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Checker Qual is the set of annotations (qualifiers) and supporting classes used by the Checker Framework to type check Java source code. Please see artifact: org.checkerframework:checker

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package org.checkerframework.framework.util.typeinference.constraint;

import org.checkerframework.framework.type.AnnotatedTypeFactory;
import org.checkerframework.framework.type.AnnotatedTypeMirror;

import java.util.Set;

/**
 * F2AReducer takes an F2A constraint that is not irreducible (@see AFConstraint.isIrreducible)
 * and reduces it by one step.  The resulting constraint may still be reducible.
 *
 * Generally reductions should map to corresponding rules in
 * http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.12.2.7
 */
public class F2AReducer implements AFReducer {

    protected final F2AReducingVisitor visitor;

    public F2AReducer(final AnnotatedTypeFactory typeFactory) {
        this.visitor = new F2AReducingVisitor(typeFactory);
    }

    @Override
    public boolean reduce(AFConstraint constraint, Set newConstraints) {
        if (constraint instanceof F2A) {
            final F2A f2A = (F2A) constraint;
            visitor.visit(f2A.formalParameter, f2A.argument, newConstraints);
            return true;

        } else {
            return false;
        }
    }

    /**
     * Given an F2A constraint of the form:
     * F2A( typeFromFormalParameter, typeFromMethodArgument )
     *
     * F2AReducingVisitor visits the constraint as follows:
     * visit ( typeFromFormalParameter, typeFromMethodArgument, newConstraints )
     *
     * The visit method will determine if the given constraint should either:
     *    a) be discarded - in this case, the visitor just returns
     *    b) reduced to a simpler constraint or set of constraints - in this case, the new constraint
     *    or set of constraints is added to newConstraints
     *
     *  Sprinkled throughout this class are comments of the form:
     *
     * {@code
     *  // If F has the form G<..., Yk-1, ? super U, Yk+1, ...>, where U involves Tj
     * }
     *
     *  These are excerpts from the JLS, if you search for them you will find the corresponding
     *  JLS description of the case being covered.
     */
    class F2AReducingVisitor extends AFReducingVisitor {

        public F2AReducingVisitor(AnnotatedTypeFactory typeFactory) {
            super(F2A.class, typeFactory);
        }

        @Override
        public AFConstraint makeConstraint(AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
            return new F2A(subtype, supertype);
        }

        @Override
        public AFConstraint makeInverseConstraint(AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
            return new A2F(subtype, supertype);
        }

        @Override
        public AFConstraint makeEqualityConstraint(AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
            return new FIsA(subtype, supertype);
        }
    }
}