Method Class | getGenericParameterTypes() method in Java

The java.lang.reflect.Method.getGenericParameterTypes() method of Method class returns an array of Type objects that represent the parameter types, declared in method at time of coding. It means that the getGenericParameterTypes() method returns array of parameters that belongs to method object. It returns an array of length 0 if the method object takes no parameters.

If a formal parameter type is a parameterized type, the Type object returned for it must accurately reflect the actual type parameters used in the source code. e.g for method public void getValue(T value){} substitute a type parameter T with a parameterized type (i.e., List) then method will return “java.util.List” as parameter type.

Syntax:

public Type[] getGenericParameterTypes()

Return Value: This method returns an array of Types that represent the formal parameter types of the method object, in declaration order.

Exception: This method throws following exceptions:

• GenericSignatureFormatError – if the generic method signature is not same as the format specified in The JVM Specification.
• TypeNotPresentException – if parameter types refers to a non-existent type declaration.
• MalformedParameterizedTypeException – if the underlying parameter types refers to a parameterized type that cannot be instantiated for any reason.

Below programs illustrates getGenericParameterTypes() method of Method class:

Program 1: Print all Parameter type declared for Method

// 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 containing many parameter
    public void setManyValues(int value1,
                              double value2,
                              String value3)
    {
        System.out.println("setManyValues");
    }
}
Output:

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

Program 2: Check if the Method object contains parameter or not

// 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");
    }
}
Output:

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

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