e.hash == hash && key.equals(e.key)

The previous sentence explains that when there is thiskey的，你看看put方法，当value为nullin the table, an exception will occur:

public V put(K key, V value) { Segment s; if (value == null) throw new NullPointerException(); int hash = hash(key); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); return s.put(key, hash, value, false); }

This is a higher version of mine. It may be different from yours, but the value cannot be empty.
So:

if (v != null) return v; return readValueUnderLock(e); // recheck

If it is not empty, it can be returned directly. If it is empty, it means that other threads are operating it. So I added it.

public V get(Object key) { Segment s; // manually integrate access methods to reduce overhead HashEntry[] tab; int h = hash(key); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry e = (HashEntry) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; } } return null; }

The current versionget方法把HashEntry e弄成UNSAFE.getObjectVolatile()获取，像是volatileis out

The poster can upgrade the JDK, I use it1.8找了一下，没发现这段代码在ConcurrentHashMap.

The object is null when it is initialized. This null is not assigned specifically in the program.