gistfile1.java

import com.google.common.base.Preconditions;

interface TokenGenerator {
    enum VerificationResult { OK, INVALID }

    byte[] generate();
    VerificationResult verify(byte[] token);

}

public class CharTokenGenerator implements TokenGenerator {

    private final String charset;
    private final int length;
    private final RNGenerator generator;
    private final int checksum;

    public CharTokenGenerator(String charset, int length, RNGenerator generator) {
        this(charset, length, generator, 0);
    }

    public CharTokenGenerator(String charset, int length, RNGenerator generator, int checksum) {
        Preconditions.checkArgument(charset.length() % 256 == 0);
        Preconditions.checkArgument(length > 0);
        Preconditions.checkArgument(checksum >= 0);
        this.charset = charset;
        this.length = length;
        this.generator = generator;
        this.checksum = checksum;
    }

    @Override
    public byte[] generate() {
        //...
    }

    @Override
    public VerificationResult verify(byte[] token) {
        //..
    }

}