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How I can access and add value from the list which is nested in the hashmap and list in Java

I am trying to add value for the List which is stored in HashMap and that has one parent List.
When I try to do so I get "The method get in type is not compatible with the List"
I am trying the following code, logic is :
If I get the matching value of tID in the txnValue List I am just adding the "Values" List otherwise I am creating the new HashMap.
List < HashMap > txnvalues = new ArrayList < HashMap > ();
for (LinkedHashMap < String, Object > linkedHashMap: resultset) {
HashMap data = new HashMap < > ();
HashMap attrData = new HashMap < > ();
List values = new ArrayList < > ();
data.put("values", new ArrayList < > ());
attrData.put("attrID", linkedHashMap.get("ID"));
attrData.put("attrVal", linkedHashMap.get("VAL"));
String txnID = linkedHashMap.get("T_ID").toString();
if (!txnvalues.stream().anyMatch(list -> list.containsValue(txnID))) {
data.put("tID", linkedHashMap.get("T_ID"));
values.add(attrData);
data.put("Values", values);
txnvalues.add(data);
} else {
txnvalues.get("Values").add(attrData); // this Line throws error
}
}
Example :
[{
"tID":123,
"Values":[{attrID:1,attrVal:123}]
}]
//Here If linkedHashmap.get("T_ID") = 123 which matches with tID then I want to add data in the Values
[{
"tID":123,
"Values":[{attrID:1,attrVal:123},{attrID:11,attrVal:467}]
}]
//If it doesn't match then I want to create new Hashmap and update txnValues Like this
[{
"tID":123,
"Values":[{attrID:1,attrVal:123},{attrID:2,attrVal:3435}]
},
{
"tID":456,
"Values":[{attrID:2,attrVal:233}]
}
]

I decided to parameterize all of your various iterables. Below is the parameterized code.
List<HashMap<String, List<HashMap<String, Object>>>> txnvalues = new ArrayList<HashMap<String, List<HashMap<String, Object>>>>();
for (LinkedHashMap<String, Object> linkedHashMap : resultset) {//Error here
HashMap<String, List<HashMap<String, Object>>> data = new HashMap<String, List<HashMap<String, Object>>>();
HashMap<String, Object> attrData = new HashMap<String, Object>();
List<HashMap<String, Object>> values = new ArrayList<HashMap<String, Object>>();
data.put("values", new ArrayList<>());
attrData.put("attrID", linkedHashMap.get("ID"));
attrData.put("attrVal", linkedHashMap.get("VAL"));
String txnID = linkedHashMap.get("T_ID").toString();
if (!txnvalues.stream().anyMatch(list -> list.containsValue(txnID))) {
data.put("tID", linkedHashMap.get("T_ID")); //Error here
values.add(attrData);
data.put("Values", values);
txnvalues.add(data);
} else {
txnvalues.get("Values").add(attrData); //Error here
}
}
First, you have multiple errors in your code such as trying to put a String key and Object value into data, which is a HashMap that only takes a String key and a List(of HashMaps of Strings and Objects) value. Another such is trying to get an item from txnvalues by a String, when txnvalues is a List and therefore requires an integer index parameter.
Second, you have a variable here which is never defined: resultset. We don't know what it is or how it is used, since it's never referenced elsewhere.
Third, there are many many ways to handle nested sets. This >-> List<HashMap<String, List<HashMap<String, Object>>>> is simply horrible.
Please re-write your code in a way that is readable, parameterized, and can properly compile without errors. Just parameterizing will help you keep track of which iterables take which parameters and will help prevent the problem you had when you came here for help.

I'm probably late with this answer. Nevertheless, I'll introduce a possible remedy accompanied by a detailed explanation.
At the first glance, such a deeply nested collection seems contrived and incomprehensible. But problems that you can see in this code aren't something unusual, they could be observed in many questions on StackOverflow, and in many repositories. The only difference is in concentration.
Let's try to examine it closely. A map is a data structure that is commonly misused by beginners because it allows to combine objects of different nature. I am pretty sure that provided code models something more or less tangible. Did you notice that PO tries to access an entry that has a string key called "id"? That's a clear indicator that collections here are used in place of objects.
If I say object graph can be far more complex, it probably wouldn't be something new. But how to reason about the code that is written in such a way?
Let's step aside for a moment and consider the following task:
there are a number of sailboats, you need to determine which of them will win the race and return its name as a result;
input provided as a plain text and consists of the following parameters: unique name, displacement, and weight (only these three for simplicity);
the speed of the vessel depends on its displacement and weight (i.e. formula is provided, we need only parse the values);
It is very likely that somebody can come up with such a solution:
create a Map<String, List<Double>>, where the key is a sailboat's name and the value is a list that contains displacement and weight;
then just iterate over the entry set, apply the formula and so find the fastest vessel.
Only a couple of methods, and it seems that a separate class for a sailboat will allegedly increase the overall complexity and amount of code. That's a common delusion for many students. The creation of a separate class will provide a logical structure to the code and will pay off if you would wish to extend or reuse it. Note that not only attributes of the sailboat must belong to this class but also the methods that allow to compute sailboat's speed and compare sailboats based on it.
Decomposition is a skill and it has to be exercised. And for those of you who didn't realize from the beginning that a sailboat in the previous example has to be represented by an object, I advise to try the next exercise: describe a university, a candy shop, a grocery store, a cat, anything you like but without using objects. First, think about a couple of use-cases that entail accessing some properties of the elements of the system that you're trying to model. Then draw diagrams and write the code using warriors collections and arrays, pay attention that the more complex your system becomes, the more cumbersome become all nested maps and lists, which make you write your code like this:
map.get(something).get(something).add(somethingElse);
And then, when you see the problems, you are ready to implement the classes that make sense in your domain model and compare the two approaches.
Disclaimer: understanding decomposition is a crucial thing but class design is a very broad topic, there are lots of things to study in this area like classic principles and design patterns. But before diving into these topics, you have to have a firm understanding of decomposition and OOP. Without this knowledge even with an object-oriented approach, your solution could become convoluted and difficult to manage. But this is a step in the right direction. The fact alone that you are using an object-oriented language doesn't automatically make your solution object-oriented. It's a skill, and it has to be exercised.
It was a very long digression, now let's get to the point.
As I already said, I'm convinced that the post author had in mind some kind of natural use case. Instead of names that describe the system in this maze of data structures we can see only dump get() and put(). But there's a clue in the usage of map. An id as a key is a clear indicator that it has to be an object which is substituted by a map.
That is a start of a journey, I'll try to provide a scenario that makes sense (at least a bit) and pieces of a system that fits into a structure depicted in the scheme provided at the start of this post.
Let's consider an organization that sells something (I'm not trying to guess what was the author's intention, but providing a use case that will allow to reason about the code). There are a bunch of departments, each with a unique identifier.
Each department has a collection of products that it sells. Department gets different products from different suppliers. And in turn, each product has a unique id a collection of suppliers represented by plain string (it looks contrived, but keep in mind it's just an illustration of what the code does).
As a use-case, let's assume that the company launches a new product and it must be accessible in all its departments. The code checks whether the department has this product already, if not, the product will be added with a default set of suppliers, otherwise it merges the existing set of suppliers and the default one.
As you can see the code in the main method is very concise. Note that all the miscellanies of data structures are still there, but we are not accessing them directly. As the information expert principle suggests, this logic is hidden inside the objects. That makes this solution reusable and less error-prone.
public static void main(String[] args) {
// this list is a rough equivalent of the "List<Map<String, List<Map<String, Object>>>> txnvalues"
List<Department> departments =
List.of(new Department("dep11"), new Department("dep12"));
Product newProd = new Product("id123"); // a NEW Product with id = "id123"
newProd.addAllSuppliers(List.of("supplierA", "supplierB"));
for (Department dep: departments) { // launching the new Product
dep.mergeProduct(newProd);
}
}
public class Department {
private final String departmentId;
private final Map<String, Product> idToProduct;
public Department(String departmentName) {
this.departmentId = departmentName;
this.idToProduct = new HashMap<>();
}
public void mergeProduct(Product prod) {
idToProduct.merge(prod.getId(), prod, Product::merge);
}
public void mergeAllProducts(Iterable<Product> products) {
for (Product prod: products) {
mergeProduct(prod);
}
}
public void addProduct(Product prod) {
idToProduct.put(prod.getId(), prod);
}
public void addAllProducts(Iterable<Product> products) {
for (Product prod: products) {
addProduct(prod);
}
}
public String getId() {
return departmentId;
}
public Map<String, Product> getIdToProduct() {
return Collections.unmodifiableMap(idToProduct);
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o instanceof Department other) {
return departmentId.equals(other.departmentId);
} else return false;
}
#Override
public int hashCode() {
return Objects.hash(departmentId);
}
}
public class Product {
private final String productId;
private final Set<String> suppliers;
public Product(String id) {
this.productId = id;
this.suppliers = new HashSet<>();
}
public boolean addSupplier(String newSup) {
return suppliers.add(newSup);
}
public boolean addAllSuppliers(Collection<String> newSup) {
return suppliers.addAll(newSup);
}
public Product merge(Product other) {
if (!this.equals(other)) throw new IllegalArgumentException();
Product merged = new Product(productId);
merged.addAllSuppliers(this.suppliers);
merged.addAllSuppliers(other.suppliers);
return merged;
}
public String getId() {
return productId;
}
public Set<String> getSuppliers() {
return Collections.unmodifiableSet(suppliers);
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o instanceof Product other) {
return this.productId.equals(other.productId);
} else return false;
}
#Override
public int hashCode() {
return Objects.hash(productId);
}
}
Further steps:
First of all make sure that you don't have gaps in the core concepts of OOP: encapsulation, inheritance, and polymorphism.
Draw before you start to code, it's not necessary to create a full-blown UML diagram. Even a rough set of named boxes with arrows will help you understand better how your system is structured and how its parts interact with each other.
Read and apply. Extend your knowledge gradually and try to apply it. High cohesion, Low coupling, SOLID, and lots of helpful reading can be found here, for instance this recent post
Write a bit, test a bit: don't wait until your code became a beast. Write a bit and give it a try, add something else and take a look at how these parts fit together.

In the else block, you call get method of txnvalues which a list of HashMaps and thus it expects an integer index. I believe you assume that at this point you've got a reference to the HashMap to which you would add the values. But you don't.
So, you need to find the index where to add the values, which means you have to look through the txnvalues list again.
For this reason, you should use a different approach:
txnvalues.stream()
.filter(m -> m.get("tID").equals(txnID))
.findFirst()
.ifPresentOrElse(
m -> m.get("Values").add(attrData),
() -> {
HashMap data = new HashMap<>();
// Other stuff to fill the data
txnvalues.add(data);
}
);
Here .filter(m -> m.get("tID").equals(txnID)) corresponds to your .anyMatch(list -> list.containsValue(txnID)) (the parameter list is actually instance of HashMap).
I changed the condition: according to your data sample, you looking for Map which has txnID value for the "tID" key, therefore getting the value of this key is faster than looking through all the values in the HashMap. (It may return null.)
So filter will return only the entries which contain match the required value of the "tID" key. Then .findFirst() “returns” the reference to that HashMap. Now .ifPresentOrElse performs the actions you want:
m.get("Values").add(attrData) into the list; this corresponds your one line of code in the else block;
the other code is what you had in the if block: if nothing is found, create the new instance.

Creating a loose relationship between multiple objects that others can search on

I have a list of car manufacturers, models, and trims. Each of those objects has different attributes on them. Ex. car makers can have import vs domestic, sport vs luxery, etc. My current setup looks something like below,
public Manufacturer {
private String manufacturerName; //Getter&Setter as well
private List<Model> modelList; //Getter&Setter as well
//additional attributes
}
public Model {
private String modelName;
private List<Trim> trimList; //Getter&Setter as well
//additional attributes
}
public Trim {
private String trimType; //Getter&Setter as well
//additional attributes
}
public ContainerClass {
public List<Manufacturer> manufacturerList;
}
Now I can create the objects as Mazda, 3, Grand Touring and associate the objects in the list. But if someone comes along and just wants a car that has a sunroof, it feels like a waste to have to dig into every possible manufacturer and every model for that manufacturer to see if the trim has a sunroof attribute. Are there any strategies I could use to make this easier for developers, including myself?
Note: I am at a point where I don't have a database because I don't have real data to fill the database, this is a stop gap until I get that data later, so please don't just say "create a database" :). Also I am loading this information from an ExamplesTable object of JBehave.

You can filter the manufacturers by some sub-streams:
container.getManufacturerList().stream()
.filter(manufacturer ->
manufacturer.getModelList().stream().anyMatch(model ->
model.getTrimList().stream().anyMatch(trim ->
trim.getTrimType().equals("sunroof"))))
.collect(Collectors.toList());

How can I replace object in java collection?

I am trying to replace element in collection with new modified version. Below is short code that aims to demonstrate what I'd like to achieve.
The whole idea is that I have one object that consists of collections of other objects. At some point in time I am expecting that this objects in collections (in my example phones) might require some modifications and I'd like to modify the code in one place only.
I know that in order to update the object's attributes I can use setters while iterating through the collection as demonstrated below. But maybe there is better, more general way to achieve that.
public class Customer {
private int id;
private Collection<Phone> phoneCollection;
public Customer() {
phoneCollection = new ArrayList<>();
}
//getters and setters
}
and Phone class
public class Phone {
private int id;
private String number;
private String name;
//getters and setters
}
and
public static void main(String[] args) {
Customer c = new Customer();
c.addPhone(new Phone(1, "12345", "aaa"));
c.addPhone(new Phone(2, "34567", "bbb"));
System.out.println(c);
Phone p = new Phone(2, "9999999", "new name");
Collection<Phone> col = c.getPhoneCollection();
for (Phone phone : col) {
if (phone.getId() == p.getId()) {
// This is working fine
// phone.setNumber(p.getNumber());
// phone.setName(p.getName());
// But I'd like to replace whole object if possible and this is not working, at least not that way
phone = p;
}
}
System.out.println(c);
}
}
Is this possible to achieve what I want?
I tried copy constructor idea and other methods I found searching the net but none of them was working like I would expect.
EDIT 1
After reading some comments I got an idea
I added the following method to my Phone class
public static void replace(Phone org, Phone dst){
org.setName(dst.getName());
org.setNumber(dst.getNumber());
}
and now my foreach part looks like that
for (Phone phone : col) {
if (phone.getId() == p.getId()) {
Phone.replace(phone, p);
}
}
And it does the job.
Now if I change the Phone class attributes I only need to change that method. Do you think it is OK solving the issue that way?

You should not modify the collection while you're iterating through it; that's likely to earn you a ConcurrentModificationException. You can scan the collection for the first object that matches your search criterion. Then you can exit the loop, remove the old object, and add the new one.
Collection<Phone> col = c.getPhoneCollection();
Phone original = null;
for (Phone phone : col) {
if (phone.getId() == p.getId()) {
original = phone;
break;
}
}
if (original != null) {
Phone replacement = new Phone(original);
replacement.setNumber(p.getNumber());
replacement.setName(p.getName());
col.remove(original);
col.add(replacement);
}
Alternatively, you could declare a more specific type of collection, such as a List, that would allow you to work with indexes, which would make the replacement step much more efficient.
If your phone IDs are unique to each phone, you should consider using a Map<Integer, Phone> that maps each phone ID to the corresponding phone. (Alternatively, you could use some sort of third-party sparse array structure that doesn't involve boxing each ID into an Integer.) Of course, if your IDs aren't unique, then you might want to modify the above to gather a secondary collection of all matching phones (and reconsider the logic of your existing code as well).

You can also use a Set (HashSet), this is only when you don't want to do the way Mike suggested.
Use the Phone as an item in the set. Don't forget to implement hashCode() and equals() in Phone. hashCode() should return the id, as it is supposed to be unique.
Since you are concerned about replacing the item, here's how HashSet will help you :
Create an instance of your object.
Remove the object you want to replace from the set.
Add the new object (you created in step 1) back to the set.
Both these operations 2 & 3 are guaranteed in O(1) / constant time.
You don't need to maintain a map for this problem, that's redundant.
If you want to get the object from the collection itself and then modify it, then HashMap would be better, search is guaranteed in O(1) time.

Instead of a list, use a map with the Phone's id as the key. Then your code looks like this:
public static void main(String[] args) {
Customer c = new Customer();
c.addPhone(new Phone(1, "12345", "aaa"));
c.addPhone(new Phone(2, "34567", "bbb"));
System.out.println(c);
Phone p = new Phone(2, "9999999", "new name");
Map<Integer, Phone> phoneMap = c.getPhoneMap();
phoneMap.put(p.getId(), p);
System.out.println(c);
}

If you take the object out from the collection and update its properties, it will get reflected in the same object in collection too.. Hence, you dont have to technically replace object after updating it.
As "Mike M." pointed out, you can use hashmap to retrieve the object quickly without iteration and update the object values.

If order matters to you, you can change Collection to List (Since you're always using an ArrayList anyway) and then:
int index = col.indexOf(phone);
col.remove(phone);
col.add(p, index);

Java: creating several instances of object in class itself or how to restructure

I'm a java beginner and have a question concerning how to best structure a cooking program.
I have a class called Ingredient, this class currently looks like this:
public class Ingredient {
private String identifier;
private double ingredientFactor;
private String titleInterface;
public Ingredient(String identifier, double ingredientFactor,String titleInterface) {
this.identifier = identifier;
this.ingredientFactor = ingredientFactor;
this.titleInterface = titleInterface;
}
I want to initialize several objects (about 40) with certain values as instance variables and save them in a Map, for example
Map<String, Ingredient> allIngredients = new HashMap<String, Ingredient>();
allIngredients.put("Almonds (ground)", new Ingredient("Almonds (ground)", 0.7185, "Almonds (ground)");
Later on I want to retrieve all these objects in the form of a Map/HashMap in a different class.
I'm not sure how to proceed best, initialize all these objects in the Ingredient class itself or provide a method that initializes it or would it be better to create an super class (AllIngredients or something like that?) that has a Map with Ingredients as instance variables?
Happy for any suggestions, thanks in advance :)

Please do not initialize all these objects in the Ingredient class itself. That would be a bad practice for oops.
Just think your class is a template from which you create copies(objects) with different values for attributes. In real world if your class represent model for a toy plane which you would use to create multiple toy planes but each bearing different name and color then think how such a system would be designed. You will have a model(class). Then a system(another class) for getting required color and name from different selection of colors and names present(like in database,files,property file ) etc.
Regarding your situation .
If predetermined values store the values in a text file,properties file,database,constants in class etc depending on the sensitivity of the data.
Create Ingredient class with constructors
Create a class which will have methods to initialize Ingredient class using predetermined values,update the values if required,save the values to text file -database etc and in your case return as map .
Also check the links below
http://www.tutorialspoint.com/design_pattern/data_access_object_pattern.htm
http://www.oracle.com/technetwork/java/dataaccessobject-138824.html

Sounds to me like you are looking for a static Map.
public class Ingredient {
private String identifier;
private double ingredientFactor;
private String titleInterface;
public Ingredient(String identifier, double ingredientFactor, String titleInterface) {
this.identifier = identifier;
this.ingredientFactor = ingredientFactor;
this.titleInterface = titleInterface;
}
static Map<String, Ingredient> allIngredients = new HashMap<String, Ingredient>();
static {
// Build my main set.
allIngredients.put("Almonds (ground)", new Ingredient("Almonds (ground)", 0.7185, "Almonds (ground)"));
}
}

Benefits of factoring a new class vs string values for elements in a Set

This is more of a design question with implications for code simplicity vs. performance.
Lets say you want to make sure a set of values for a given user id are the same between two systems. The example here is to check that a student id has the same number of course enrollments in System A and System B.
For this we create:
List<String> studentList = new ArrayList<String>();
Set<String> sysAEnrollments = new HashSet<String>();
Set<String> sysBEnrollments = new HashSet<String>();
private Map<String, String> badEnrollList = new HashMap<String, String>();
And fill them appropriately, given a list of student ids(studentList):
studentList = getCurrentStudentList();
for (String id : studentList){
sysAEnrollments = getSysAEnrollments(id);
sysBEnrollments = getSysBEnrollments(id);
if (!sysAEnrollments.containsAll(sysBEnrollments)){
badEnrollList.put(id, getBadEnrollmentsById(id, sysAEnrollments, sysBEnrollments));
}
}
Question: What should the method 'getBadEnrollmentsById' return?
Either a concatenated string with enough meaning so it can just be printed out.
Or have a new object, for example another collection with the list of course ids that could be used for further processing but harder to use for printed output.
Is it worth designing thoroughly all expected objects or replace some of them with concatenated strings for clarity and performance?
NOTES:
System A is preferred as the authoritative source
Output from getBadEnrollmentsById should have all courses and flag those missing in system B.
PROPOSED SOLUTION: (2012-SEP-14)
EDIT (2012-SEP-17): Updated the Course class to include hashCode and equals
As suggested by user351721 I continued modelling the remaining objects that match the expected results/requirements.
Slight changes made a big difference and allowed me to go over this design flaw and finish with the implementation.
The revised collections are:
List<String> studentList = new ArrayList<String>();
Enrollment sysAEnrollments;
Enrollment sysBEnrollments;
Map<String, List<String>> badEnrollList = new HashMap<String, List<String>>();
And we populate the Enrollments:
for (String id : studentList){
sysAEnrollments = getSysAEnrollments(id);
sysBEnrollments = getSysBEnrollments(id);
if (!sysAEnrollments.getCourses().containsAll(sysBEnrollments.getCourses())){
List<String> missingCourses = getProblemEnrollmentListById(id, sysAEnrollments, sysBEnrollments);
badEnrollList.put(id, missingCourses);
}
}
So for now the output can be printed from badEnrollList by getting at each ArrayList and printing the course names. A course name with a * will mean that it's missing in sysB.
The Enrollment class looks like this:
public class Enrollment {
private Set<Course> courses = new HashSet<Course>();
public void setCourses(Set<Course> courses){
this.courses = courses;
}
public Set<Course> getCourses(){
return this.courses;
}
}
And the Course class ended up like this:
public class Course {
private String id;
private String name;
public String getId() {
return id;
}
public void setId(final String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
// Must override hashCode() and equals()
#Override
public boolean equals(Object o){
if (o == this)
return true;
if (!(o instanceof Course))
return false;
Course c = (Course) o;
return c.id.equals(this.id) && c.name.equals(this.name);
}
#Override
public int hashCode(){
// Magic numbers as shown on Joshua Bloch's book "Effective Java" 2nd Edition, p.48
int result = 17;
result = 31 * this.id.hashCode();
result = 31 * this.name.hashCode();
return result;
}
}
The changes might look subtle but the important clue is that Enrollments are not a collection of strings, Enrollments are a collection of Courses AND each Course has a name and a availability property. They don't seem to do much but by using them I am defining the objects that I'm working with and documenting how these classes can be reused in the future.

"Growing Object-Oriented Software, Guided by Tests" addresses this question: chapter 7, "Value Types". Worth reading. An excerpt:
The more code we write, the more we’re convinced that we should define types to represent value concepts in the domain, even if they don’t do much. It helps to create a consistent domain model that is more self-explanatory. If we create, for example, an Item type in a system, instead of just using String, we can f ind all the code that’s relevant for a change without having to chase through the method calls

concatenated strings
would mean you have to define a pattern and corresponding set of valid strings and implement validation and translation to entity classes. Providing an interface or class would make it easier to update your code in a year or so, not to mention other programmers that might work with your application. Why not store student, enrollment or course objects in badEnrollList? How do these objects look like and what do you want to do with them?
In general: Yes, designing thoroughly all expected objects is worth it.

I feel that a collection, such as List<String> would be a desirable return value. This allows you to more efficiently capture multiple discrepancies between the two sets, and process the missing courses in your second object more intuitively. Printing the list wouldn't be that hard, either - depending on how you wished to convey the information.
It's also worth mentioning that the .equals() method for Set is a cleaner and more intuitive way to ensure equivalence between two sets.

Instead of using all these sets and maps, I'd use Plain Old Java Objects (POJOs) that reflect the actual business objects in question. From what you've indicated, you have Students who have an id of some sort, and who are enrolled in classes on System A and on System B. I would build up a set of Student objects defined like so:
public class Student {
private String id;
private List<String> enrollmentsA;
private List<String> enrollmentsB;
// appropriate getters and setters
}
Depending on if you want to do anything else with Classes, it may even be preferable to create some form of EnrolledClass object to represent that too.
Within the students class, I'd then have a method that would determine the "bad" enrollments. If all that you want to do with this data is generate an email message, it may even be as simple as a String:
public String getBadEnrollmentsMessage() {
List<String> enrolledBoth = getCommonEnrollments();
List<String> enrolledOnlyA = getAOnlyEnrollments();
List<String> enrolledOnlyB = getBOnlyEnrollments();
StringBuilder output;
// format the contents of the above lists into output
// format should be however you want it in the email.
return output.toString();
}
Then you could have a map of Students to email enrollments messages:
HashMap<Student, String> studentEmails;
for (Student s : allStudents) {
studentEmails.put(s, s.getBadEnrollmentsMessage());
}
Of course, if you have a method like getBadEnrollmentsMessage(), I'm not even sure you need the Map of students and strings in the first place. Frankly you could just create a sendEnrollmentEmail method, pass in a Student, and extract the message via getBadEnrollmentsMessage() right there.