https://openjdk.java.net/ 
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package java.security.interfaces;

import java.security.PrivateKey;
import java.util.Optional;


An interface for an elliptic curve private key as defined by RFC 7748. These keys are distinct from the keys represented by ECPrivateKey, and they are intended for use with algorithms based on RFC 7748 such as the XDH KeyAgreement algorithm. An XEC private key is an encoded scalar value as described in RFC 7748. The decoding procedure defined in this RFC includes an operation that forces certain bits of the key to either 1 or 0. This operation is known as "pruning" or "clamping" the private key. Arrays returned by this interface are unpruned, and implementations will need to prune the array before using it in any numerical operations. 
Since:11
/**
 * An interface for an elliptic curve private key as defined by RFC 7748.
 * These keys are distinct from the keys represented by {@code ECPrivateKey},
 * and they are intended for use with algorithms based on RFC 7748 such as the
 * XDH {@code KeyAgreement} algorithm.
 *
 * An XEC private key is an encoded scalar value as described in RFC 7748.
 * The decoding procedure defined in this RFC includes an operation that forces
 * certain bits of the key to either 1 or 0. This operation is known as
 * "pruning" or "clamping" the private key. Arrays returned by this interface
 * are unpruned, and implementations will need to prune the array before
 * using it in any numerical operations.
 *
 * @since 11
 */
public interface XECPrivateKey extends XECKey, PrivateKey {

    
Get the scalar value encoded as an unpruned byte array. A new copy of
the array is returned each time this method is called.

Returns:the unpruned encoded scalar value, or an empty Optional if the
    scalar cannot be extracted (e.g. if the provider is a hardware token
    and the private key is not allowed to leave the crypto boundary).
/**
     * Get the scalar value encoded as an unpruned byte array. A new copy of
     * the array is returned each time this method is called.
     *
     * @return the unpruned encoded scalar value, or an empty Optional if the
     *     scalar cannot be extracted (e.g. if the provider is a hardware token
     *     and the private key is not allowed to leave the crypto boundary).
     */
    Optional<byte[]> getScalar();
}