Decrypt my private key using KeyCrypter
Attempting to implement the interface KeyCrypter is a worthy exercise but it seems to require more than a few simple lines of code. bitcoinj offers one implementation with the class KeyCrypterScrypt and you may wish to look at the source file for this class. However, because this implementation exists, you do not need to provide your own implementation of KeyCrypter if all you want to do is have the ability to encrypt and decrypt keys. This can be done as follows:

import org.bitcoinj.core.ECKey;
import org.bitcoinj.crypto.KeyCrypter;
import org.bitcoinj.crypto.KeyCrypterScrypt;
import org.spongycastle.crypto.params.KeyParameter;

public class Test {

  public static void main(String[] args){

  ECKey k1 = new ECKey(); // some random key

  // encrypting a key
  KeyCrypter crypter1 = new KeyCrypterScrypt();
  KeyParameter aesKey1 = crypter1.deriveKey("some arbitrary passphrase");
  ECKey k2 = k1.encrypt(crypter1, aesKey1);
  System.out.println(k2.isEncrypted()); // true

  // decrypting a key
  KeyCrypter crypter2 = k2.getKeyCrypter();
  KeyParameter aesKey2 = crypter2.deriveKey("some arbitrary passphrase");
  ECKey k3 = k2.decrypt(aesKey2);
  System.out.println(k1.equals(k3));  // true
  }
}