Read all about how I wrote this and understanding BIP39 here.

Apart from generating a seed, only English, French, Spanish and Japanese currently packaged, but as WordList is an interface and you can provide your own.

Install

Use either of these repositories:

repositories {
    jcenter()
}

Or:

repositories {
    maven {
        url 'https://dl.bintray.com/novacrypto/BIP/'
    }
}

Add dependency:

dependencies {
    compile 'io.github.novacrypto:BIP39:2019.01.27'
}

Usage

Generate a mnemonic

Using a StringBuilder:

StringBuilder sb = new StringBuilder();
byte[] entropy = new byte[Words.TWELVE.byteLength()];
new SecureRandom().nextBytes(entropy);
new MnemonicGenerator(English.INSTANCE)
    .createMnemonic(entropy, sb::append);
System.out.println(sb.toString());

If you're paranoid and/or need higher than normal memory security, consider using a SecureCharBuffer:

try (SecureCharBuffer secure = new SecureCharBuffer()) {
    byte[] entropy = new byte[Words.TWELVE.byteLength()];
    new SecureRandom().nextBytes(entropy);
    new MnemonicGenerator(English.INSTANCE)
        .createMnemonic(entropy, secure::append);
    Arrays.fill(entropy, (byte) 0); //empty entropy
    //do something with your secure mnemonic
}

Validate a mnemonic

try {
    MnemonicValidator
        .ofWordList(English.INSTANCE)
        .validate(mnemonic);
} catch (UnexpectedWhiteSpaceException e) {
   ...
} catch (InvalidWordCountException e) {
    ...
} catch (InvalidChecksumException e) {
     ...
} catch (WordNotFoundException e) {
    ...
    //e.getSuggestion1()
    //e.getSuggestion2()
}

Or if you have a list of words from a word list:

MnemonicValidator
        .ofWordList(English.INSTANCE)
        .validate(mnemonicWordsInAList);

Generate a seed

As does not use a word list, can be used now for any language.

byte[] seed = new SeedCalculator().calculateSeed(mnemonic, passphrase);

Or if you have a list of words from a word list:

byte[] seed = new SeedCalculator()
                     .withWordsFromWordList(English.INSTANCE)
                     .calculateSeed(mnemonicWordsInAList, passphrase);

Note: it will work for words off of the word list, but it allows use of secure CharSequences if they match the wordlist, normalized or not (as they are never toStringed)

Those examples both use SpongyCastle, if you don't need or want that dependency, you can use javax.crypto like so:

byte[] seed = new SeedCalculator(JavaxPBKDF2WithHmacSHA512.INSTANCE).calculateSeed(mnemonic, passphrase);

That will not work on Android API < 26 https://developer.android.com/reference/javax/crypto/SecretKeyFactory.html and see Issue #17.