Java Glossary : initialisation

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initialisation

Java automatically initialises all non-local variables with a default value, i.e. 0 or null, even chars. However Java does not automatically initialise local variables, though it will initialise any arrays or objects newly created to be stored in local variables. The Java compiler is clever. If you forget to initialise when it is required, it will tell you at compile time. You never need to worry about inadvertently accessing uninitialised variables. Java always protects you from your folly. When you create an array of objects, Java automatically initialises all the slots to null, for both singly and doubly dimensioned arrays e.g. String [ ] or String [ ] [ ]. It does not create a new object for each slot.

There are three kinds of variable, local, instance (one member per object), and static (one per class). When you first use a class, its class variables are initialised. When you first use a new SomeConstructor, the object's instance variables are initialised. There is no automatic initialisation for local variables. Usually you initialise them inline. Here is the order in which the six types of initialisation are applied. The types of initialisation near the top of the table are applied first.

Type of Initialisation	Local	Instance	Static	Notes
automatic null or zero	No	Yes	Yes	No code needed. char = 0, not ' '; reference = null, int = 0;
inline known finals e.g. final int MAX_WEIGHT = 1000;	Yes	Yes	Yes	If the compiler can figure out the value of the expression at compile time, it gets done first ahead of the other inline initialisations. If it can't, then it is treated as an ordinary inline initialisation, even though it is final.
inline e.g. int a = 1;	Yes	Yes	Yes	Interleaved in textual order with static {} or instance {}.
static init block e.g. static {a = 1;}	Yes^*	No	Yes	Interleaved in textual order with inline initialisations.
init block e.g. {a = 1;}	Yes^*	Yes	Yes	Interleaved in textual order with inline initialisations.
constructor e.g. a = 1;	Yes	Yes	Yes	constructor to see how constructors are chained to do a series of initialisations
assignment in a method e.g. a = 1;	Yes	Yes	Yes	procedural code, not really initialisation.

* local variables initialised in an init block or static init block must be declared in that block, and have scope confined to that block.

According the the Java language specification, after the zero initialisations, the class variable initializers and static initializers of the class, or the field initializers of the interface, are executed in textual order, as though they were a single block, except that final class variables and fields of interfaces whose values are compile-time constants are initialized first. In other words static final constants known at compile time are logically done first.

Beware of depending on inline initialisations of member or class variables being done in the order the variables are listed because various beautifier tools such as VAJ may reoreder them. When the value of one variable depends on another, put the dependent initialisations in an init or static init block, where you have explicit control of the ordering, rather than using inline initialisations. The compiler is not smart like a spreadsheet to figure out the dependencies and calculate them in natural order. The constructor must call the constructor of the superclass as the very first thing it does. If you leave out this code, the compiler inserts it for you. This means when you invoke a constructor, first the base fields are initialised, then the subclass. You are invoking a daisy chain of constructors of the ancestor classes when you invoke a constructor.

Canadian Spelling ¤ constructor initialisation gotchas ¤ constructor ¤ Instance Initialisation gotchas ¤ instance ¤ local scope ¤ natural order ¤ static initialisation gotchas ¤ static

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