java.txt

1)Singleton:
->Lazy loading
private constructor
double null check block with synchronized block,
volatile(to make lazy loading atomic,utilize jmm) so that two thread cannot cache the instance in their local memory
Singleton can still be broken by:
	1. Reflection:
		still loophole for lazy loading
		Avoidance: throw Runtime Exception from private constructor.
	2. Cloning
		override clone(). Either return same instance or throw exception
	3. Deserializable
		While deserializing readObject() is called which internally call readResolve().
		Override readResolve().Either return same instance or throw exception

2)CopyOnWriteArraySet
->Internally it uses CopyOnWriteArrayList which implements Set for unique elements
Fail safe iterator

3)IntegerCache
->Like Stringconstantpool, integer also have integer cache to store Integer Object of range -128 to 127 because the value used in this range are most comonly used
thus.same object will be returned if already stored in integercache
we also have ByteCache, ShortCache, LongCache, CharacterCache for Byte, Short, Long, Character respectively.

4)Immutable:
-> class final
states private final
initialize states in public constructor;
only getters no setters
override toString(),hashcode(),equals()
For Mutable state. create new object ,avoid data leakage
For list, use  Collections.unmodifiableList().

5)map.put(key, value): returns null if key not found else the previous value if key already present
set.add(key): return boolean, true if key not present and add key, false if already present and does not replace

6)Intern:
->String constant pool also called as String intern pool store String literals
intern(): can be used to add String object from heap to inten pool if already not exist.If already exist return address of constant pool string