Java Method類 getGenericParameterTypes()用法及代碼示例

Method類的java.lang.reflect.Method.getGenericParameterTypes()方法返回表示參數類型的Type對象數組，這些對象類型在編碼時在method中聲明。這意味著getGenericParameterTypes()方法返回屬於方法對象的參數數組。如果方法對象不帶參數，則返回長度為0的數組。

如果形式參數類型是參數化類型，則為其返回的Type對象必須準確反映源代碼中使用的實際類型參數。例如，對於方法public void getValue(T value){}，將類型參數T替換為參數化類型(即List)，然後方法將返回“java.util.List”作為參數類型。

用法:

public Type[] getGenericParameterTypes()

返回值：此方法以聲明順序返回表示方法對象的形式參數類型的Types數組。

異常：此方法引發以下異常：

• GenericSignatureFormatError-如果通用方法簽名與JVM規範中指定的格式不同。
• TypeNotPresentException-如果參數類型引用了不存在的類型聲明。
• MalformedParameterizedTypeException-如果基礎參數類型引用由於某種原因而無法實例化的參數化類型。

以下示例程序旨在說明Method類的getGenericParameterTypes()方法：

程序1:打印為方法聲明的所有參數類型

// Program Demonstrate how to apply getGenericParameterTypes() 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 = demoClass.class; 
  
            // get Method Object 
            Method[] methods = classobj.getMethods(); 
  
            // iterate through methods 
            for (Method method:methods) { 
  
                // only taking method defined in the demo class 
                if (method.getName().equals("setValue") 
                    || method.getName().equals("getValue") 
                    || method.getName().equals("setManyValues")) { 
  
                    // apply getGenericParameterTypes() method 
                    Type[] parameters = method.getGenericParameterTypes(); 
  
                    // print parameter Types of method Object 
                    System.out.println("\nMethod Name:"
                                       + method.getName()); 
                    System.out.println("No of Parameters:"
                                       + parameters.length); 
                    System.out.println("Parameter object details:"); 
                    for (Type type:parameters) { 
  
                        System.out.println(type.getTypeName()); 
                    } 
                } 
            } 
        } 
        catch (Exception e) { 
            e.printStackTrace(); 
        } 
    } 
} 
// a simple class 
class demoClass { 
  
    // method containing two parameter 
    public void setValue(String value1, String value2) 
    { 
        System.out.println("setValue"); 
    } 
  
    // method containing no parameter 
    public String getValue() 
    { 
        System.out.println("getValue"); 
        return "getValue"; 
    } 
  
    // method containg many parameter 
    public void setManyValues(int value1, 
                              double value2, 
                              String value3) 
    { 
        System.out.println("setManyValues"); 
    } 
}

輸出：

Method Name:setManyValues
No of Parameters:3
Parameter object details:
int
double
java.lang.String

Method Name:getValue
No of Parameters:0
Parameter object details:

Method Name:setValue
No of Parameters:2
Parameter object details:
java.lang.String
java.lang.String

程序2:檢查Method對象是否包含參數

// Program Demonstrate how to apply getGenericParameterTypes() 
// 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 = sample.class; 
  
            Method[] methods = classobj.getMethods(); 
  
            /*check whether setManyValues() method contains  
                         int parameter or not 
                and print no of string parameter it contains*/
  
            for (Method method:methods) { 
                if (method.getName().equals("setValue")) { 
  
                    int count = containsParameter( 
                        method, 
                        (Type)java.lang.String.class); 
                    System.out.println("No of String"
                                       + " Parameters in setValue():"
                                       + count); 
                } 
            } 
  
            // check whether setManyValues() method 
            // contains int parameter or not 
            // and print no of string parameter it contains 
  
            for (Method method:methods) { 
  
                if (method.getName().equals("setManyValues")) { 
  
                    int count = containsParameter(method, 
                                                  (Type) int.class); 
  
                    System.out.println("No of int Parameters"
                                       + " in setManyValues():"
                                       + count); 
                } 
            } 
        } 
        catch (Exception e) { 
            e.printStackTrace(); 
        } 
    } 
  
    // count no of parameters contain by 
    // method same as passed to method 
    private static int
    containsParameter(Method method, 
                      Type parameterName) 
    { 
        int count = 0; 
  
        // get all parameter types list 
        // using getGenericParameterTypes() 
        Type parameters[] = method.getGenericParameterTypes(); 
  
        for (int i = 0; i < parameters.length; i++) { 
            // check contains parameter or not 
            if (parameters[i] == parameterName) { 
                count++; 
            } 
        } 
  
        return count; 
    } 
} 
// a simple class 
class sample { 
  
    // method containing two parameter 
    public void setValue(String value1, 
                         String value2) 
    { 
        System.out.println("setValue"); 
    } 
  
    // method containing many parameter 
    public void setManyValues(int value1, 
                              double value2, 
                              String value3) 
    { 
        System.out.println("setManyValues"); 
    } 
}

輸出：

No of String Parameters in setValue():2
No of int Parameters in setManyValues():1

參考： https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Method.html#getGenericParameterTypes-