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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 org.checkerframework.checker.initialization.InitializationChecker;
import org.checkerframework.checker.nullness.AbstractNullnessChecker;
import org.checkerframework.framework.qual.DefaultFor;
import org.checkerframework.framework.qual.DefaultInUncheckedCodeFor;
import org.checkerframework.framework.qual.DefaultQualifierInHierarchy;
import org.checkerframework.framework.qual.ImplicitFor;
import org.checkerframework.framework.qual.LiteralKind;
import org.checkerframework.framework.qual.SubtypeOf;
import org.checkerframework.framework.qual.TypeUseLocation;

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 javax.lang.model.type.TypeKind;

/**
 * {@link NonNull} is a type annotation that indicates that an expression is
 * never {@code null}.
 *
 * 
 * For fields of a class, the {@link NonNull} annotation indicates that this
 * field is never {@code null}
 * after the class has been fully initialized. Class initialization is
 * controlled by the Freedom Before Commitment type system, see
 * {@link InitializationChecker} for more details.
 *
 * 

 * For static fields, the {@link NonNull} annotation indicates that this field
 * is never {@code null} after the containing class is initialized.
 *
 * 

 * This annotation is rarely written in source code, because it is the default.
 *
 * 
 * This annotation is associated with the {@link AbstractNullnessChecker}.
 *
 * @see Nullable
 * @see MonotonicNonNull
 * @see AbstractNullnessChecker
 * @checker_framework.manual #nullness-checker Nullness Checker
 */
@SubtypeOf(MonotonicNonNull.class)
@ImplicitFor(types = { TypeKind.PACKAGE,
                       TypeKind.INT, TypeKind.BOOLEAN, TypeKind.CHAR,
                       TypeKind.DOUBLE, TypeKind.FLOAT, TypeKind.LONG,
                       TypeKind.SHORT, TypeKind.BYTE },
    // All literals except NULL_LITERAL:
    literals = { LiteralKind.STRING })
@DefaultQualifierInHierarchy
@DefaultFor({ TypeUseLocation.EXCEPTION_PARAMETER })
@DefaultInUncheckedCodeFor({ TypeUseLocation.PARAMETER, TypeUseLocation.LOWER_BOUND })
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE_USE, ElementType.TYPE_PARAMETER })
public @interface NonNull {
}