src/java.base/share/classes/java/security/spec/ECFieldFp.java

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package java.security.spec;

import java.math.BigInteger;

/**
 * This immutable class defines an elliptic curve (EC) prime
 * finite field.
 *
 * @see ECField
 *
 * @author Valerie Peng
 *
 * @since 1.5
 */
public class ECFieldFp implements ECField {

    private final BigInteger p;

    /**
     * Creates an elliptic curve prime finite field
     * with the specified prime {@code p}.
     * @param p the prime.
     * @throws    NullPointerException if {@code p} is null.
     * @throws    IllegalArgumentException if {@code p}
     * is not positive.
     */
    public ECFieldFp(BigInteger p) {
        if (p.signum() != 1) {
            throw new IllegalArgumentException("p is not positive");
        }
        this.p = p;
    }

    /**
     * Returns the field size in bits which is size of prime p
     * for this prime finite field.
     * @return the field size in bits.
     */
    public int getFieldSize() {
        return p.bitLength();
    }

    /**
     * Returns the prime {@code p} of this prime finite field.
     * @return the prime.
     */
    public BigInteger getP() {
        return p;
    }

    /**
     * Compares this prime finite field for equality with the
     * specified object.
     * @param obj the object to be compared.
     * @return true if {@code obj} is an instance
     * of ECFieldFp and the prime value match, false otherwise.
     */
    @Override
    public boolean equals(Object obj) {
        if (this == obj)  return true;

        return obj instanceof ECFieldFp other
                && p.equals(other.p);
    }

    /**
     * {@return a hash code value for this prime finite field}
     */
    @Override
    public int hashCode() {
        return p.hashCode();
    }
}