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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.javacutil;

/*>>>
import org.checkerframework.dataflow.qual.Pure;
import org.checkerframework.dataflow.qual.SideEffectFree;
*/

/**
 * Simple pair class for multiple returns.
 *
 * TODO: as class is immutable, use @Covariant annotation.
 */
public class Pair {
    public final V1 first;
    public final V2 second;

    private Pair(V1 v1, V2 v2) {
        this.first = v1;
        this.second = v2;
    }

    public static  Pair of(V1 v1, V2 v2) {
        return new Pair(v1, v2);
    }

    /*@SideEffectFree*/
    @Override
    public String toString() {
        return "Pair(" + first + ", " + second + ")";
    }

    private int hashCode = -1;

    /*@Pure*/
    @Override
    public int hashCode() {
        if (hashCode == -1) {
            hashCode = 31;
            if (first != null) {
                hashCode += 17 * first.hashCode();
            }
            if (second != null) {
                hashCode += 17 * second.hashCode();
            }
        }
        return hashCode;
    }

    @Override
    public boolean equals(Object o) {
        if (!(o instanceof Pair)) {
            return false;
        }
        @SuppressWarnings("unchecked")
        Pair other = (Pair) o;
        if (this.first == null) {
            if (other.first != null) return false;
        } else {
            if (!this.first.equals(other.first)) return false;
        }
        if (this.second == null) {
            if (other.second != null) return false;
        } else {
            if (!this.second.equals(other.second)) return false;
        }

        return true;
    }
}