Infinite Primes Generator

Primes.java

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.function.LongPredicate;
import java.util.function.LongSupplier;
import java.util.stream.LongStream;

/**
 * Primes generator.
 */
public class Primes {

    private final Map<Long,Boolean> cache;
    private final LongPredicate isPrime;

    /**
     * Creates a new basic prime generator.
     */
    public Primes(){
        this.cache = Collections.emptyMap();
        this.isPrime = Primes::isPrime;
    }

    /**
     * Creates a memoized prime generator with a cache of
     * the provided size.
     *
     * @param cacheSize - the cache size.
     */
    public Primes(int cacheSize) {
        if(cacheSize < 1) {
            throw new IllegalArgumentException("The cache size must be a positive integer bigger than 0: " + cacheSize);
        }
        this.cache = new HashMap<>(cacheSize);
        this.isPrime = memoize(Primes::isPrime);
    }

    private LongPredicate memoize(LongPredicate source) {
        return x -> cache.computeIfAbsent(x, source::test);
    }

    private static boolean isEven(long n){
        return n % 2 == 0;
    }

    /**
     * Determines the smallest divisor of n starting at k.
     */
    private static long ldf(long n, long k) {
        while(true){
            if(n % k == 0) return k;
            else if(k * k > n) return n;
            else k = k + 2;
        }
    }

    private static boolean isPrime(long n) {
        return n==2 || ((n >= 2) && !isEven(n) && ldf(n, 3) == n);
    }

    /**
     * Infinite generator of prime numbers.
     */
    private static class PrimeGenerator implements LongSupplier {

        private long prime = 2;
        private final LongPredicate isPrime;

        PrimeGenerator(LongPredicate isPrime) {
            this.isPrime = isPrime;
        }

        @Override
        public long getAsLong() {
            if(prime == 2) {
                return prime++;
            }
            while(true){
                long next = prime;
                prime = prime + 2;
                if(isPrime(next)){
                    return next;
                }
            }
        }
    }

    /**
     * Provides an infinite stream of prime numbers.
     */
    public LongStream stream(){
        return LongStream.generate(new PrimeGenerator(this.isPrime));
    }

    /**
     * Usage example
     */
    public static void main(String[] args) {

        Primes primes = new Primes();
        primes.stream().limit(30).forEach(n -> System.out.printf("%d ", n));
        System.out.println();
    }

}