LambdaExample.java

private final ConcurrentMap<Integer, ConcurrentMap<Integer, AtomicLong>> sizeDeltas = new ConcurrentHashMap<>();

// Good old approach
@Override public void accumulateSizeDelta(int cacheId, int part, long delta) {
    ConcurrentMap<Integer, AtomicLong> partDeltas = sizeDeltas.get(cacheId);
    if (partDeltas == null) {
        ConcurrentMap<Integer, AtomicLong> partDeltas0 =
            sizeDeltas.putIfAbsent(cacheId, partDeltas = new ConcurrentHashMap<>());
        if (partDeltas0 != null)
            partDeltas = partDeltas0;
    }

    AtomicLong accDelta = partDeltas.get(part);
    if (accDelta == null) {
        AtomicLong accDelta0 = partDeltas.putIfAbsent(part, accDelta = new AtomicLong());
        if (accDelta0 != null)
            accDelta = accDelta0;
    }

    accDelta.addAndGet(delta);
}

// Lambda approach
@Override public void accumulateSizeDelta1(int cacheId, int part, long delta) {
    ConcurrentMap<Integer, AtomicLong> partDeltas = sizeDeltas.computeIfAbsent(cacheId, k -> new ConcurrentHashMap<>());
    AtomicLong accDelta = partDeltas.computeIfAbsent(part, k -> new AtomicLong());
    accDelta.addAndGet(delta);
}

// Lambda one-liner approach
@Override public void accumulateSizeDelta2(int cacheId, int part, long delta) {
    sizeDeltas
        .computeIfAbsent(cacheId, k -> new ConcurrentHashMap<>())
        .computeIfAbsent(part, k -> new AtomicLong())
        .addAndGet(delta);
}