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checker-qual contains annotations (type qualifiers) that a programmer writes to specify Java code for type-checking by the Checker Framework.

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package org.checkerframework.checker.signature.qual;

import org.checkerframework.framework.qual.SubtypeOf;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Canonical names have the same syntactic form as {@link FullyQualifiedName fully-qualified name}s.
 * Every canonical name is a fully-qualified name, but not every fully-qualified name is a canonical
 * name.
 *
 * JLS section
 * 6.7 gives the following example:
 *
 * 

 *
 * The difference between a fully qualified name and a canonical name can be seen in code such as:
 *
 * {@code
 * package p;
 * class O1 { class I {} }
 * class O2 extends O1 {}
 * }
 *
 * Both {@code p.O1.I} and {@code p.O2.I} are fully qualified names that denote the member class
 * {@code I}, but only {@code p.O1.I} is its canonical name.
 *
 * 
 *
 * Given a character sequence that is a fully-qualified name, there is no way to know whether or not
 * it is a canonical name, without examining the program it refers to. Type-checking determines that
 * a string is a {@code CanonicalName} based on provenance (what method produced the string), rather
 * than the contents of the string.
 *
 * @see FullyQualifiedName
 * @checker_framework.manual #signature-checker Signature Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@SubtypeOf({
    FullyQualifiedName.class,
    CanonicalNameOrEmpty.class,
    CanonicalNameOrPrimitiveType.class
})
public @interface CanonicalName {}