Getting inner class field values using reflection

I am trying to access inner class field values using reflection.

Suppose the classes are:-

public class OuterClass  {
    int a;
    public class InnerClass {
        int b;
        InnerClass(int b){
            this.b = b;
       }
    }

    public void setInnerField(int val){
        InnerClass a = new InnerClass(val);
    }
}

And the main function has:-

OuterClass obj = new OuterClass();
obj.a = 10;
obj.setInnerField(5);

Following is what I am trying to get the inner class field value i.e. b Referred from How i can access inner class variable value using reflection in java?

Class[] classes = obj.getClass().getDeclaredClasses();     
for(Class innerClass: classes){
    //System.out.println(innerClass.getName());
    Field[] fields = innerClass.getDeclaredFields();
    for(Field field : fields){
        //System.out.println(field.getName());
        System.out.println(field.get(obj));  //<-----IllegalArgumentException    
//Tried with below line too as suggested in above link                                                                     
System.out.println(field.get(innerClass.getInterfaces())); //<-----IllegalArgumentException
    }           
}

Then what can be the ways to access the inner class field values at runtime?