How to generate the PublicKey from a RSA PrivateKey

PublicKeyGenerator.java

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.RSAPublicKeySpec;

import sun.security.rsa.RSAPrivateCrtKeyImpl;

public class PublicKeyGenerator {
	public static void main(String[] args) throws Exception {
		//convert pem to der with "openssl pkcs8 -topk8 -nocrypt -in key.pem -inform"
		byte[] privateKeyBytes = getPrivateKeyBytes("yourprivatecertificate.der"); 
		PrivateKey privateKey = getPrivate(privateKeyBytes); //get private key

		//you need to know the Implementation. the interfaces doesn't have all informations or parse it out of the privateKey.toString()
		RSAPrivateCrtKeyImpl rsaPrivateKey = (RSAPrivateCrtKeyImpl)privateKey; 
		
		//create a KeySpec and let the Factory due the Rest. You could also create the KeyImpl by your own.
		PublicKey publicKey = KeyFactory.getInstance("RSA").generatePublic(new RSAPublicKeySpec(rsaPrivateKey.getModulus(), rsaPrivateKey.getPublicExponent()));
		System.out.println(publicKey.getEncoded()); //store it - that's it
	}

	public static byte[] getPrivateKeyBytes(String filename) throws Exception {
		File f = new File(filename);
		FileInputStream fis = new FileInputStream(f);
		DataInputStream dis = new DataInputStream(fis);
		byte[] keyBytes = new byte[(int) f.length()];
		dis.readFully(keyBytes);
		dis.close();

		return keyBytes;
	}

	public static PrivateKey getPrivate(byte[] privateKeyBytes)
			throws Exception {
		PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(privateKeyBytes);
		KeyFactory kf = KeyFactory.getInstance("RSA");
		return kf.generatePrivate(spec);
	}
}

This code assume no other security providers (only JCE) - e.g., if the private key is generated by Bouncycastle then it does not implement RSAPrivateCrtKeyImpl.