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public static class MyKEMImpl implements KEMSpi {

    @Override
    public KEMSpi.EncapsulatorSpi engineNewEncapsulator(PublicKey publicKey,
            AlgorithmParameterSpec spec, SecureRandom secureRandom)
            throws InvalidAlgorithmParameterException, InvalidKeyException {
        if (!checkPublicKey(publicKey)) {
            throw new InvalidKeyException("unsupported key");
        }
        if (!checkParameters(spec)) {
            throw new InvalidAlgorithmParameterException("unsupported params");
        }
        return new MyEncapsulator(publicKey, spec, secureRandom);
    }

    class MyEncapsulator implements KEMSpi.EncapsulatorSpi {
        MyEncapsulator(PublicKey publicKey, AlgorithmParameterSpec spec,
                SecureRandom secureRandom){
            this.spec = spec != null ? spec : getDefaultParameters();
            this.secureRandom = secureRandom != null
                    ? secureRandom
                    : getDefaultSecureRandom();
            this.publicKey = publicKey;
        }

        @Override
        public KEM.Encapsulated encapsulate(int from, int to, String algorithm) {
            byte[] encapsulation;
            byte[] secret;
            // calculating...
            return new KEM.Encapsulated(
                    new SecretKeySpec(secret, from, to - from, algorithm),
                    encapsulation, null);
        }

        // ...
    }

    // ...
}