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Annotations

Custom Annotations

Learn to declare custom annotations, define parameters, and set default values.

Interview: Commonly tested on annotation declaration syntax, permitted parameter types, and default values.

Last Updated: June 13, 2026 • 10 min read

Java allows declaring custom annotations using the @interface keyword. Custom annotations can hold key-value attributes to supply metadata properties to runtime frameworks.

Core Idea

Custom annotations declare attributes as parameterless methods that can optionally have default values.

Why It Matters

Allows designing custom routing or permission rules (e.g. @RequiresRole) for enterprise frameworks.

Interview Lens

Tests permitted annotation parameter types and default value rules.

Custom Annotation Constraints

When declaring attributes inside custom annotations:

  • Allowed Types: Only primitives, String, Class, enums, other annotation types, and arrays of these types are permitted. Complex objects or nested collections are not allowed.
  • Defaults: You can specify default values using the default keyword.
  • Value Attribute: If an annotation contains a single attribute named value(), callers can omit the key name when declaring it (e.g. @MyAnnotation("test")).

Code Walkthrough

This class declares a custom annotation representing database columns, with default settings. 

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Column {
    String name(); // Required parameter
    boolean nullable() default true; // Optional with default value
    int length() default 255;
}

Interview-Relevant Information

Q: Can custom annotation parameters have null as a default value?
Answer: No. Annotation attributes cannot be null. This is a design constraint in the Java language specification. If you want to represent an unassigned state, you must use sentinel values like empty strings (default "") or negative values.

Quick Checklist

What types can be attributes inside annotations? Can you use null as a default? If yes, you understand custom annotations.

Use Cases

Creating ORM annotation parameters for entity fields.

Custom testing frameworks to label execution categories (@TestCategory).

Common Mistakes

Defining complex class types as attributes (only primitives, String, Class, enums, and arrays are allowed).

Forgetting to set a default value when a parameter should be optional, forcing callers to specify it on every tag.