android-35/java/security/spec/ECPoint.java

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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package java.security.spec;

import java.math.BigInteger;

/**
 * This immutable class represents a point on an elliptic curve (EC)
 * in affine coordinates. Other coordinate systems can
 * extend this class to represent this point in other
 * coordinates.
 *
 * @author Valerie Peng
 *
 * @since 1.5
 */
public class ECPoint {

    private final BigInteger x;
    private final BigInteger y;

    /**
     * This defines the point at infinity.
     */
    public static final ECPoint POINT_INFINITY = new ECPoint();

    // private constructor for constructing point at infinity
    private ECPoint() {
        this.x = null;
        this.y = null;
    }

    /**
     * Creates an ECPoint from the specified affine x-coordinate
     * {@code x} and affine y-coordinate {@code y}.
     * @param x the affine x-coordinate.
     * @param y the affine y-coordinate.
     * @throws    NullPointerException if {@code x} or
     * {@code y} is null.
     */
    public ECPoint(BigInteger x, BigInteger y) {
        if ((x==null) || (y==null)) {
            throw new NullPointerException("affine coordinate x or y is null");
        }
        this.x = x;
        this.y = y;
    }

    /**
     * Returns the affine x-coordinate {@code x}.
     * Note: POINT_INFINITY has a null affine x-coordinate.
     * @return the affine x-coordinate.
     */
    public BigInteger getAffineX() {
        return x;
    }

    /**
     * Returns the affine y-coordinate {@code y}.
     * Note: POINT_INFINITY has a null affine y-coordinate.
     * @return the affine y-coordinate.
     */
    public BigInteger getAffineY() {
        return y;
    }

    /**
     * Compares this elliptic curve point for equality with
     * the specified object.
     * @param obj the object to be compared.
     * @return true if {@code obj} is an instance of
     * ECPoint and the affine coordinates match, false otherwise.
     */
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (this == POINT_INFINITY) return false;

        return obj instanceof ECPoint other
                && ((x.equals(other.x))
                && (y.equals(other.y)));
    }

    /**
     * Returns a hash code value for this elliptic curve point.
     * @return a hash code value.
     */
    public int hashCode() {
        if (this == POINT_INFINITY) return 0;
        return x.hashCode() << 5 + y.hashCode();
    }
}