Java Exception Handling

Java Exception Handling – AbstractMethodError

Moving along through our in-depth Java Exception Handling series, today we’ll be going over the AbstractMethodError. This error is thrown when there are incompatibilities between compiled classes/JAR files using abstract methods.

Throughout this article we’ll examine the AbstractMethodError by looking at where it fits into the overall Java Exception Hierarchy. We’ll also dig into a functional code example that illustrates the basics of using abstract classes and methods in Java, and how their use may lead to AbstractMethodErrors in your own code. Let’s get to it!

The Technical Rundown

All Java errors implement the java.lang.Throwable interface, or are extended from another inherited class therein. The full exception hierarchy of this error is:

Full Code Sample

Below is the full code sample we’ll be using in this article. It can be copied and pasted if you’d like to play with the code yourself and see how everything works.

package io.airbrake;

import io.airbrake.utility.Logging;

import java.util.GregorianCalendar;

public class Main {

    public static void main(String[] args) {
        Logging.lineSeparator("TEST A");
        createBookTestA();
        Logging.lineSeparator("TEST B");
        createBookTestB();
    }

    private static void createBookTestA() {
        try {
            AbstractBook book = new AbstractBook(
                    "A Game of Thrones",
                    "George R.R. Martin",
                    848,
                    new GregorianCalendar(1996, 8, 6).getTime()
            );
            Logging.log(book.getTagline());
        } catch (AbstractMethodError error) {
            // Output expected AbstractMethodErrors.
            Logging.log(error);
        } catch (Exception | Error exception) {
            // Output unexpected Exceptions/Errors.
            Logging.log(exception, false);
        }
    }

    private static void createBookTestB() {
        try {
            AbstractBook book = new AbstractBook(
                    "A Clash of Kings",
                    "George R.R. Martin",
                    761,
                    new GregorianCalendar(1998, 10, 16).getTime()
            );
            Logging.log(book.getTagline());
        } catch (AbstractMethodError error) {
            // Output expected AbstractMethodErrors.
            Logging.log(error);
        } catch (Exception | Error exception) {
            // Output unexpected Exceptions/Errors.
            Logging.log(exception, false);
        }
    }
}
package io.airbrake;

import java.util.Date;

abstract class Publication {
    /**
     * Get author of book.
     *
     * @return Author name.
     */
    abstract String getAuthor();

    /**
     * Get publication type of book.
     *
     * @return Publication type.
     */
    abstract PublicationType getPublicationType();

    /**
     * Get published date of book.
     *
     * @return Published date.
     */
    abstract Date getPublishedAt();

    /**
     * Get a formatted tagline with author, title, page count, publication date, and publication type.
     *
     * @return Formatted tagline.
     */
    abstract String getTagline();

    /**
     * Get title of book.
     *
     * @return Title.
     */
    abstract String getTitle();

    /**
     * Set author of book.
     *
     * @param author Author name.
     */
    abstract void setAuthor(String author);

    /**
     * Set publication type of book.
     *
     * @param type Publication type.
     */
    abstract void setPublicationType(PublicationType type);

    /**
     * Set published date of book.
     *
     * @param publishedAt Page count.
     */
    abstract void setPublishedAt(Date publishedAt);

    /**
     * Set title of book.
     *
     * @param title Title.
     */
    abstract void setTitle(String title);
}
package io.airbrake;

import java.text.DateFormat;
import java.util.Date;

public class AbstractBook extends Publication {
    private String author;
    private String title;
    private Integer pageCount;
    private Date publishedAt;
    private PublicationType publicationType = PublicationType.BOOK;

    private static final Integer maximumPageCount = 4000;

    /**
     * Constructs an empty book.
     */
    public AbstractBook() { }

    /**
     * Constructs a basic book.
     *
     * @param title Book title.
     * @param author Book author.
     */
    public AbstractBook(String title, String author) {
        setAuthor(author);
        setTitle(title);

    }

    /**
     * Constructs a basic book, with page count.
     *
     * @param title Book title.
     * @param author Book author.
     * @param pageCount Book page count.
     */
    public AbstractBook(String title, String author, Integer pageCount) {
        setAuthor(author);
        setPageCount(pageCount);
        setTitle(title);
    }

    /**
     * Constructs a basic book, with page count.
     *
     * @param title Book title.
     * @param author Book author.
     * @param pageCount Book page count.
     */
    public AbstractBook(String title, String author, Integer pageCount, Date publishedAt) {
        setAuthor(author);
        setPageCount(pageCount);
        setTitle(title);
        setPublishedAt(publishedAt);
    }

    /**
     * Constructs a basic book, with page count, publication date, and publication type.
     *
     * @param title Book title.
     * @param author Book author.
     * @param pageCount Book page count.
     */
    public AbstractBook(String title, String author, Integer pageCount, Date publishedAt, PublicationType publicationType) {
        setAuthor(author);
        setPageCount(pageCount);
        setTitle(title);
        setPublishedAt(publishedAt);
        setPublicationType(publicationType);
    }

    /**
     * Get author of book.
     *
     * @return Author name.
     */
    public String getAuthor() {
        return author;
    }

    /**
     * Get page count of book.
     *
     * @return Page count.
     */
    public Integer getPageCount() {
        return pageCount;
    }

    /**
     * Get publication type of book.
     *
     * @return Publication type.
     */
    public PublicationType getPublicationType() { return publicationType; }

    /**
     * Get published date of book.
     *
     * @return Published date.
     */
    public Date getPublishedAt() { return publishedAt; }

    /**
     * Get a formatted tagline with author, title, page count, publication date, and publication type.
     *
     * @return Formatted tagline.
     */
    public String getTagline() {
        return String.format("'%s' by %s is %d pages, published %s as %s type.",
                getTitle(),
                getAuthor(),
                getPageCount(),
                DateFormat.getDateInstance().format(getPublishedAt()),
                getPublicationType());
    }

    /**
     * Get title of book.
     *
     * @return Title.
     */
    public String getTitle() {
        return title;
    }

    /**
     * Publish current book.
     * If book already published, throws IllegalStateException.
     */
    public void publish() throws IllegalStateException {
        Date publishedAt = getPublishedAt();
        if (publishedAt == null) {
            setPublishedAt(new Date());
            System.out.println(String.format("Published '%s' by %s.", getTitle(), getAuthor()));
        } else {
            throw new IllegalStateException(
                    String.format("Cannot publish '%s' by %s (already published on %s).",
                            getTitle(),
                            getAuthor(),
                            publishedAt));
        }
    }

    /**
     * Set author of book.
     *
     * @param author Author name.
     */
    public void setAuthor(String author) {
        this.author = author;
    }

    /**
     * Set page count of book.
     *
     * @param pageCount Page count.
     */
    public void setPageCount(Integer pageCount) throws IllegalArgumentException {
        if (pageCount > maximumPageCount) {
            throw new IllegalArgumentException(String.format("Page count value [%d] exceeds maximum limit [%d].", pageCount, maximumPageCount));
        }
        this.pageCount = pageCount;
    }

    /**
     * Set publication type of book.
     *
     * @param type Publication type.
     */
    public void setPublicationType(PublicationType type) { this.publicationType = type; }

    /**
     * Set published date of book.
     *
     * @param publishedAt Page count.
     */
    public void setPublishedAt(Date publishedAt) {
        this.publishedAt = publishedAt;
    }

    /**
     * Set title of book.
     *
     * @param title Title.
     */
    public void setTitle(String title) {
        this.title = title;
    }

    /**
     * Get string representation of Book.
     *
     * @return String representation.
     */
    public String toString() {
        return getTagline();
    }
}

This code sample also uses the Logging utility class, the source of which can be found here on GitHub.

When Should You Use It?

In most situations you’ll rarely experience AbstractMethodErrors during runtime, because they are typically caught by the compiler prior to execution. However, there are certain scenarios in which code is referencing outdated abstract classes, which can lead to AbstractMethodErrors. Most commonly, consider a situation where an abstract class and an inherited class are compiled. Next, modifications are made to the abstract class and only that abstract class is recompiled, while the extended class is left alone. This can cause AbstractMethodErrors, since the runtime doesn’t know how to handle the incompatibility between the two classes.

To illustrate we’ve created a basic Publication abstract class, which defines a handful of methods that should be implemented in all extension classes:

package io.airbrake;

import java.util.Date;

abstract class Publication {
    abstract String getAuthor();

    abstract PublicationType getPublicationType();

    abstract Date getPublishedAt();

    abstract String getTagline();

    abstract String getTitle();

    abstract void setAuthor(String author);

    abstract void setPublicationType(PublicationType type);

    abstract void setPublishedAt(Date publishedAt);

    abstract void setTitle(String title);
}

The AbstractBook class then extends Publication and provides actual implementation of the abstract methods above:

package io.airbrake;

import java.text.DateFormat;
import java.util.Date;

public class AbstractBook extends Publication {
    private String author;
    private String title;
    private Integer pageCount;
    private Date publishedAt;
    private PublicationType publicationType = PublicationType.BOOK;

    private static final Integer maximumPageCount = 4000;

    /**
     * Constructs an empty book.
     */
    public AbstractBook() { }

    /**
     * Constructs a basic book.
     *
     * @param title Book title.
     * @param author Book author.
     */
    public AbstractBook(String title, String author) {
        setAuthor(author);
        setTitle(title);

    }

    /**
     * Constructs a basic book, with page count.
     *
     * @param title Book title.
     * @param author Book author.
     * @param pageCount Book page count.
     */
    public AbstractBook(String title, String author, Integer pageCount) {
        setAuthor(author);
        setPageCount(pageCount);
        setTitle(title);
    }

    /**
     * Constructs a basic book, with page count.
     *
     * @param title Book title.
     * @param author Book author.
     * @param pageCount Book page count.
     */
    public AbstractBook(String title, String author, Integer pageCount, Date publishedAt) {
        setAuthor(author);
        setPageCount(pageCount);
        setTitle(title);
        setPublishedAt(publishedAt);
    }

    /**
     * Constructs a basic book, with page count, publication date, and publication type.
     *
     * @param title Book title.
     * @param author Book author.
     * @param pageCount Book page count.
     */
    public AbstractBook(String title, String author, Integer pageCount, Date publishedAt, PublicationType publicationType) {
        setAuthor(author);
        setPageCount(pageCount);
        setTitle(title);
        setPublishedAt(publishedAt);
        setPublicationType(publicationType);
    }

    /**
     * Get author of book.
     *
     * @return Author name.
     */
    public String getAuthor() {
        return author;
    }

    /**
     * Get page count of book.
     *
     * @return Page count.
     */
    public Integer getPageCount() {
        return pageCount;
    }

    /**
     * Get publication type of book.
     *
     * @return Publication type.
     */
    public PublicationType getPublicationType() { return publicationType; }

    /**
     * Get published date of book.
     *
     * @return Published date.
     */
    public Date getPublishedAt() { return publishedAt; }

    /**
     * Get a formatted tagline with author, title, page count, publication date, and publication type.
     *
     * @return Formatted tagline.
     */
    public String getTagline() {
        return String.format("'%s' by %s is %d pages, published %s as %s type.",
                getTitle(),
                getAuthor(),
                getPageCount(),
                DateFormat.getDateInstance().format(getPublishedAt()),
                getPublicationType());
    }

    /**
     * Get title of book.
     *
     * @return Title.
     */
    public String getTitle() {
        return title;
    }

    /**
     * Publish current book.
     * If book already published, throws IllegalStateException.
     */
    public void publish() throws IllegalStateException {
        Date publishedAt = getPublishedAt();
        if (publishedAt == null) {
            setPublishedAt(new Date());
            System.out.println(String.format("Published '%s' by %s.", getTitle(), getAuthor()));
        } else {
            throw new IllegalStateException(
                    String.format("Cannot publish '%s' by %s (already published on %s).",
                            getTitle(),
                            getAuthor(),
                            publishedAt));
        }
    }

    /**
     * Set author of book.
     *
     * @param author Author name.
     */
    public void setAuthor(String author) {
        this.author = author;
    }

    /**
     * Set page count of book.
     *
     * @param pageCount Page count.
     */
    public void setPageCount(Integer pageCount) throws IllegalArgumentException {
        if (pageCount > maximumPageCount) {
            throw new IllegalArgumentException(String.format("Page count value [%d] exceeds maximum limit [%d].", pageCount, maximumPageCount));
        }
        this.pageCount = pageCount;
    }

    /**
     * Set publication type of book.
     *
     * @param type Publication type.
     */
    public void setPublicationType(PublicationType type) { this.publicationType = type; }

    /**
     * Set published date of book.
     *
     * @param publishedAt Page count.
     */
    public void setPublishedAt(Date publishedAt) {
        this.publishedAt = publishedAt;
    }

    /**
     * Set title of book.
     *
     * @param title Title.
     */
    public void setTitle(String title) {
        this.title = title;
    }

    /**
     * Get string representation of Book.
     *
     * @return String representation.
     */
    public String toString() {
        return getTagline();
    }
}

We can test this out by creating a new AbstractBook instance and outputting it to the log:

private static void createBookTestA() {
    try {
        AbstractBook book = new AbstractBook(
                "A Game of Thrones",
                "George R.R. Martin",
                848,
                new GregorianCalendar(1996, 8, 6).getTime()
        );
        Logging.log(book.getTagline());
    } catch (AbstractMethodError error) {
        // Output expected AbstractMethodErrors.
        Logging.log(error);
    } catch (Exception | Error exception) {
        // Output unexpected Exceptions/Errors.
        Logging.log(exception, false);
    }
}

Executing the createBookTestA() method produces the following output:

---------------- TEST A ----------------
'A Game of Thrones' by George R.R. Martin is 848 pages, published Sep 6, 1996 as BOOK type.

That’s all well and good, but what happens if we modify one of these classes but not the other (effectively causing their versions to be incompatible)? For example, let’s modify the Publication abstract class and add another abstract method:

abstract class Publication {

    // ...

    abstract String getDetails();

    // ...
}

If we compile just this Publication class, but not also the inherited AbstractBook class, executing our test code will result in an AbstractMethodError, since the secondary class hasn’t been recompiled and made aware of the changes:

----- TEST B -----
[EXPECTED] java.lang.AbstractMethodError: io.airbrake.Publication.getDetails()Ljava/lang/String;

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