Set private field value with reflection

I have 2 classes: Father and Child

public class Father implements Serializable, JSONInterface {

    private String a_field;

    //setter and getter here

}

public class Child extends Father {
    //empty class
}

With reflection I want to set a_field in Child class:

Class<?> clazz = Class.forName("Child");
Object cc = clazz.newInstance();

Field f1 = cc.getClass().getField("a_field");
f1.set(cc, "reflecting on life");
String str1 = (String) f1.get(cc.getClass());
System.out.println("field: " + str1);

but I have an exception:

Exception in thread "main" java.lang.NoSuchFieldException: a_field

But if I try:

Child child = new Child();
child.setA_field("123");

it works.

Using setter method I have same problem:

method = cc.getClass().getMethod("setA_field");
method.invoke(cc, new Object[] { "aaaaaaaaaaaaaa" });