Custom Errors in Karel

Custom error types in Java are typically implemented by extending the Exception class. Here’s an example that demonstrates a custom exception type and its usage:

import java.util.function.Function;

// A custom exception type with the suffix "Exception"
class ArgException extends Exception {
    private int arg;

    public ArgException(int arg, String message) {
        super(message);
        this.arg = arg;
    }

    public int getArg() {
        return arg;
    }

    @Override
    public String toString() {
        return arg + " - " + getMessage();
    }
}

public class CustomExceptions {
    public static int f(int arg) throws ArgException {
        if (arg == 42) {
            // Throw our custom exception
            throw new ArgException(arg, "can't work with it");
        }
        return arg + 3;
    }

    public static void main(String[] args) {
        try {
            f(42);
        } catch (Exception e) {
            // In Java, we use instanceof to check the exception type
            if (e instanceof ArgException) {
                ArgException ae = (ArgException) e;
                System.out.println(ae.getArg());
                System.out.println(ae.getMessage());
            } else {
                System.out.println("Exception doesn't match ArgException");
            }
        }
    }
}

In this Java example, we define a custom exception ArgException that extends the built-in Exception class. This is analogous to implementing the error interface in Go.

The f method throws our custom exception when the input is 42, similar to the Go example.

In the main method, we use a try-catch block to handle exceptions. We use instanceof to check if the caught exception is of type ArgException. This is similar to using errors.As in Go, but Java’s exception handling is more straightforward in this regard.

If we run this program, it will output:

42
can't work with it

This demonstrates how to create and use custom exceptions in Java, which serves a similar purpose to custom errors in Go.

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Custom Errors in Karel