Java Initialisation

Daniel Weibel
Created 1 Sep 2017
Last updated 13 Nov 2017

Variable Default Values

Fields (i.e. variables declared at the class level) are automatically initialised to a default value:

  • 0 for numeric types
  • false for boolean
  • \u0000 for char
  • null for reference types (e.g. String or any other object)

Thus, fields may be used without ever explicitely assigning a value to them. This applies to static as well as non-static fields.

Local variables

Local variables (i.e. variables declared within a method or block) are not automatically initialised.

Local variables must be assigned a value before using them. Otherwise the following compiler error occurs:

error: variable <X> might not have been initialized

Array Initialisation

Declare and initialise an array in a single statement.

Note that for initialising an array, you must provide its size. You cannot just initialise an array, leaving its size undetermined, and then add any number of elements, like you can do with a List.

Initialise With Default Values

int[] a = new int[n];

Creates an array of size n. All array elements are set to the default values of the array’s type (see Variable Default Values).

Initialise With Custom Values

int[] a = {1, 1, 2, 3, 5, 8};

Creates an array of size 6. The array elements are set to the provided values.

List Initialisation

Declare and initialise a List in a single statement with Arrays.asList:

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

Array of Lists

It’s possible to create an array of Lists (or of any other Collection) in the following way:

List<Integer>[] array = new ArrayList[10];

This compiles, but results in the following “unchecked” warning, if compiling with Xlint:unckecked: warning: [unchecked] unchecked conversion
                List<Integer>[] array = new ArrayList[10];
  required: List<Integer>[]
  found:    ArrayList[]

In general, it is not recommended to use an array of Collection objects (see here).

It’s probably better to use a list of lists:

List<List<Integer>> list = new ArrayList<>();

List of Arrays

It’s possible to create a list (or any other Collection) of arrays like this:

List<int[]> list = new ArrayList<>();