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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.optional.qual;

import org.checkerframework.framework.qual.InheritedAnnotation;
import org.checkerframework.framework.qual.PostconditionAnnotation;

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;

/**
 * Indicates that the expression evaluates to a non-empty Optional, if the method terminates
 * successfully.
 *
 * This postcondition annotation is useful for methods that construct a non-empty Optional:
 *
 * 

 *   {@literal @}EnsuresPresent("optStr")
 *   void initialize() {
 *     optStr = Optional.of("abc");
 *   }
 * 
 *
 * It can also be used for a method that fails if a given Optional value is empty, indicating that
 * the argument is present if the method returns normally:
 *
 * 
 *   /** Throws an exception if the argument is empty. */
 *   {@literal @}EnsuresPresent("#1")
 *   void useTheOptional(Optional<T> arg) { ... }
 * 
 *
 * @see Present
 * @see org.checkerframework.checker.optional.OptionalChecker
 * @checker_framework.manual #optional-checker Optional Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@PostconditionAnnotation(qualifier = Present.class)
@InheritedAnnotation
public @interface EnsuresPresent {
    /**
     * The expression (of Optional type) that is present, if the method returns normally.
     *
     * @return the expression (of Optional type) that is present, if the method returns normally
     */
    String[] value();
}