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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.initialization.qual;

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;
import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.framework.qual.DefaultFor;
import org.checkerframework.framework.qual.SubtypeOf;
import org.checkerframework.framework.qual.TypeUseLocation;

/**
 * This type qualifier indicates how much of an object has been fully initialized. An object is
 * fully initialized when each of its fields contains a value that satisfies the field's
 * declaration.
 *
 * An expression of type {@code @UnknownInitialization(T.class)} refers to an object that has all
 * fields of {@code T} (and any super-classes) initialized. Just {@code @UnknownInitialization} is
 * equivalent to {@code @UnknownInitialization(Object.class)}. Please see the manual for examples of
 * how to use the annotation (the link appears below).
 *
 * 
Reading a field of an object of type {@code @UnknownInitialization} might yield a value that
 * does not correspond to the declared type qualifier for that field. For instance, consider a
 * non-null field:
 *
 * 
@NonNull Object f;
 *
 * In a partially-initialized object, field {@code f} might be {@code null} despite its
 * {@literal @}{@link NonNull} type annotation.
 *
 * What type qualifiers on the field are considered depends on the checker; for instance, the
 * {@link org.checkerframework.checker.nullness.NullnessChecker} considers {@link NonNull}. The
 * initialization type system is not used on its own, but in conjunction with some other type-system
 * that wants to ensure safe initialization.
 *
 * @checker_framework.manual #initialization-checker Initialization Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@SubtypeOf({})
@DefaultFor({TypeUseLocation.LOCAL_VARIABLE, TypeUseLocation.RESOURCE_VARIABLE})
public @interface UnknownInitialization {
  /**
   * The type-frame down to which the expression (of this type) has been initialized at least
   * (inclusive). That is, an expression of type {@code @UnknownInitialization(T.class)} has all
   * type-frames initialized starting at {@code Object} down to (and including) {@code T}.
   *
   * @return the type whose fields are fully initialized
   */
  Class value() default Object.class;
}