为什么列出 <generic> 在使用Java的mybatis的情况下无法正常工作？

Question

My service code here

public HashMap<String, Object> syncEmployees(Long updatedAt, String userId) {
    HashMap<String, Object> outputMap = new LinkedHashMap<String, Object>();
    **List<String>** outputEmployee = employeeDao.getEmployeeSyncDetails(updatedAt);

        System.out.println("\n\n Size : "+outputEmployee.size()+"\nOutput : "+outputEmployee);

    outputMap.put("employee", outputEmployee);
    return outputMap;
}

Dao code which is return type is list of string List(String)

**List<String>** getEmployeeSyncDetails(@Param("updatedAt") long updatedAt);

.XML mapper and query where method returns list of xyzModel

<resultMap id="xyzmap" type="xyzModel" >
        <result property="userId" column="user_id" />
        <result property="employeeCode" column="employee_code" />
        <result property="designationId" column="designation_id" />

    </resultMap>


<select id="getxyzDetails" resultMap="xyzmap">
    SELECT   
             user_id, ua.employee_code, designation_id
    FROM users
    WHERE  updated_at &gt; #{updatedAt} 
    ORDER BY  updated_at ASC
</select>

And output is like Size : 3 Output : [com.webapp.models. xyzModel @1567524c, com.webapp.models. xyzModel @7744c2cd, com.webapp.models. xyzModel @43515de7]

My question is how its is working? How xyzmodel is getting populated in list of String actually my problem is XML mapper return xyzModel but if I write any other model like UserModel or String or any other model it is not showing any error or execute perfectly with proper output. If we process this list then only he send error java.lang.ClassCastException

Answer 1

In runtime, there are no types for generics.

If you would write the code for getEmployeeSyncDetails yourself, your compiler would warn you about this.

Read here about type erasure in java: https://docs.oracle.com/javase/tutorial/java/generics/erasure.html