Annotations Basics

Annotations are metadata tags added to code elements like classes, methods, variables, or parameters. They do not change code execution directly, but are read by compilers, build tools, or runtime frameworks.

Annotation Retention Policies

An annotation's Retention Policy determines how long the metadata survives:

• SOURCE: Discarded by the compiler. Useful for checks (e.g. @Override, @SuppressWarnings) or code generator plugins (Lombok).
• CLASS: Kept in the compiled .class file, but discarded by the JVM classloader at runtime. This is the default.
• RUNTIME: Preserved in the compiled class file and loaded into JVM memory. These can be inspected at runtime using Reflection.

Code Walkthrough

This class demonstrates using standard compiler-instructing annotations.

public class AnnotationDemo {
    @Override // Compiler verifies method matches parent declaration
    public String toString() {
        return "AnnotationDemo instance";
    }
@Deprecated // Compiler triggers warning if other classes invoke this
    public void legacyMethod() {
        System.out.println("Use newMethod() instead.");
    }
}

Interview-Relevant Information