方法类 | Java中的 getGenericReturnType() 方法

Java.lang.reflect.Method 类的getGenericReturnType()方法返回一个表示返回类型的 Type 对象，在编码时在方法中声明。因此，getGenericReturnType() 方法返回方法对象的返回类型。

如果形式返回类型是参数化类型，则为其返回的 Type 对象必须准确反映源代码中使用的实际类型参数。例如，对于方法 public T getValue(){}，如果将类型 T 替换为参数化类型（即 List），则 getGenericReturnType() 将返回“Java.util.List< Java.lang.String>”为返回类型。

句法：

public Type getGenericReturnType()

返回值：它返回一个 Type 对象，表示方法对象的正式返回类型。

异常：此方法抛出以下异常：

• GenericSignatureFormatError – 如果通用方法签名与 JVM 规范中指定的格式不同。
• TypeNotPresentException – 如果返回类型指的是不存在的类型声明。
• MalformedParameterizedTypeException – 如果基础返回类型引用了由于任何原因无法实例化的参数化类型。

例子：

Code:
public class demo{
     public T getValue(){}
}
Explanation:
In the above method when we going to apply getGenericReturnType() method
it is going to return T as a generic return type.

Code:
public class demo{
     public List getValue(){}
}
Explanation:
In the above method when we going to apply getGenericReturnType() method
it is going to return java.util.List as a generic return type

下面的程序说明了 Method 类的 getGenericReturnType() 方法

程序 1：通过在方法上应用 getGenericReturnType() 打印方法对象的返回类型。

Java

// Program to apply getGenericReturnType() method
// of Method Class.
import java.lang.reflect.Method;
import java.lang.reflect.Type;
 
public class GFG {
 
    // Main method
    public static void main(String[] args)
    {
        try {
            // create class object
            Class classobj = demoForReturnParam.class;
 
            // get Method Object
            Method[] methods = classobj.getMethods();
 
            // iterate through methods
            for (Method method : methods) {
 
                // taking only method defined in the demo class
                if (method.getName().equals("setValue")
                    || method.getName().equals("getValue")
                    || method.getName().equals("setManyValues")) {
                    // apply getGenericReturnType() method
                    Type returnParam = method.getGenericReturnType();
 
                    // print return Types of method Object
                    System.out.println("\nMethod Name : "
                                       + method.getName());
 
                    System.out.println("Return Type Details: "
                                       + returnParam);
                }
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}
 
// a simple class
class demoForReturnParam {
 
    // method returning int value
    public int setValue()
    {
        System.out.println("setValue");
        return 24;
    }
 
    // method returning string value
    public String getValue()
    {
        System.out.println("getValue");
        return "getValue";
    }
 
    // method returning nothing
    public void setManyValues(int value1,
                              String value3)
    {
        System.out.println("setManyValues");
    }
}

输出：

Method Name : setManyValues
Return Type Details: void

Method Name : getValue
Return Type Details: class java.lang.String

Method Name : setValue
Return Type Details: int

程序 2：给一个 Return 参数类型作为输入来检查方法对象是否具有相同的返回类型。

Java

// Program to show how to apply
// getGenericReturnType() method of Method Class
import java.lang.reflect.Method;
import java.lang.reflect.Type;
 
public class GFG {
 
    // Main method
    public static void main(String[] args)
    {
        try {
            // create class object
            Class classobj = DemoForReturnParam.class;
 
            Method[] methods = classobj.getMethods();
 
            // check whether setManyValues() method
            // contains string parameter or not
            for (Method method : methods) {
                if (method.getName().equals("setValue")) {
 
                    boolean flag = containsReturnParameter(method,
                                                           (Type)java.lang.String.class);
 
                    System.out.println("setValue()"
                                       + " contains int return type: "
                                       + flag);
                }
            }
 
            // check whether setManyValues() method
            // contains int parameter or not
            for (Method method : methods) {
                if (method.getName().equals("setManyValues")) {
 
                    boolean flag = containsReturnParameter(method,
                                                           (Type) int.class);
 
                    System.out.println("setManyValues()"
                                       + " contains int return type: "
                                       + flag);
                }
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
 
    // check whether method object
    // have same return type or not
    private static boolean
    containsReturnParameter(Method method, Type parameterName)
    {
 
        // get return type using getGenericReturnType()
        Type returnParameter = method.getGenericReturnType();
 
        // check contains return parameter or not
        if (returnParameter == parameterName) {
            return true;
        }
 
        return false;
    }
}
 
// a simple class
class DemoForReturnParam {
 
    // method returning int value
    public void setValue()
    {
        System.out.println("setValue");
    }
 
    // method returning nothing
    public int setManyValues(int value1, String value3)
    {
        System.out.println("setManyValues");
        return 21;
    }
}

输出：

setValue() contains int return type: false
setManyValues() contains int return type: true

参考：
https://docs.oracle.com/javase/8/docs/api/java Java