Using reflection to get the description of a class.

https://github.com/mtumilowicz/java-reflection

We will show how to obtain basic info about class (such as modifiers, implemented interfaces, etc.).

1. We have simple class hierarchy in the family module:
   • module-info

     module family {
     }
     

   • parent

     public class Parent implements ParentInterface {
     }
     
     public interface ParentInterface {
     }
     

   • child

     public abstract class Child<T> extends Parent implements ChildInterface, Serializable {
     }
     
     public interface ParentInterface {
     }
     

All tests are in reflection.ClassReflectionTest class:

• modifiers

  assertEquals(Modifier.toString(childClass.getModifiers() & Modifier.classModifiers()), "public abstract");
  

  • int getModifiers() - returns modifiers as int (https://github.com/mtumilowicz/java11-ORed-container)
  • int classModifiers() - returns possible modifiers for class
    note, that we have other similar methods:
    • interfaceModifiers()
    • constructorModifiers()
    • methodModifiers()
    • fieldModifiers()
    • parameterModifiers() (since java 8)
  • Modifier.toString(int mod) converts from int to string representation

• all modifiers at once (since java 8)

  assertEquals(childClass.toGenericString(), "public abstract class child.Child<T>");
  

• name

  assertEquals(childClass.getName(), "child.Child");
  

• simple name

  assertEquals(childClass.getSimpleName(), "Child");
  

• superclass

  assertEquals(childClass.getSuperclass(), Parent.class);
  

  • if there is no explicit superclasses and class differs from Object.class

    assertEquals(Parent.class.getSuperclass(), Object.class);
    

  • superclass of Object.class is null

    assertNull(Object.class.getSuperclass());
    

• interfaces

  • only interfaces explicitly declared (inherited are not returned)

    assertArrayEquals(childClass.getInterfaces(), new Class<?>[]{ChildInterface.class, Serializable.class});
    

  • if there is no interfaces, the array is empty

    assertArrayEquals(Object.class.getInterfaces(), new Class<?>[]{});
    

• type parameters

  assertEquals(Arrays.toString(childClass.getTypeParameters()), "[T]");
  

  • if there is no type parameters, the array is empty

    var typeParameters = Object.class.getTypeParameters();
    assertEquals(typeParameters.length, 0);
    

• package name

  assertEquals(childClass.getPackageName(), "child");
  

• module name

  • in tests:

    assertTrue(childClass.getModule().toString().contains("unnamed module"));
    assertNull(childClass.getModule().getName());
    

  • however in main:

    public static void main(String[] args) {
            System.out.println(Child.class.getModule().getName());
    }
    

    will print: family