1. Arrays and Collections
Intro to the Java Collections Framework
CS-2851
Dr. Mark L. Hornick

2. CS-1020
12/4/2018
Java Array Review
Declaration of an array: <datatype>[] <variable>; // approach 1
<datatype> <variable>[]; // approach 2
Creation of the actual array: <variable> = new <datatype>[<size>];
int[] scores;
scores = new int[12];
Variation 1
int scores[];
scores = new int[12];
Variation 2
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate.
Arrays are created like objects!
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

3. The distinction between declaration and creation
CS-1020
12/4/2018
The distinction between declaration and creation
int[] scores; // or int scores[]
After Declaration of an array, a variable that references an array is created somewhere in memory
But since there’s no actual array (yet), the variable has a value of null.
Start of Memory …
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate.
scores
null …
End of Memory
Dr. Mark L. Hornick

4. The distinction between declaration and creation
CS-1020
12/4/2018
The distinction between declaration and creation
int[] scores; // or int scores[]
scores = new int[12];
After Creation of the array, the memory for the array elements is allocated and the variable is assigned to reference the first element of the array
All elements of the array are initialized (in this case) to 0. …
scores
addr
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate. …
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

5. After assigning values to the array elements:
CS-1020
12/4/2018
After assigning values to the array elements:
int[] scores; // or int scores[]
scores = new int[12];
for( int i=0; i<scores.length; i++ )
scores[i] = i;
scores
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate.
The index of the first
position in an array is 0.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

6. CS-1020
12/4/2018
Another approach: Simultaneous declaration creation, and initialization of an array
int[] scores ={8,7,10,4,5,6,5,8,9,10,5,8};
scores 1 2 3 4 5 6 7 8 9 10 11
scores[2]
This indexed expression refers to the element at position #2 8 7 10 4 5 6 5 8 9 10 5 8
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

7. Declaring an array of objects
CS-1020
12/4/2018
Declaring an array of objects
Person[] people;
people = new Person[3];
The people array is actually an array of references to Person objects
After Creation of the array, the memory for the array elements is allocated and the variable is assigned to reference the first element of the array
All elements of the array are initialized (in this case) to null. …
people
addr
null
null
null
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate. …
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

8. Creating an array of objects
CS-1020
12/4/2018
Creating an array of objects
Person[] people;
people = new Person[3];
people[0] = new Person(A);
When a Person object is created, the object is placed somewhere in memory. The new operator returns this address in memory.
After initializing an array element to reference an instance of a Person object, the array element is assigned to reference to the Person object …
people
addr
addr
null
null
Person
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate. …
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

9. Another way of looking at it
CS-1020
12/4/2018
Another way of looking at it
Code
Person[ ] people;
people = new Person[20];
people[0] = new Person(A);
One Person object is created and the reference to this object is placed in position 0. C 1 2 3 4 16 17 18 19
person
people 1 2 3 4 16 17 18 19
After is executed C
Person A
State of
Memory
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

10. CS-1020
12/4/2018
Simultaneous declaration creation, and initialization of an object array
Person[] people ={new Person(A), new Person(B), new Person(C)};
people 1 2
Person A
Person B
Person C
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

11. Element Deletion A Before is executed After is executed 1 2 3 people 1
CS-1020
12/4/2018
Element Deletion
int index= 1;
people[index] = null;
Delete Person B by setting the reference in position 1 to null. What problem does this introduce? A 1 2 3
people A B C D 1 2 3
people A C D
Before is executed
After is executed
In this approach, we simply leave the position of a deleted object to a null. With this approach, an array index positions will be a mixture of null and real pointers.
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

12. Element Insertion (?) A Before is executed A 1 2 3 people A B C D
CS-1020
12/4/2018
Element Insertion (?)
int index= 1;
people[index] = new Person(E);
What happens?? A 1 2 3
people A B C D
In this approach, we simply leave the position of a deleted object to a null. With this approach, an array index positions will be a mixture of null and real pointers.
Before is executed A
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick

13. Element Insertion problems
CS-1020
12/4/2018
Element Insertion problems
int index= 1;
people[index] = new Person(E);
The old element is simply replaced. What happened to B??? A 1 2 3
people A B C D
people 1 2 3 A E C D
In this approach, we simply leave the position of a deleted object to a null. With this approach, an array index positions will be a mixture of null and real pointers.
Before is executed A
After is executed A
Portions adapted with permission from the textbook author.
CS-2851
Dr. Mark L. Hornick
Dr. Mark L. Hornick