Speaker: Dan Smith

See the table of contents for more posts

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General

• Dictionary says initializing is to set something to a starting position/value/configuration
• Dangerous things happen if don’t satisfy invariants
• Initialize – local variables, fields, arrays, classes, class instances, specific objects, components, modules, frameworks, systems

Variables

• Well behaved variable always initialized before used
• If final, only one initialization
• Local variables checked at compile time
• Whether the default value is an initial value depends on programmer intent
• Final fields are good because must be initialized/can’t be mutated. However don’t prevent reading before initialization. (via constructor calling a method)

Class initialization state

• Uninitialized – init hasn’t started yet, class can’t be used
• Larval – init code running in specific thread; other threads block. Can see default values or final values changing. Can see accidental state
• Initialized – init completed, class can be used
• Erroneous – init threw exception. Class may never be used

Instance initialization

• Some code runs early in construction like passing things to another constructor
• Other code runs later like instance initializers or constructor bodies

JEP 492 – Flexible constructor bodies

• Allows more code to run in early phase.
• Can have lines of code before this/super call
• Larval state now split into early and late larval
• Early larval – constructor code is running up the hierarchy
• Late larval – nrolling down the hierarchy. starting from when Object superclass initialized
• In early larval, can’t use this or invoke instance methods
• In late larval, be careful. this can be shared; field may change value, final fields may change
• Instance initializers considered early larval
• Array creation expressions produced an initialized array. Programmers don’t see a larval state.

Why change?

• Initialization bugs are subtle and hard to see
• Value classes must initialize fields before sharing larval object so JVM can freely make copies
• Allows null checked variables to ensure cannot be null so can’t have a default value

Future

• Want to change the timing so field initializers and implicit super() run in early phase.

My take

It was like getting a pick behind the scenes. I also learned that larval is a state. (said in Q&A that concept exists now for classes but doesn’t use term. And for instances the concept is there but don’t use the term state) The beginning was slow and I was worried I would be bored. But it got better