是否可以在不使 class 通用于 return List<T> 的情况下使方法通用

Question

是否可以在不使 class 通用的情况下使方法通用？无论如何使用通用类型可以实现以下目标吗？

下面的代码无法编译，这只是为了展示我想要实现的目标。

public class Test {

    public List<T> execute(String name, Map<String, Object> params,   List<T> type) {
          List<Object> emps = new ArrayList<Object>();
          emps.add(new Employee());
         //emps object will be generated dynamically 
         return (List<T>)emps;
    }

    public static void main(String[] args) {
        Test test = new Test();
        test.execute("testing", null, new ArrayList<Employee>());
    }
}

Answer 1

试试这个：

public <T> List<T> execute(String name, Map<String, Object> params, List<T> type) {
    List<Object> emps = new ArrayList<Object>();
    emps.add(new Employee());
    //emps object will be generated dynamically
    return (List<T>) emps;
}

看看这个解释：

The Java compiler takes advantage of target typing to infer the type parameters of a generic method invocation. The target type of an expression is the data type that the Java compiler expects depending on where the expression appears. Consider the method Collections.emptyList, which is declared as follows:

static <T> List<T> emptyList(); Consider the following assignment statement:

List<String> listOne = Collections.emptyList(); This statement is expecting an instance of List<String>; this data type is the target type. Because the method emptyList returns a value of type List<T>, the compiler infers that the type argument T must be the value String. This works in both Java SE 7 and 8. Alternatively, you could use a type witness and specify the value of T as follows:

List<String> listOne = Collections.<String>emptyList();

https://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html

Answer 2

是的，您可以在非泛型 class 中通过使用 <T> 表示法将其声明为泛型来将方法定义为泛型。

简单地将方法签名更改为：

public <T> List<T> execute(String name, Map<String, Object> params, List<T> type)

并且该方法将是 return 类型为 T 的通用 List 对象。

Answer 3

尝试下面它应该工作

public List<?> execute(String name, Map<String, Object> params,   List<?> type) {

}

是否可以在不使 class 通用于 return List<T> 的情况下使方法通用

Is it possible to make method generic without making class generic to return List<T>

java

generics

generic-list