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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.nullness.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.framework.qual.DefaultFor;
import org.checkerframework.framework.qual.DefaultQualifierInHierarchy;
import org.checkerframework.framework.qual.LiteralKind;
import org.checkerframework.framework.qual.QualifierForLiterals;
import org.checkerframework.framework.qual.SubtypeOf;
import org.checkerframework.framework.qual.TypeKind;
import org.checkerframework.framework.qual.TypeUseLocation;
import org.checkerframework.framework.qual.UpperBoundFor;

/**
 * If an expression's type is qualified by {@code @NonNull}, then the expression never evaluates to
 * {@code null}. (Unless the program has a bug; annotations specify intended behavior.)
 *
 * For fields of a class, the {@link NonNull} annotation indicates that this field is never
 * {@code null} after the class has been fully initialized. For static fields, the {@link
 * NonNull} annotation indicates that this field is never {@code null} after the containing
 * class is initialized. "Fully initialized" essentially means that the Java constructor has
 * completed. See the Initialization Checker
 * documentation for more details.
 *
 * This annotation is rarely written in source code, because it is the default.
 *
 * @see Nullable
 * @see MonotonicNonNull
 * @checker_framework.manual #nullness-checker Nullness Checker
 * @checker_framework.manual #initialization-checker Initialization Checker
 * @checker_framework.manual #bottom-type the bottom type
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@SubtypeOf(MonotonicNonNull.class)
@QualifierForLiterals(LiteralKind.STRING)
@DefaultQualifierInHierarchy
@DefaultFor(TypeUseLocation.EXCEPTION_PARAMETER)
@UpperBoundFor(
    typeKinds = {
      TypeKind.PACKAGE,
      TypeKind.INT,
      TypeKind.BOOLEAN,
      TypeKind.CHAR,
      TypeKind.DOUBLE,
      TypeKind.FLOAT,
      TypeKind.LONG,
      TypeKind.SHORT,
      TypeKind.BYTE
    })
public @interface NonNull {}