The introduction of Java records has provided developers with a new data structure that simplifies the handling and creation of data objects. With the latest addition of record patterns and nested patterns, record handling has become even more efficient and dynamic.

Let's start by reviewing the basics - a record is a lightweight alternative to traditional classes that serves as a transparent carrier for data with built-in accessor methods. Its main purpose is to hold data rather than implement complex behaviors.

However, with the introduction of record patterns in Java 21, the traditional instanceof-and-cast idiom has been simplified. Instead of checking if an object is an instance of a specific record class and then casting it to use its components, we can now use a record pattern to combine both steps. For example, instead of writing this:

if (obj instanceof String) {
    String name = (String)obj;
    ... use name ...
}
In Java 21, we can simply write:

if (obj instanceof String name) {
    ... use name ...
}

This makes the code more concise and less error-prone, as the type pattern String s is used to match against the value of obj and initialize the pattern variable s to the value of obj that has been cast to String.

Furthermore, Java 21 also introduces nested patterns, which allows for the matching and decomposition of more complex object graphs, such as nested records. Let's say we have the following declarations:

Record BankAccount(String accountNumber, String accountHolder, double balance) {}
Record Transaction(String transactionId, double amount, String type, String timestamp) {}
Record BankStatement(List transactions, BankAccount account) {}

We can now use a nested record pattern to calculate the total transaction amount from a BankStatement object as follows:

static double calculateTotalTransactions(BankStatement statement) {
    if (statement instanceof BankStatement(List transactions, BankAccount account)) {
        double total = 0;
        for (Transaction t : transactions) {
            total  = t.amount;
        }
        return total;
    }
    return 0;
}

In this example, the outer pattern BankStatement(List transactions, BankAccount account) contains a nested pattern BankAccount account which is used to extract the account component from the record BankStatement. This allows for more concise and efficient code, as we don't have to handle each individual subpattern match separately.

Another useful feature of record patterns is type inference. When a record pattern names a generic record class without providing any type arguments, the compiler will automatically infer the type arguments when the pattern is used. This simplifies the code, as shown in the following example:

record MyBankAccount(L limitFactor, M money){};

static void recordInference(MyBankAccount bankaccount){
    switch(bankaccount){
        case MyBankAccount(var moneylimit, var amount) ->
            System.out.println("Messages: Inferred that account has balance "   amount   " with a limit of "   moneylimit);
        case MyBankAccount(var moneylimit, null)->
            System.out.println("Messages: Inferred that account has a balance of zero with a limit of "   moneylimit);
        ...
    }
}

This feature is also supported with nested record patterns, making the code even more concise and readable.

Finally, record patterns are fully integrated with switch expressions and statements in Java 21. However, it is important to note that when using record patterns in switch statements, the switch block must be exhaustive. This means that all possible values of the selector expression must be accounted for in the switch block. With record patterns, this is determined by analyzing the types of the patterns used. For example, the case label case Bar b will match values of type Bar and all possible subtypes of Bar.

In conclusion, understanding and mastering record patterns in Java 21 is crucial for anyone preparing for the Java SE 21 Developer Certification. These new features allow for more streamlined and efficient record handling, making it an essential skill for Java developers.