Array Creation and Access: Revision Notes (Computer Science A 4.3)

These notes recap Topic 4.3. Full explanations and worked examples are in the study guide.

Recap

An array stores multiple values of the same type: primitives or object references.
The length is fixed at creation and is read with the length attribute (no parentheses).
new sets every element to a default: 0, 0.0, false or null.
An initializer list such as {72, 65, 90} creates and fills an array in one statement.
Square brackets read or assign an element using an index.
Valid indices are 0 through length - 1. Outside that range, the program throws ArrayIndexOutOfBoundsException at run time.

Key relationships

Task	Code
Create with defaults	`int[] counts = new int[4];`
Create with values	`int[] scores = {72, 65, 90, 58, 81};`
Number of elements	`scores.length`
First element	`scores[0]`
Last element	`scores[scores.length - 1]`
Assign	`scores[1] = 70;`
Computed index	`w[w[0]] = 8;` evaluates the inner `w[0]` first

Default values

Element type	Default
`int`	0
`double`	0.0
`boolean`	false
`String` (any reference type)	`null`

Worked reminder

int[] w = new int[5]; w[0] = 3; w[1] = w[0] * 2; w[w[0]] = 8; leaves [3, 6, 0, 8, 0], because w[w[0]] is w[3]. For scores = {72, 65, 90, 58, 81}, scores.length is 5 and scores[5] throws ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 5. Swapping needs a temporary: int temp = v[0]; v[0] = v[2]; v[2] = temp;.

Tracing arrays in the exam

Draw the array as a row of boxes with the indices underneath. Update the boxes as each assignment runs. When an index is itself an array element, evaluate the inner one first. Before you answer, check every index against 0 to length - 1: if one is outside, the answer is the exception, not a value.

Compile and run-time errors

Message or exception	Cause
`cannot find symbol ... method length()`	Writing `scores.length()` for an array
`incompatible types: int cannot be converted to String`	Storing an `int` in a `String[]`
`incompatible types: possible lossy conversion from double to int`	Using a `double` as an index or length
`ArrayIndexOutOfBoundsException` (run time)	An index below 0 or at least `length`

Mistakes to avoid

Using length as the last index.
Writing length() for an array.
Expecting null Strings to behave like "".
Assuming b = a copies an array.
Forgetting a temporary variable when swapping.

Quick self-check

What is the default value of a boolean array element? Answer: false.
For int[] a = new int[8];, what are the valid indices? Answer: 0 to 7.
After int[] x = {5, 6, 7}; x[x[0] - 3] = 1;, what is x[2]? Answer: 1, because x[0] - 3 is 2.
What does new String[2] contain? Answer: two null elements.