:#eeeeee;vertical-align:top;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:right;"> 
 1 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 2 
 </div> 
 <div style="margin-left:0;text-align:right;"> 
 3 
 </div> 
 </div> </td> 
 <td style="vertical-align:top;width:696.188px;"> 
 <div style="margin-left:0;text-align:left;"> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 Optional&lt;Employee&gt; optCar = Optional.ofNullable(employee); 
 </div> 
 <div style="margin-left:0;text-align:left;"> 
 &nbsp; 
 </div> 
 </div> </td> 
 </tr> 
 </tbody> 
</table>

--------------------

### **从 Optional 中获取价值** ###

您可以使用 get() 方法从 Optional 中检索值，但它最不安全。如果 value 不存在，那么它将抛出 NoSuchElementException，因此您需要确保在调用 get() 方法之前调用 isPresent() 方法。

--------------------

### **检查可选中的值** ###

您可以使用 isPresent 方法检查 Optional 中是否包含值。

--------------------

### **Optional 中的条件操作** ###

如果值存在于 Optional 中，您可以使用 ifPresent 方法执行操作。  
如下更改 JavaOptionalMain 中的 main 方法

当你运行这个程序时，你会得到下面的输出：

员工姓名：Dummy

员工姓名：John 在列表中找到

正如您在此处看到的，如果列表中存在员工姓名，那么只有我们正在打印员工姓名，否则它不会执行任何操作。

--------------------

### **使用 orElse 在 Optional 中的默认值** ###

如果 Optional 中没有值，可以使用 orElse 方法返回默认值。  
如下更改 JavaOptionalMain 中的 main 方法

当你运行这个程序时，你会得到下面的输出：

员工姓名：Dummy  
员工姓名：Martin

请注意，即使 Optional 中存在值，也会创建默认对象。

--------------------

### **使用 orElseGet 在 Optional 中的默认值** ###

orElseGet 是 orElse 的惰性计数器部分。它将[供应商][Link 1]作为参数，并且仅当 Optional 中不存在 value 时才会调用。  
如下更改 JavaOptionalMain 中的 main 方法

当你运行这个程序时，你会得到下面的输出：

员工姓名：Dummy  
员工姓名：Martin

--------------------

### **从可选中抛出异常** ###

如果 Optional 为空，您可以使用 orElseThrow 抛出异常。它类似于 get() 方法，但在这种情况下，您可以选择抛出任何异常而不是 NoSuchMethodException。  
如下更改 JavaOptionalMain 中的 main 方法

### **在 Java 8 API 中使用 Optional** ###

Java 8 API 中有很多 Optional 的用法。让我举一个例子。  
Stream.findFirst() 方法返回一个带有此流的第一个元素的 Optional，如果流为空，则返回一个空的 Optional。

[这就是Java 8][Java 8]中的 Optional 的全部内容。

学习愉快！！

[NullPointerException]: https://java2blog.com/java-lang-nullpointerexception/
[Java]: https://java2blog.com/core-java-tutorial-for-beginners-experienced/
[Link 1]: https://java2blog.com/java-8-supplier-example/
[Java 8]: https://java2blog.com/java-8-tutorial/