COMP-202: Foundations of Programming

Lecture 5: Arrays, Reference Type, and Methods

Sandeep Manjanna, Summer 2015

Announcements

• Assignment 2 posted and due on 30th of May

(23:30).

• Extra class tomorrow - 13:05 to 15:25 in TR0100.

• Midterm Exams : June 4th (12:35 – 14:35)

Rooms : 1B45 and 306 (Burnside)

• Final Exams : June 26th (14:00 – 17:00)

Room : MAAS 112

Review• Loops

1. In a group of nested loops, which loop is executed the most

number of times?

A. The outermost loop

B. The innermost loop

C. All loops are executed same number of times

2. What value is stored in x at the end of this loop?

for (x = 1; x

Review• While Loops

int x = 0;

while (x < 4)

{

System.out.println("Write this again");

x++;

}

Step

1

Step

2

Step

3

Initialize

Condition Check

Loop body

Execution

Once the

condition fails

Condition

Fail

Review• For Loops

for (int x = 0; x < 4; x++)

{

System.out.println("Write this again");

}

Step

1

Step

2

Initialize

Condition Check

Loop body

Execution

Once the

condition fails

Condition

Fail

Step

4 Update

Step

3

• Array of Scores of 8 students:int [ ] scores = {100, 95, 25, 99, 39, 100, 100, 90}

• Here is an array with 8 values in it.

• Each spot of the array has a value and an index.

• The value is any legitimate value of the type of the array, in this case int.

• The index is an integer. Interestingly the counting starts from 0 instead of 1.

• To get or set values of an array, we will use a similar syntax to a normal variable, except we have to specify which value of the array we wish to get or set. We do this by using the index.

100 95 25 99 39 100 100 90

[0] [1] [2] [3] [4] [5] [6] [7]

scores

Indices

Review

Creating Array (Method 1)

int[] myArray = {1, 5, 6, 3};

1) Allocate this memory. Find a free spot in memory that has

enough space to hold four integers.

2) Store the address of the beginning of the memory that was just

allocated into the variable myArray.

3) Set values to this newly allocated memory.

1 5 6 3

myArray

myArray

Review

Creating Array (Method 2)

This method is useful when we don’t know the values that

needs to be saved in the array:

int[] myArray;

myArray = new int[4];

myArray[0] = 1;

……1

Variable “myArray” is created

to point to an array.

myArray

myArray

[0] [1] [2] [3]

Indices

Review

This Lecture

� Reference Types

� Reference vs Primitive Types

� Multidimensional Arrays

� Methods

One step at a time….

Adding to our vocabulary -“break”

• ‘break’ is used to stop the execution of a loop before all the iterations are completed.

• Example:int i;

for(i=0; i

Objects

• Arrays, Scanners, and Strings are Objects.• You can tell because you need new to create them.

(Well, except Strings. Strings are special.)

• In fact, except for the primitive data types (the ones that start with lowercase, like int, double, float, byte, boolean, etc.), everything in Java is an Object.

• Objects are data bundled with methods to work with the data, and properties.

Methods and Properties of Objects

• Some String methods:• .equals(), .length(), .toLowerCase()

• Some Scanner methods:• .nextLine(), .nextInt(), .nextDouble()

• In general: object.method_name()

• Arrays have no methods. Instead, they have a property called length

• .length

• Because this is not a method, there is no () after

Reference Types

Reference Types vs Primitive Types

When we write

int x = 10;

what we are technically doing is the following:

1) Creating a space in memory that will store an int

2) Specifying that whenever we use the identifier x later in our program, we want to access that memory location.

3) Storing the value of 10 into that memory location.

10

x

Objects Using Reference Types

• The variables of objects, including arrays, don't directly store the values of the objects.

• Instead, they store a reference to the location in memory containing the value.

• Think of this as storing an address, which points to where the data is actually located in memory.

Reference Types vs Primitive Types

When we write

int x[] = {10, 3, 5};

what we are technically doing is the following:

1) Creating a space in memory that will store the address of an int array.

2) Specifying that whenever we use the identifier x later in our program, we want to access that space in memory (which stores an address).

3) Creating an array somewhere else in memory. That array will store the values 10, 3, and 5 and have length of 3. That array must be located somewhere in memory. Call that address ‘a’.

4) Setting the value of x to be ‘a’ (In other words, the value of a is stored into the space of memory reserved for the variable x).

Why references can get Confusing?

• There are some interesting consequences of storing addresses into variables instead of the values.

1. 2.int[] x = {1, 2, 3}; int x = 5;

int[] y = x; int y = x;

y[0] = 2; y = 10;

System.out.println(x[0]); System.out.println(x);

• What will the above print?

1. 2 ! Because changing y[0] also changes x[0]. x and y are

reference types.

2. 5 Because x and y are primitive types in here.

One Step at a time…

Memory

0 4 …. 1000 1004 1008 …. 2004 …. 2112

Addresses

Values

int[] x = {1,2,3}

1 2 3

The {1, 2, 3} creates an array in memory. The address of the start

of this spot in memory is 1000.

One Step at a time…

Memory

0 4 …. 1000 1004 1008 …. 2004 …. 2112

Addresses

Values

int[] x = {1,2,3}

1 2 3

int[] x creates a variable of type int array. This means we create

space in memory to store the address of an int array. In this case,

we use the storage space in memory of 2004, to store the address

1000.

x

1000

One Step at a time…

Memory

0 4 …. 1000 1004 1008 …. 2004 …. 2112

1000

Addresses

Valuesint[] x = {1,2,3}

1 2 3

int[] y = x;

This means:

1) Create space in memory to store an int[] variable

2) Store into that int[] variable the value of the expression “x”

yx

1000

One Step at a time…

Memory

0 4 …. 1000 1004 1008 …. 2004 …. 2112

1000 1000

Addresses

Values

1 2 3

int[] x = {1,2,3}

int[] y = x;

y[0] = 2;

This means:

1) Assign to the first value of the array referred to by y, the value

of the expression “2”

yx

2

Can also be seen as…

int[] x = {1,2,3}

int[] y = x;

y[0] = 2;

y

x

1 2 32

Consequences: Equality

int[] arr1 = {1, 2, 3};

int[] arr2 = {1, 2, 3};

System.out.println(arr1 == arr2);

Here the addresses stored in arr1 and arr2 variables are getting compared. Not the values.

This is also why we have to use .equals() to compare the values of Strings.

Why would one EVER make things so complicated?

It turns out there are some very good reasons that will be

crucial to the idea of object oriented programming.

One useful feature is the ability to pass addresses of data

means it's easier to share (and modify!) data between

methods!

We will see an example about how this helps once we

learn about methods.

Why so Confusing!!?

Try it out!!

• Write some code that actually copies an array of ints (i.e., not just make it point to the same part of memory).

e.g.,int[] arr1 = {1, 4, 3, 6, 7};

int[] arr2;

// make arr2 have a copy of the data in arr1

Common Array Problems 1

• Referring to an index that doesn't exist.ArrayIndexOutOfBounds exception

int[] x = {1, 2, 3};

System.out.println(x[-1]);

System.out.println(x[3]);

Common Array Problems 2

• Trying to do operations on a null pointer.

NullPointerException

String[] names = new String[35]; System.out.println(names[0].length());

Multidimensional Arrays

• So far, the elements of all the arrays we have seen have been simple values: primitive types or Strings.

Such arrays are one-dimensional

• However, we can create arrays whose elements are themselves one-dimensional arrays

Such an array is really an array of arrays

• These arrays are two-dimensional arrays (or 2D arrays)

Two Dimensional Arrays

• Elements in a two dimensional array are accessed using two indices.

• The first index specifies the one-dimensional array containing the element we want to access.

• The second index specifies the element we want to access within the one-dimensional array specified by the first index.

Two Dimensional Arrays

Creating 2D Arrays

To create an array using an initializer list, you could write:

type[] variableName = {exp1, exp2, exp3, ...}, where exp1, exp2, exp3, etc all evaluated to type

• Well, in this case type is just an array. But we need array of arrays,

int[][] board = { {1,2,3}, {1,4}, {6,2} }

This is creating an array of int[] . The first array in the array is {1,2,3} the second is {1,4} and the third is {6,2}

Creating a 2D Array

int[][] board = { {1,2,3}, {1,4}, {6,2} }

We can use new operator to create multidimensional arrays.

Example:

type[][] variableName= new type[size_1][size_2];

1 2 3

1 4

6 2

board[0]

board[1]

board[2]

board

Accessing a 2D Array

int[][] x = new int[3][4];

//creates an array which contains 3 arrays of size 4.

x[0][1] = 3;

//assigns to the 2nd spot of the 1st array, the value 3;

x[2][3] = 10;

//assigns to the 4th spot of the 3rd array, the value of 10;

x[10][2] = 5;

//causes an array out of bounds exception

int[] y = x[2];

//stores into y the address of the 3rd array

What would be printed???

int [][] x = {{1,2,3},{4,5,2,7},{9,3,5,2,6}};

System.out.println(x.length);

System.out.println(x[2].length);

System.out.println(x[0]);

System.out.println(x[0][1]);

Two Dimensional Arrays

String(More about Strings)

String vs char[]

• A String is just a fancy version of char[] –both store an array of char values.

• A String has in addition:• Useful methods like .equals(), .toLowerCase()

• A special way to be created, using "" syntax

• (So we don't have to go {'H', 'e', 'l', 'l', 'o', ',', ' ', 'W', 'o', 'r', 'l', 'd'} as for a char[].)

More String Methods

.charAt(int i) Gets the ith char of a String (starting from 0)

.substring(int a, int b) Gets the substring starting at position a ending just before position b.

.compareTo(String other) Tells you which String comes first, orthographically.

.indexOf(char c) Gets the index of the first occurrence of c

.toLowerCase() Gets a lower-case version of the String.

.toUpperCase() Gets an upper-case version of the String

A Note on Describing Methods

• When you see something like:.substring(int a, int b)

• This means this is a method that:1. takes two inputs (or arguments, parameters).

2. the first is of type int

3. the second is of type int as well

• You call the method by:"some string".substring(1, 4)

• The "int"s are just there as reminders. Don't type them in!

Try it out!!

• Write some code that turns a String into a format that is like a name—i.e., the first letter is capitalized, and all subsequent ones are not.

"bob" � "Bob"

"BOB" � "Bob"

"BoB" � "Bob"

"bOb" � "Bob"

• Hint: use the methods Character.isLowerCase(char ch) and/or Character.isUpperCase(char ch) to check the case of a char.

Caesar Cipher

• Used in the ancient Roman army to obscure important messages

ATTACKATMIDNIGHT*

DWWDFNDWPLGQLJKW

• Encrypt by shifting all letters by three positions

A (1st letter) � D (4th letter)

T (20th letter) � W (23rd letter)

*Ancient Romans didn't use lowercase letters, or spaces

• Implement the encryption algorithm for a Caesar cipher

• Steps:

• Read each character of plaintext (String.charAt(int i))

• Convert it to its numerical value (by casting)

• Add 3

• Tricky: implement wrap-around (e.g., Z � C)

• Convert it back to a char, and save it or print it out

Try it out!!

String[] args

• So what exactly is args there for?

public static void main(String[] args)

• Another way to pass input data to a program

Command line: java NameOfClass arg1 arg2 … argN

DrJava: run NameOfClass arg1 arg2 … argN

• Examples of uses:

• Name of file to open when you run the program

Command-line Calculator

• This program takes three arguments. The first represents an int , the second is a single character, in {'+', '-', '*', or '/'}, and the third is another int. It prints the result of computing the calculation.

e.g., run Calc 3 + 4 � 7

run Calc 88 / 5 � 17

run Calc 2 * -111� -222

Methods

Calling Methods

• You have been calling a lot of methods already

• Some examples?String.equals()

String.length()

Using Java Libraries

• The JDK comes with many libraries, which contain classes and methods for you to use.

• String library

• Math library

• Swing library (used for graphics)

• Libraries for networking

Math Library

• Contains useful and common methods you can use off-the-shelf (i.e., don't have to write yourself)

• Also, constants like E or PI

• How do you access them?

import java.lang.math;

Documentation

• Read the Application Programming Interface(API) for the specification of a library.

• e.g., Math library: http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html

• What information do you get from an API?

Sample Entry

static double abs(double a)

Returns the absolute value of

a double value.

Name of the method

Sample Entry

static double abs(double a)

Returns the absolute value of

a double value.

The number and type

of input arguments

The name of the method together with the

number and type of input arguments is called the

signature of the method.

static double abs(double a)

Returns the absolute value of

a double value.

Description of what the

method does.

Sample Entry

Sample Entry

static double abs(double a)

Returns the absolute value of

a double value.

The return type of the method

static double abs(double a)

Returns the absolute value of

a double value.

This keyword means this method does

not have to be called on a particular

object. Don't worry too much about it

for now.

Sample Entry

static double abs(double a)

Returns the absolute value of

a double value.

Based on this API entry, we know that we call

use this method with something like:

Math.abs(-4.0)

and that this expression evaluates into the value

4.0 with type double.

Sample Entry

static double abs(double a)

Returns the absolute value of

a double value.

We also know that

Math.abs("5.0")

would result in an error.

Sample Entry

Try it out!!

• Give the input arguments (with types), and return types of the following Math library methods, and describe what they do.

Math.pow

Math.random

Math.max

Summary

� Reference Types

� Multidimensional Arrays

� Introduction to Methods