Java

Write robust Java avoiding null traps, equality bugs, and concurrency pitfalls.

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Last updated: April 29, 2026

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Quick Reference

Topic	File
Nulls, Optional, autoboxing	`nulls.md`
Collections and iteration traps	`collections.md`
Generics and type erasure	`generics.md`
Concurrency and synchronization	`concurrency.md`
Classes, inheritance, memory	`classes.md`
Streams and CompletableFuture	`streams.md`
Testing (JUnit, Mockito)	`testing.md`
JVM, GC, modules	`jvm.md`

Critical Rules

```
==
```
compares references, not content — always use
```
.equals()
```
for strings
Override
```
equals()
```
must also override
```
hashCode()
```
— HashMap/HashSet break otherwise

Optional.get()

throws if empty — use

orElse()

orElseGet()

, or

ifPresent()

Modifying while iterating throws
```
ConcurrentModificationException
```
— use Iterator.remove()
Type erasure: generic type info gone at runtime — can't do
```
new T()
```
or
```
instanceof List<String>
```
```
volatile
```
ensures visibility, not atomicity —
```
count++
```
still needs synchronization
Unboxing null throws NPE —
```
Integer i = null; int x = i;
```
crashes
```
Integer == Integer
```
uses reference for values outside -128 to 127 — use
```
.equals()
```
Try-with-resources auto-closes — implement
```
AutoCloseable
```
, Java 7+
Inner classes hold reference to outer — use static nested class if not needed
Streams are single-use — can't reuse after terminal operation
```
thenApply
```
vs
```
thenCompose
```
— compose for chaining CompletableFutures
Records are implicitly final — can't extend, components are final
```
serialVersionUID
```
mismatch breaks deserialization — always declare explicitly

Java

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