You should pass the object to get method of the field, so

  import java.lang.reflect.Field;

  Field field = object.getClass().getDeclaredField(fieldName);    
  field.setAccessible(true);
  Object value = field.get(object);

You can use:

String value = (String) this.getClass().getDeclaredField("str").get(this);

Or in a more generalized and safer form:

Field field = anObject.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
String value = (String) field.get(anObject);

And for your example, this should be enough:

String value = this.str; 

But you probably know of that one.

Note: anObject.getClass().getDeclaredField() is potentially unsafe as anObject.getClass() will return the actual class of anObject. See this example:

Object anObject = "Some string";
Class<?> clazz = anObject.getClass();
System.out.println(clazz);

Will print:

class java.lang.String

And not:

class java.lang.Object

So for your code's safety (and to avoid nasty errors when your code grows), you should use the actual class of the object you're trying to extract the field from:

Field field = YourObject.class.getDeclaredField(fieldName);

Something like this...

import java.lang.reflect.Field;

public class Test {
    public static void main(String... args) {
        try {
            Foobar foobar = new Foobar("Peter");
            System.out.println("Name: " + foobar.getName());
            Class<?> clazz = Class.forName("com.csa.mdm.Foobar");
            System.out.println("Class: " + clazz);
            Field field = clazz.getDeclaredField("name");
            field.setAccessible(true);
            String value = (String) field.get(foobar);
            System.out.println("Value: " + value);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

class Foobar {
    private final String name;

    public Foobar(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }
}

Or, you can use the newInstance method of class to get an instance of your object at runtime. You'll still need to set that instance variable first though, otherwise it won't have any value.

E.g.

Class<?> clazz = Class.forName("com.something.Foobar");
Object object = clazz.newInstance();

Or, where it has two parameters in its constructor, String and int for example...

Class<?> clazz = Class.forName("com.something.Foobar");
Constructor<?> constructor = clazz.getConstructor(String.class, int.class);
Object obj = constructor.newInstance("Meaning Of Life", 42);

Or you can interrogate it for its constructors at runtime using clazz.getConstructors()

NB I deliberately omitted the casting of the object created here to the kind expected, as that would defeat the point of the reflection, as you'd already be aware of the class if you do that, which would negate the need for reflection in the first place.