Custom Exception Hierarchy

// Base — extend RuntimeException for unchecked (most modern Java code)
public class AppException extends RuntimeException {
    public AppException(String message) { super(message); }
    public AppException(String message, Throwable cause) { super(message, cause); }
}

public class NotFoundException extends AppException {
    private final String resource;
    private final String id;

    public NotFoundException(String resource, String id) {
        super(resource + " not found: " + id);
        this.resource = resource;
        this.id = id;
    }
    public String resource() { return resource; }
    public String id()       { return id; }
}

public class ValidationException extends AppException {
    private final String field;
    public ValidationException(String field, String message) {
        super("invalid " + field + ": " + message);
        this.field = field;
    }
    public String field() { return field; }
}

// Caller can match by type
class Service {
    void handle() {
        try {
            // ... do work ...
        } catch (NotFoundException nfe) {
            System.out.printf("404 %s/%s%n", nfe.resource(), nfe.id());
        } catch (ValidationException ve) {
            System.out.printf("400 field=%s%n", ve.field());
        } catch (AppException ae) {
            System.err.println("500 " + ae.getMessage());
        }
    }
}