Sometimes you are writing a test class and want to check that the class you are testing is being changed the way you expect.  When a class has private menbers that you don't want to open up to anyone,  you can use reflection to see them.  This is a simple example. 

import java.lang.reflect.Field;

class ExampleClass {
    private String cantTouchThis = "Secret Values"; // private field
}

public class PrivateMemberAccessTest {
    public static void main(String[] args) throws Exception {
        ExampleClass example = new ExampleClass();
        Class c = example.getClass();

        // get the reflected object 
        Field field = c.getDeclaredField("cantTouchThis");

        // set accessible true 
        field.setAccessible(true);
        System.out.println("Value of cantTouchThis: " + field.get(example)); 

        // modify the member varaible
        field.set(keyPair, "Yes I can");
        System.out.println("Value of cantTouchThis: " + field.get(example));     }
}