How to sort a Collection of objects based on 2 properties

Question

I have a class like this:

    static class Transactions{
    private int type;
    private String to;
    private String from;
    private double amount;
    public Transactions (int type, String to, String from, double amount) {
        if(type>=1 || type<=3) {
            this.type=type;
            this.to=to;
            this.from=from;
            this.amount=amount;
        }
        else
            throw new InvalidParamaterException(type);
    }
//This is for deposits and withdrawal
    public Transactions (int type, String para, double amount) {
        
    }
    public int getType() {
        return type;
    }
    public String getTo() {
        return to;
    }
    public String getFrom() {
        return from;
    }
    public double getAmount() {
        return amount;
    }
    
        
    }

static class Bank {
private String Name;
private ArrayList<Customer> customers = new ArrayList<>();
private ArrayList<Company> companies = new ArrayList<>();
private ArrayList<Account> accounts = new ArrayList<>();
private String Address;
public Bank(String Name, String Address) {
    this.Name=Name;
    this.Address=Address;
}
public void processTransactions(Collections ts) {
    Comparator<Transactions> byTypeAndTo = 
            Comparator.comparing(Transactions::getType)
                .thenComparing(Transactions::getTo);
    ts.sort(byTypeAndTo);
}

What I want to do is create a Transactions collection and then sort that collection by type. Type only have values 1, 2, 3, and sorting should happen in that order.

If two transactions have the same type, I want to sort them by String attribute to (that is an account number, so it is all numerical).

How can I sort a collection with two parameters like this?

processTransactions() method should take unsorted collection as a parameter, sort them and process them.

But the last line of code gives an error:

The method sort(List<T>) in the type Collections is not 
applicable for the arguments (Comparator<Assignment02_20190808022.Transactions>)

Answer 1

Use Java 8 methods of the Comparator interface comparing() and thenComparing():

Comparator<Transactions> byTypeAndTo = 
            Comparator.comparing(Transactions::getType)
                .thenComparing(Transactions::getTo);

In order to sort a list of objects, you can apply method sort() on it, available since Java 8:

transactions.sort(byTypeAndTo);

You can play around with this Online demo.

For more information on how to build comparators using Java 8, have a look at this tutorial.

But the last line of code gives an error:

The method signature is incorrect, I believe it should be:

public void processTransactions(Collection<Transactions> ts)

Don't confuse utility class Collections with the Collection interface, and don't omit the generic type parameter.

You can invoke method sort() only on the object of type List (the same with Collections.sort() it expects a list and comparator).

In order to sort the list provided in the guise of Collection you need to apply casting. Since you have a requirement that you method should expect an argument of type Collection there's no other way around it (but it's not good design). That's how it might look like:

public void processTransactions(Collection<Transactions> ts) {
    if (!(ts instanceof List<Transactions>)) {
        throw new IllegalArgumentException();
    }
    List<Transactions> transactions = (List<Transactions>) tr;
    
    transactions.sort(byTypeAndTo);

    // process the `transactions` list
}

Note: it's better to define a comparator inside the Bank class as a public static final field.