Introspection example to change private field and call private method

Account.java

public class Account {

	// #1 : Safe private field... 
	private long myMoney;

	public Account() {
		this.myMoney = 100;
	}
	
  public long getMoney(){
    return myMoney;
  }
    
  // #2 : Safe private method... 
  private void resetAccount(){
  	myMoney = 0;
  }
    
}

AccountHackUtil.java

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public final class AccountHackUtil {

	// Change a private field
    public static void changeField(Object object,String fieldName,Object newValue) throws IllegalArgumentException, IllegalAccessException {
        Class<?> objectClass = object.getClass();
        Field[] fields = objectClass.getDeclaredFields();
        for (Field field : fields) {
            String localFieldName = field.getName();
            if (fieldName.equals(localFieldName)){
                field.setAccessible(true);
                field.set(object, newValue);
            }

        }
    }
	
    // call a private method
    public static void changeMethod(Object object,String methodName) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException{
        Class<?> objectClass = object.getClass();
        Method[] methods = objectClass.getDeclaredMethods();
        for (Method method : methods) {
            String name = method.getName();
           if (methodName.equals(name)){
        	   method.setAccessible(true);
        	   method.invoke(object);
            }
        }
    }
}

AccountTest.java

import java.lang.reflect.InvocationTargetException;

import org.junit.Assert;
import org.junit.Test;

public class AccountTest {

	@Test
	public void changeAccountMoneyField() throws IllegalArgumentException, IllegalAccessException {
		Account account = new Account();
		Assert.assertEquals(100, account.getMoney());
		AccountHackUtil.changeField(account, "myMoney", 0);
		Assert.assertEquals(0, account.getMoney());
	}

	@Test
	public void callAccountResetMethod() throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {
		Account account = new Account();
		Assert.assertEquals(100, account.getMoney());
		AccountHackUtil.changeMethod(account, "resetAccount");
		Assert.assertEquals(0, account.getMoney());		
	}
	
}