== Java ==

What

• The constant pool contains the constants associated with the class or interface defined by the file.
• Constants are stored in the constant pool.
  • literal strings
  • final variable values
  • fully qualified names of classes and interfaces
  • field names and descriptors
  • method names and descriptors
• Symbolic references to all types, fields, and methods used by a type are stored in the constant pool
  • it plays a central role in the dynamic linking of Java programs
• The constant pool is organized as a list of entries. A count of the number of entries in the list, constant_pool_count, precedes the actual list, constant_pool.

Why

• For quick object creation. Just get one from pool if it exists, otherwise create one in the pool
• The runtime constant pool is a subset of the method area , different to heap memory for “new” object creation

How

• Wrapper classes of primitive (Byte, Short, Integer, Long, Character, Boolean, no Float and Double) are implemented the constant pool.
• Byte, Short, Integer, Long, Character only create constants object in pool when value (-128~127]
• IntegerCache
  • default range (-128, 127]
  • high can be configured when VM init by property java.lang.Integer.IntegerCache.high property, but MAX is Integer.MAX_VALUE
• All literal strings and string-valued constant expressions are interned.
  • String.intern() // get from pool if exists or add this new String object to the pool

Reference

• Java (JVM) Memory Types

== python ==

How

• int cache range [-5, 257)
• range() vs. xrange(). xrange uses an iterator instead of generating the entire list, so uses less memory
• String poolization
  • intern(string)

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>>> a=258;print hex(id(a))
0x6000838f8
>>> b=258;print hex(id(b))
0x60007bfd8
>>> a is b
False
>>> a=-6;print hex(id(a))
0x60007c008
>>> b=-6;print hex(id(b))
0x6000838f8
>>> a is b
False
>>> a=37;print hex(id(a))
0x600028c40
>>> b=37;print hex(id(a))
0x600028c40
>>> a is b
True

>>> a=1001;b=1001;print hex(id(a));print hex(id(b)); a is b;
0x6000838e0
0x6000838e0
True

== golang ==

• TBD. The memory model of golang is something differents with other programming language.