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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.qualframework.poly;

import java.util.*;

/** A qualifier wildcard, bounded above and below by {@link PolyQual}s.
 */
public class Wildcard {
    private final PolyQual lower;
    private final PolyQual upper;

    public Wildcard(PolyQual lower, PolyQual upper) {
        // Don't let the user pass in `null`.  We use `null` to represent the
        // special empty wildcard.
        if (lower == null || upper == null) {
            throw new IllegalArgumentException("wildcard bounds may not be null");
        }
        this.lower = lower;
        this.upper = upper;
    }

    public Wildcard(PolyQual qual) {
        this(qual, qual);
    }

    public Wildcard(Q groundLower, Q groundUpper) {
        this(new PolyQual.GroundQual(groundLower), new PolyQual.GroundQual(groundUpper));
    }

    public Wildcard(Q groundQual) {
        this(new PolyQual.GroundQual(groundQual));
    }


    /**
     * Force the user to write `Wildcard.empty()` instead of `new Wildcard()`,
     * to make it clear that they're getting something special, rather than
     * a normal wildcard with default bounds or something like that.
     */
    private Wildcard() {
        this.lower = null;
        this.upper = null;
    }

    /** Produce the empty wildcard. */
    public static  Wildcard empty() {
        return new Wildcard();
    }


    /** Get the lower bound of the wildcard, or {@code null} if this is the
     * empty wildcard. */
    public PolyQual getLowerBound() {
        return this.lower;
    }

    /** Get the upper bound of the wildcard, or {@code null} if this is the
     * empty wildcard. */
    public PolyQual getUpperBound() {
        return this.upper;
    }

    /** Check if this is the empty wildcard. */
    public boolean isEmpty() {
        return this.lower == null;
    }

    /** Substitute wildcards for qualifier parameters. */
    public Wildcard substitute(Map> substs) {
        Map> lowerSubsts = new HashMap<>();
        Map> upperSubsts = new HashMap<>();

        for (String k : substs.keySet()) {
            lowerSubsts.put(k, substs.get(k).getLowerBound());
            upperSubsts.put(k, substs.get(k).getUpperBound());
        }

        PolyQual newLower = lower.substitute(lowerSubsts);
        PolyQual newUpper = upper.substitute(upperSubsts);
        return new Wildcard(newLower, newUpper);
    }

    /** Combine with another wildcard, using the provided {@link
     * CombiningOperation}s for the upper and lower bounds.
     */
    public Wildcard combineWith(Wildcard other,
            CombiningOperation lowerOp, CombiningOperation upperOp) {
        return new Wildcard(
                this.getLowerBound().combineWith(other.getLowerBound(), lowerOp),
                this.getUpperBound().combineWith(other.getUpperBound(), upperOp));
    }

    @Override
    public boolean equals(Object o) {
        if (o == null || o.getClass() != this.getClass()) {
            return false;
        }
        @SuppressWarnings("rawtypes")
        Wildcard other = (Wildcard)o;
        return this.lower.equals(other.lower)
            && this.upper.equals(other.upper);
    }

    @Override
    public int hashCode() {
        return this.lower.hashCode() * 37
            + this.upper.hashCode() * 59;
    }

    @Override
    public String toString() {
        if (this.isEmpty()) {
            return "ø";
        } else if (lower.equals(upper)) {
            return lower.toString();
        } else {
            return "? ∈ [" + lower + ".." + upper + "]";
        }
    }
}