Java - 在另一个 Map 中创建 Map 时出现问题

Question

I'm trying to create a program that keeps track of a bunch of different Strings, and "ties them together" with the current user's entered name (another String), meaning every person should have their own wallets. I tried to do this using a Map inside another Map, but this is where my brain overloads. How do I tie every wallet to the correct name and then display all of that? The comment in my code gives a good example of it. Here is what I have so far:

    Scanner sysin = new Scanner(System.in);
    boolean firstTime = true;
    Map<String, Set<Long>> walletTracker = new HashMap<String, Set<Long>>();
    Map<String, Map<String, Set<Long>>> nameTracker = new HashMap<String, Map<String, Set<Long>>>();
    
    if(!firstTime) {
        /* Here it should display every entered name, wallet and time of deposit, like this:
        
        Jack:
            JacksWallet:
                [12345], [123456], [1234567]
            JacksOtherWallet:
                [123], [1234]
        Jonathan:
            JonsWallet:
                [12345678]
        */
    }
    
    for(int i = 0; i < 1;) {
        System.out.print("Enter your name: ");
        String name = sysin.nextLine();
        System.out.print("Please enter a wallet name: ");
        String wallet = sysin.nextLine();
        
        Set<Long> deposits = walletTracker.getOrDefault(name, new HashSet<>());
        deposits.add(System.currentTimeMillis());
        walletTracker.put(wallet, deposits);
        nameTracker.put(name, walletTracker);
        
        System.out.println("You successfully withdrew money from "+ wallet +". Press enter to continue...");
        firstTime = false;
        String enter = sysin.nextLine();
    }

Answer 1

Set<Long> deposits = walletTracker.getOrDefault(name, new HashSet<>());

This returns new HashSet<>() if there is no mapping for name but it does not add that to the map. What you want is .computeIfAbsent , which will just return the mapping that goes with name , but if it is not there at all, it evaluates your lambda and then adds that to the map and then returns that:

Set<Long> deposits = walletTracker.computeIfAbsent(name, k -> new HashSet<>());

That's 'lambda' syntax - you don't write an expression that resolves to a new hashset, you write an expression that resolves to a function that produces a new hashset when provided with the key (which we don't need here, and is equal to name anyway).

Java-esque strategy here is to make a Wallet class at least. In general, once you start putting <> inside `<>, especially if you're 3 levels deep, stop, and start writing classes instead.

That should be a Map<String, Wallet> and a Map<String, Person> or whatnot.

Answer 2

Here's what I found & noticed:

• The if(!firstTime) {} block should be within the for loop, so that it actually prints on every iteration.
• You attempt to use walletTracker.getOrDefault(name, new HashSet<>()); , but the name variable is not the correct variable to use here. You should be using the wallet input variable.

Here is my "print it out" code, that matches your recommended formatting:

nameTracker.forEach((name, wallet) -> {
    System.out.println(name);
    wallet.forEach((walletName, dates) -> {
        System.out.printf("\t%s\n\t\t%s\n",
            walletName, Arrays.toString(dates.toArray(new Long[0])));
    });
});

Outside of this, the code you used to actually populate the map(s) is correct.

@rzwitserloot Made some good points about using OOP to your advantage, and I would recommend those suggestions as well.