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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.optional;

import com.sun.source.tree.ExpressionTree;
import com.sun.source.tree.MemberReferenceTree;
import com.sun.source.tree.MethodInvocationTree;
import com.sun.source.tree.Tree;
import com.sun.source.tree.Tree.Kind;

import org.checkerframework.checker.optional.qual.Present;
import org.checkerframework.common.basetype.BaseAnnotatedTypeFactory;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.framework.flow.CFAbstractAnalysis;
import org.checkerframework.framework.flow.CFStore;
import org.checkerframework.framework.flow.CFTransfer;
import org.checkerframework.framework.flow.CFValue;
import org.checkerframework.framework.type.AnnotatedTypeMirror;
import org.checkerframework.javacutil.AnnotationBuilder;
import org.checkerframework.javacutil.TreeUtils;

import java.util.Collection;
import java.util.function.Function;

import javax.lang.model.element.AnnotationMirror;
import javax.lang.model.element.ElementKind;
import javax.lang.model.element.ExecutableElement;

/** OptionalAnnotatedTypeFactory for the Optional Checker. */
public class OptionalAnnotatedTypeFactory extends BaseAnnotatedTypeFactory {

    /** The element for java.util.Optional.map(). */
    private final ExecutableElement optionalMap;

    /** The @{@link Present} annotation. */
    protected final AnnotationMirror PRESENT = AnnotationBuilder.fromClass(elements, Present.class);

    /**
     * Creates an OptionalAnnotatedTypeFactory.
     *
     * @param checker the Optional Checker associated with this type factory
     */
    public OptionalAnnotatedTypeFactory(BaseTypeChecker checker) {
        super(checker);
        optionalMap = TreeUtils.getMethodOrNull("java.util.Optional", "map", 1, getProcessingEnv());
        postInit();
    }

    @Override
    public AnnotatedTypeMirror getAnnotatedType(Tree tree) {
        AnnotatedTypeMirror result = super.getAnnotatedType(tree);
        optionalMapNonNull(tree, result);
        return result;
    }

    /**
     * If {@code tree} is a call to {@link java.util.Optional#map(Function)} whose argument is a
     * method reference, then this method adds {@code @Present} to {@code type} if the following is
     * true:
     *
     * 
     *   The type of the receiver to {@link java.util.Optional#map(Function)} is
     *       {@code @Present}, and
     *   
{@link #returnHasNullable(MemberReferenceTree)} returns false.
     * 
     *
     * @param tree a tree
     * @param type the type of the tree, which may be side-effected by this method
     */
    private void optionalMapNonNull(Tree tree, AnnotatedTypeMirror type) {
        if (!TreeUtils.isMethodInvocation(tree, optionalMap, processingEnv)) {
            return;
        }
        MethodInvocationTree mapTree = (MethodInvocationTree) tree;
        ExpressionTree argTree = mapTree.getArguments().get(0);
        if (argTree.getKind() == Kind.MEMBER_REFERENCE) {
            MemberReferenceTree memberReferenceTree = (MemberReferenceTree) argTree;
            AnnotatedTypeMirror optType = getReceiverType(mapTree);
            if (optType == null || !optType.hasEffectiveAnnotation(Present.class)) {
                return;
            }
            if (!returnHasNullable(memberReferenceTree)) {
                // The method still could have a @PolyNull on the return and might return null.
                // If @PolyNull is the primary annotation on the parameter and not on any type
                // arguments or array elements, then it is still safe to mark the optional type as
                // present.
                // TODO: Add the check for poly null on arguments.
                type.replaceAnnotation(PRESENT);
            }
        }
    }

    /**
     * Returns true if the return type of the function type of {@code memberReferenceTree} is
     * annotated with {@code @Nullable}.
     *
     * @param memberReferenceTree a member reference
     * @return true if the return type of the function type of {@code memberReferenceTree} is
     *     annotated with {@code @Nullable}
     */
    private boolean returnHasNullable(MemberReferenceTree memberReferenceTree) {
        if (TreeUtils.MemberReferenceKind.getMemberReferenceKind(memberReferenceTree)
                .isConstructorReference()) {
            return false;
        }
        ExecutableElement memberReferenceFuncType = TreeUtils.elementFromUse(memberReferenceTree);
        if (memberReferenceFuncType.getEnclosingElement().getKind()
                == ElementKind.ANNOTATION_TYPE) {
            // Annotation element accessor are always non-null;
            return false;
        }

        if (!checker.hasOption("optionalMapAssumeNonNull")) {
            return true;
        }
        return containsNullable(memberReferenceFuncType.getAnnotationMirrors())
                || containsNullable(memberReferenceFuncType.getReturnType().getAnnotationMirrors());
    }

    /**
     * Returns true if {@code annos} contains a nullable annotation.
     *
     * @param annos a collection of annotations
     * @return true if {@code annos} contains a nullable annotation
     */
    private boolean containsNullable(Collection annos) {
        for (AnnotationMirror anno : annos) {
            if (anno.getAnnotationType().asElement().getSimpleName().contentEquals("Nullable")) {
                return true;
            }
        }
        return false;
    }

    @Override
    public CFTransfer createFlowTransferFunction(
            CFAbstractAnalysis analysis) {
        return new OptionalTransfer(analysis);
    }
}