Session 04 - Arrays in Java

Java & JEE Training
Session 4 – Handling Arrays in Java

Classification: Restricted
Agenda
• Autoboxing and Unboxing in Java
• Handling Arrays

Autoboxing and Unboxing in Java

• Autoboxing is the automatic conversion that the Java compiler makes
between the primitive types and their corresponding object wrapper
classes. For example, converting an int to an Integer, a double to a Double,
and so on.
• If the conversion goes the other way, this is called unboxing.
• Introduced in JDK 5

Arrays
• We need a way to store and manipulate huge quantities of data.
• “An array is an indexed sequence of values of the same type.”
Examples.
• 52 playing cards in a deck.
• 5 thousand undergrads at Princeton.
• 1 million characters in a book.
• 10 million audio samples in an MP3 file.
• 4 billion nucleotides in a DNA strand.
• 73 billion Google queries per year.
• 50 trillion cells in the human body.

Goal: Many variables of the same type

Arrays in Java
Java has special language support for arrays.
• To make an array: declare, create, and initialize it.
• To access element i of array named a, use a[i].
• Array indices start at 0.
Compact alternative.
• Declare, create, and initialize in one statement.
• Default initialization: all numbers automatically set to zero.

Declaring Arrays – Two ways

Exercise: Vector dot product
Dot product. Given two vectors x[] and y[] of length N, their dot product is
the sum of the products of their corresponding components.

Array Processing Examples

Example: Card shuffling algorithm
Goal. Given an array, rearrange its elements in random order.
• Shuffling algorithm.
• In iteration i, pick random card from deck[i] through deck[N-1], with each
card equally likely.
• Exchange it with deck[i].

Two Dimensional Arrays
Two-dimensional arrays.
• Table of data for each experiment and outcome.
• Table of grades for each student and assignments.
• Table of grayscale values for each pixel in a 2D image.
Mathematical abstraction. Matrix.
Java abstraction. 2D array.

Two Dimensional Arrays in Java
• Array access. Use a[i][j] to access element in row i and column j.
• Zero-based indexing. Row and column indices start at 0.
• a[i] is the i’th row, which is a one dimensional array here.

Initialize 2D array : Inline initialization

Example: Matrix Addition

Example: Matrix Multiplication

Example: Using arrays to simplify repetitive code
How to simplify writing the above code?

Example: Using arrays to simplify repetitive code

Default Initialization: Class / instance variable & Arrays
Each class variable, instance variable, or array component is initialized with a
default value when it is created:

Exercise: What is the output? Why?

Exercise: What is the output? Why?
Attempts to create an array of size n^4 for n = 1000.
This program compiles cleans.
When n is 1000, it leads to the following error
java.lang.NegativeArraySizeException
because 1000^4 overflows an int and results in a negative integer.
When n is 200, it leads to the following error
java.lang.OutOfMemoryError: Requested array size exceeds VM limit

Exercise: What is the output?
What is wrong with the following code fragment?

What is wrong with the following code fragment?
It does not allocate memory for a[] with new. The code results in a variable
might not have been initialized compile-time error.

It prints false. The == operator compares whether the (memory addresses
of the) two arrays are identical, not whether their corresponding values are
equal.

What is the output?
• What happens when you try to compile a program with the following
statement?
int[] a = new int[-17];

What is the output?
• What happens when you try to compile a program with the following
statement?
int[] a = new int[-17];
It compiles cleanly, but throws a java.lang.NegativeArraySizeException
when you execute it.

What is the output?
• Suppose that b[] is an array of 100 elements, with all entries initialized to 0,
and that a[] is an array of N elements, each of which is an integer between
0 and 99. What is the effect of the following loop?

Iterating through elements in an Array – Two ways
• for Loop
• foreach Loop (introduced in Java 1.5)

Arrays – Sorting and Searching
• java.util package has API for search and sort
• Arrays.sort() –
• Default sorting is ascending order
• But this can be used with sorting order Collections.reverseOrder()
which can be used for sorting in descending order, but can be used
only with Object type and not primitive type
• Arrays.binarySearch()

Exercise
• Given an array of sorted elements, search for the element requested.
• Given a 2D array that is sorted column wise and row wise, write an
algorithm to find if an element exists in the matrix or not. Keep the
temporal complexity of the algorithm in mind.

Topics to be covered in next session
• Memory Allocation & Garbage Collection
• Strings in Java

Thank You!

Session 04 - Arrays in Java

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