CS2851

1
Arrays and Collections

• Intro to the Java Collections Framework

2
Java Array Review

• Declaration of an array ltdatatypegt
  ltvariablegt // approach 1
• ltdatatypegt ltvariablegt // approach 2
• Creation of the actual array ltvariablegt
  new ltdatatypegtltsizegt

3
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 theres no actual array (yet), the
  variable has a value of null.

Start of Memory

scores
null

End of Memory
4
The distinction between declaration and creation

• int scores // or int scores
• scores new int12
• 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
0
0
0
0
0
0
0
0
0
0
0
0

5
After assigning values to the array elements

• int scores // or int scores
• scores new int12
• for( int i0 iltscores.length i )
• scoresi i

The index of the first position in an array is 0.
6
Another approach Simultaneous declaration
creation, and initialization of an array
8
7
4
5
6
5
8
9
10
5
8
10
7
Declaring an array of objects

• Person people
• people new Person3
• 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

8
Creating an array of objects

• Person people
• people new Person3
• people0 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

9
Another way of looking at it
Code
One Person object is created and the reference to
this object is placed in position 0.
State of Memory
10
Simultaneous declaration creation, and
initialization of an object array
0
1
2
Person B
Person C
11
Element Deletion
Delete Person B by setting the reference in
position 1 to null. What problem does this
introduce?
12
Element Insertion (?)
What happens??
13
Element Insertion problems
The old element is simply replaced. What happened
to B???
0
1
2
3
D
A
C
E