college/Summer-2024/CS-3443/Slides/txt/05_Arrays-and-ArrayLists.txt

Application
Programming
Hend Alkittawi

Basic Java Data Structures
Arrays and ArrayLists, Wrapper
Classes and Static Methods

JAVA ARRAYS
-

Arrays in Java are objects which can be declared
-

by size
int[] myNumbers = new int[4];
Employee[] employees = new Employee[10];

-

by values directly (using an array initializer)
double[] myNumbers = {10.5, 20.34, 30.8, 40.12};
Account[] accounts = {accountOne, accountTwo};

-

Access values in an array by providing an index
double x = myNumbers[0];

-

Overwrite values at an index by using assignment
x = x + 5;
myNumbers[1] = x;

-

Loop over array elements to fill array, modify elements,… etc.
for (int i = 0; i < myNumbers.length; i++){
myNumbers[i] = i * 10;
System.out.println(myNumbers[i]);
}

JAVA ARRAYS
-

Recall the String manipulation methods
String stringA = "I’m out of candy corn, send help!";
for( int i = 0; i < stringA.length(); i++ ){
char c = stringA.charAt(i);
System.out.print( c );
}
String[] sentences = stringA.split( "," );
System.out.println( sentences.length );
System.out.println( sentences[1].trim() );

JAVA ARRAYLISTS
-

An ArrayList object is an array that can grow or shrink as
needed! Use an ArrayList when you don’t know how many of
something you need.

-

To create an ArrayList:
ArrayList<Integer> grades = new ArrayList<Integer>();
ArrayList<String> letters = new ArrayList<String>();
ArrayList<Employee> employees = new ArrayList<Account>();
ArrayList<Account> accounts = new ArrayList<Account>();
More on generics later!

JAVA ARRAYLISTS
-

Some useful methods for working with ArrayLists:
-

add() to add an object to the ArrayList

-

get(int index) to get an object from the ArrayList

-

contains() to check if an element is in the ArrayList

-

size() to get the number of elements currently in the ArrayList

-

remove(int index) to remove an object from an index in the
ArrayList
ArrayList<Integer> grades = new ArrayList<Integer>();
grades.add(5);
boolean present = grades.contains(7);
ArrayList<String> letters = new ArrayList<String>();
letters.add("CS3443");

WRAPPER CLASSES
-

Each primitive data type has a corresponding wrapper class,
which enables you to manipulate primitive type values as
objects. For example:
-

double has Double

-

int has Integer

-

char has Character

-

boolean has Boolean

WRAPPER CLASSES
-

The conversion between the primitive data type and wrapper
class type is mostly automatic
-

converting a primitive type to wrapper class is called
autoboxing.

-

converting a wrapper class object to primitive type is called
unboxing.

Double dbox = Math.sqrt(2);
double d = 1.0 / dbox;

// autoboxing
// unboxing

WRAPPER CLASSES
-

Wrapper classes provide several methods for manipulating data.

-

Some of the methods provided by these classes:
-

Double.parseDouble() to translate a String into a double value

-

Integer.parseInt() to translate a String into a int value

-

Character.getNumericValue() to translate a specified Unicode
character into the int value that it represents.

-

Note that there is no object associated with these methods

STATIC METHODS
-

Methods we have seen so far execute in response to method
calls on specific objects.

-

Sometimes a method performs a task that does not depend on an
object. These methods are called static/class methods.

-

To declare a method as static, place the static keyword before
the return type in the method’s declaration.
public static void myMethod( arguments ) { method body }

-

To call a class’s static method, specify the class name
followed by a dot (.), and the method name.
ClassName.methodName(arguments);

STATIC METHODS
-

Some of the static methods in the String class:
-

String.valueOf() to get the String value of a given variable
of a primitive type
String s = String.valueOf( 350.4 );
System.out.println( s.charAt(3) );

-

String.format() to format a string, similar to sprintf in C.

String.format("Account object: name = %s, balance =
$%.2f", name, balance);

STATIC METHODS
-

The class Math contains static methods for performing basic
numeric operations such as the elementary exponential,
logarithm, square root, and trigonometric functions

-

Here are a few class methods to try:
double absValPos = Math.abs(13)
double absValNeg = Math.abs(-13)
double minVal
= Math.min(3, 4)

OBJECT VS. STATIC METHODS
-

There are two types of methods in Java
-

-

●

Object methods
-

Associated with an object

-

Sent as a message to an object

-

Implicitly passed to the current object

-

Keyword: this

Class/Static methods
-

Not associated with a particular object

-

Sent as a message to a class

-

Keyword: static

When to use static methods - Stack Overflow

CLASS ACTIVITY
-

Given the strings below, which of the following lines contain
an object method?
String greeting = "HI";
String obvious = "This is a string";
String strWithSpace = "
1.

greeting.toLowerCase();

2.

String.valueOf(55);

3.

obvious.indexOf( "is");

4.

String.valueOf(17.8);

5.

strWithSpace.trim();

This is a string.

";

CLASS ACTIVITY
-

Given the strings below, which of the following lines contain
an object method?
String greeting =

"HI";

String obvious = "This is a string";
String strWithSpace =᳚ "
1.

greeting.toLowerCase();

2.

String.valueOf(55);

3.

obvious.indexOf("is");

4.

String.valueOf(17.8);

5.

strWithSpace.trim();

This is a string.

";

ARRAYS AND ARRAYLISTS IN A JAVA CLASS
-

In the case where a class variable contains a data structure,
multiple setter methods should be created.
-

Setter to set the value of the entire data structure

-

Setter/Adder to add just one value to the data structure
public class HelloWorld{
private String[] messages;
public void setMessages(String[] texts){
this.messages = texts;
}
public void addMessage (String text){
This.messages[0] = text;
}
public void addMessage (String text, int index){
// code to add the value of text to the array
}
}

Method Overloading

ENHANCED for STATEMENT
-

The enhanced for statement iterates through the elements of an
array/arraylist without using a counter!

-

for (paramType parameter : arrayName){
/* statements that read/obtain array elements, cannot
modify elements with the enhanced for statement */
}
parameter has a type and an identifier.

-

the type of the parameter must be consistent with the the type
of elements in the array
for (int x : myNumbers){
sum = sum + x;
}

JAVA PACKAGES
-

Related classes are typically grouped into packages so that
they can be imported into programs and reused.

-

The ArrayList class is part of the java.util package, so the
package need to be imported to your class to be able to use
the ArrayList class.
import java.util.ArrayList;

-

The package java.lang is implicitly imported by the compiler,
so it is not necessary to import classes in that package to
use them.

-

String and Math are examples of classes in java.lang package

CODE DEMO
-

Create class(es) to
demo the use of arrays
and arraylists!
Create a class to demo
static methods and
variables.

DO YOU HAVE ANY
QUESTIONS?

THANK
YOU!

@

hend.alkittawi@utsa.edu

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