Defer in Mercury

import java.io.*;

public class DeferExample {
    public static void main(String[] args) {
        // Suppose we wanted to create a file, write to it,
        // and then close when we're done. Here's how we could
        // do that with try-with-resources in Java.
        try (FileWriter f = createFile("/tmp/defer.txt")) {
            writeFile(f);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static FileWriter createFile(String path) throws IOException {
        System.out.println("creating");
        return new FileWriter(path);
    }

    private static void writeFile(FileWriter f) throws IOException {
        System.out.println("writing");
        f.write("data\n");
    }

    // The closing of the file is handled automatically by the try-with-resources statement
}

In Java, we don’t have a direct equivalent to Go’s defer keyword. However, we can achieve similar functionality using the try-with-resources statement, which was introduced in Java 7. This feature ensures that each resource is closed at the end of the statement.

In the example above, we’ve created a FileWriter in the try statement. Java will automatically close this resource when the try block is exited, either normally or due to an exception.

The createFile method creates a new FileWriter and returns it. If there’s an error creating the file, it will throw an IOException.

The writeFile method writes some data to the file. Again, if there’s an error writing to the file, it will throw an IOException.

In Java, we don’t need to explicitly close the file as we do in Go, because the try-with-resources statement handles this for us. It’s equivalent to Go’s defer in this context, ensuring that the file is closed after we’re done with it, even if an exception occurs.

Running the program would produce output similar to this:

creating
writing

The file is automatically closed after the try block is exited, which is similar to the behavior of defer in Go.

It’s worth noting that while try-with-resources is a powerful feature in Java for resource management, it’s not as flexible as Go’s defer. defer in Go can be used for any function calls, not just resource cleanup. For more general deferred execution in Java, you might need to use finally blocks or create your own utility methods.