Why would one declare a Java interface method as abstract?

java

best-practices

java-8

singleton

byAnton Shumikhin·Mar 14, 2025

In Java, method declarations inside interfaces are inherently abstract, thus negating any need for the explicit abstract keyword. This implicit keyword represents the underlying semantics of an interface, which anticipates implementation in a child class but doesn't provide one natively.

Consider the code snippet:

// When verbosity is your best friend...but should it be?
public interface VerboseExample {
    abstract void churnButter();
}

// Brevity is the soul of code...at least in interfaces
public interface SuccinctExample {
    void churnButter(); // 'abstract` has left the chat
}

In summary, when sketching out interface methods, find serenity in simplicity—omit the abstract keyword and spotlight on the method signatures awaiting implementation.

Abstract: The Ghost Keyword

Once upon a time, the abstract keyword was a common sight in interface methods, but the Java Language Specification since Java SE 7 has pushed this specter into the shadows; all interface methods are now implicitly considered as abstract.

Trace the roots:

Prior to Java SE 7, some tools like Eclipse's refactor utility in a bid to ensure compliance with then standards, would playfully insert abstract during a "pull interface" action.
In the days of ancient technology, specifically JDK 1.3, the abstract keyword provided a crumb trail to differentiate between interface and default methods.
However, with Java's maturity, the abstract keyword has fallen into disuse and now risks confusing budding Java developers.

Navigating to the Oracle Java Language Specification (you can spot it here) would yield more insights on these evolutions.

Debunking Misconceptions

There are several misconceptions lurking around Java interfaces:

Default Methods: After the default methods swung into the scene in Java 8, it became critical to distinguish these methods (boasting of provided implementation) from their abstract counterparts (still awaiting implementation). Kicking abstract to the curb facilitated a cleaner distinction.
Marker Interfaces: Even in marker interfaces, interfaces that moonlight as method-free zones, the abstract keyword becomes an unwanted guest.
A Matter of Compatibility: Brandishing the abstract keyword for interface methods does nothing to improve your standing with modern Java versions, although it once served a purpose in the legacy era.

Adopting Modern Practices

Switching to modern Java? Stick to these best practices:

Cut the Redundancy: Keep your codebase neat and tidy. Streamlined, succinct declarations are your best friends. Shoo away any needless modifiers to improve readability and maintainability.
Style Master: Don't disregard popular code style guides. They frown upon pointless modifiers in interface definitions. Listen to them.
Document Reader: Turn to official Java Language Specification and other dependable sources to stay updated on language functionalities and best practices.

A journey through time: `abstract`'s rise and fall

The story of abstract in interface methods has all the elements of an epic—rise, dominance, and eventual fall. Embrace this story not only as a historical chronicle but a testament to the evolving nature of Java:

In the early days, marking interface methods as abstract was akin to wearing a badge stating "I am yet to be implemented". As Java matured, the badge became unnecessary—we all knew the code of conduct and the badge was merely an eyesore.
Understand that behaviors of IDEs too have evolved over time. The flirtation with abstract is now seen as an unnecessary complexity, a hurdle in the path of progress toward cleaner, crisper code.
With Java 8 allowing interfaces to house concrete methods alongside abstract ones (default and static methods), appreciating the evolution of Java become crucial. After all, those who cannot remember the past are condemned to code inefficiently.