Sometimes you just need a small simple cache for calculated values because the value generation is expensive in time or resources. In this post I show how such a cache can be implemented in a thread safe way with the Java8 version of the ConcurrentHashMap class with just 2 lines of relevant code.

For this example I use the calculation of the n-th fibonacci number and calculate the values for there input of 1 to 20.

using no cache

The sample program and the first version of the fibonacci() method without caching are as follows:

package com.sothawo;

import java.math.BigInteger;

public class App {

    private BigInteger fibonacci(int n) {
        return (n <= 2) ? BigInteger.ONE : fibonacci(n - 1).add(fibonacci(n - 2));
    }

    public static void main(String[] args) {
        new App().run();
    }

    private void run() {
        for (int i = 1; i <= 20; i++) {
            System.out.println("fibonacci(" + i + ") is " + fibonacci(i));
        }
    }
}

This program produces the desired results, but due to the recursion there are 35.400 calls of the fibonacci() method, as already calculated values are recalculated again and again.

pre-Java 8 cache

As a cache I use a HashMap and guard it’s use with a lock object to ensure thread safety. The modified program looks like this:

package com.sothawo;

import java.math.BigInteger;
import java.util.HashMap;
import java.util.Map;

public class App {

    private Map<Integer, BigInteger> simpleCache = new HashMap<>();
    private Object simpleCacheLock = new Object();

    private BigInteger fibonacci(int n) {
        BigInteger fib;
        synchronized (simpleCacheLock) {
            fib = simpleCache.get(n);
            if (null == fib) {
                fib = (n <= 2) ? BigInteger.ONE : fibonacci(n - 1).add(fibonacci(n - 2));
                simpleCache.put(n, fib);
            }
        }
        return fib;
    }

    public static void main(String[] args) {
        new App().run();
    }

    private void run() {
        for (int i = 1; i <= 20; i++) {
            System.out.println("fibonacci(" + i + ") is " + fibonacci(i));
        }
    }
}

This works as expected and gets the number of calls to the fibonacci() method down to the minimum of 20, but is a lot of code.

Java 8 version

The ConcurrentHashMap in Java 8 has got a nice addition, the computeIfAbsent method. It takes two arguments: the key value and a function to calculate the value if not present in the cache. This leads to the following code:

package com.sothawo;

import java.math.BigInteger;
import java.util.concurrent.ConcurrentHashMap;

public class App {
    private ConcurrentHashMap<Integer, BigInteger> betterCache = new ConcurrentHashMap<>();

    private BigInteger fibonacci(int n) {
        return betterCache.computeIfAbsent(n, (i) -> (i <= 2) ? BigInteger.ONE : fibonacci(i - 1).add(fibonacci(i - 2)));
    }

    public static void main(String[] args) {
        new App().run();
    }

    private void run() {
        for (int i = 1; i <= 20; i++) {
            System.out.println("fibonacci(" + i + ") is " + fibonacci(i));
        }
    }
}

The same result as before, but with just two lines of code.