Arrays

1

• Arrays

2
Arrays What good are they?

• Arrays are a list of data
• They are very good at storing a number of
  variables of the same data type.
• These can be of any data type, primitive or
  reference

3
Arrays

• Declaration
• ltdata typegt ltnamegt
• Initialization
• ltnamegt new ltdata typegtltsizegt
• Data type can be primitive or reference
• char alphabet
• alphabet new char26

4
Arrays

• Arrays are objects in java
• They have methods like toString( ) and data
  members like length
• They are special objects, and like strings have a
  number of ways they can be initialized
• alphabet a, b, c, , y, z

5
Arrays

• What does this look like in memory?

6
Arrays

• What does this look like in memory?

alphabet
a
b
c
d
e
f
g


y
z
7
Arrays

• How do we index (access) the individual ellements?

alphabet
a
b
c
d
e
f
g


y
z
8
Arrays

• How do we index (access) the individual ellements?

alphabet
a
b
c
alphabet0
d
e
f
g


y
z
9
Arrays

• How do we index (access) the individual ellements?

alphabet
a
b
c
alphabet0
d
e
alphabet1
f
g


y
z
10
Arrays

• How do we index (access) the individual ellements?

alphabet
a
b
c
alphabet0
d
e
alphabet1
f
g

alphabet25

y
z
11
Arrays

• These are the indices

alphabet
a
b
c
alphabet0
d
e
alphabet1
f
g

alphabet25

y
z
12
Arrays

• The index to an array can be any expression as
  long as it is within the valid range
• alphabet0
• alphabet27
• alphabetxy2
• alphabet(int) Math.rand()25
• The only problem occurs if the index is
• Less than 0
• Greater than or equal to the length

13
Arrays What good are they?

• Example
• int testGrade new int9
• for(int i 0 i lt testGrade.length i)
• testGradei inBox.getInteger(
• Enter test score for student i)
• Notice length is a public data member, not a
  method!

14
Arrays What good are they?

• Example
• double average 0
• for(int i 0 i lt testGrade.length i)
• average testGradei
• average average / testGrade.length
• System.out.println(Test Average average)
• Notice length is a public data member, not a
  method!

15
Arrays What good are they?

• Questions???

16
Arrays Objects

• Arrays of Objects are just as useful as arrays of
  primitives
• They are declared and initialized the exact same
  way
• String names new String9
• String names Katie, Andy, Gil

17
Arrays Objects

• Using
• String names new String9
• Initially, all elements are null
• Each element must be defined

18
Arrays Objects

• String names new String9

names
null
null
null
null
null
null
null
null
null
19
Arrays Objects

• String names new String9
• names0 new String(Gil)

names
Gil
null
names0
null
null
null
null
null
null
null
20
Arrays Objects

• String names new String9
• names0 new String(Gil)
• names8 Phil

names
Gil
null
names8
null
null
null
null
null
null
Phil
21
Arrays Objects

• String names new String9
• names0 new String(Gil)
• names8 Phil
• for(int i1 ilt8 i)
• namesi alphabeti il

names
Gil
names8
null
bil
null
null
null
null
null
Phil
22
Arrays Objects

• String names new String9
• names0 new String(Gil)
• names8 Phil
• for(int i1 ilt8 i)
• namesi alphabeti il

names
Gil
names8
bil
null
null
null
cil
null
null
Phil
23
Arrays Objects

• String names new String9
• names0 new String(Gil)
• names8 Phil
• for(int i1 ilt8 i)
• namesi alphabeti il

This would eventually initialize all the elements.
names
Gil
names8
bil
null
null
null
cil
null
null
Phil
24
Arrays Objects

• Elements must be initialized before they can be
  used
• After an element has been initialized, they can
  be used just like regular variables
• names1.equals(names7)
• System.out.println(names5
• received a grade of alphabet0)

25
Arrays Objects

• Questions???

26
Arrays

• Exercise
• Write a code segment that
• declares a String array of size 10
• Initializes it with 10 Strings entered by the
  user (hint use an InputBox)
• Prints the string with the smallest length
• Prints the string with the max length

27
Arrays

• Removing objects from arrays
• You can simply replace it with a new object
• names0 new String(Gil 2)

names
Gil
null
names0
null
null
null
Gil 2
null
null
null
null
28
Arrays

• Also like other objects, two elements can point
  to the same object
• names0 Moe
• names1 names0

names
Moe
names0
null
null
null
null
null
null
null
29
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(squares)
• public void method1(int pattern)
• // do something here

squares
1
2
4
9
16
25
36
49
64
30
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(squares)
• public void method1(int pattern)
• // do something here

squares
1
2
pattern
4
9
16
25
36
49
64
31
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(squares)
• public void method1(int pattern)
• // do something here

squares
1
2
pattern
4
9
16
25
36
49
pattern0 squares0 ? true pattern1
squares1 ? true pattern ? squares
64
32
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(squares)
• public void method1(int pattern)
• // do something here

squares
1
2
pattern
4
9
16
25
36
49
pattern0 squares0 ? true pattern1
squares1 ? true pattern squares ? true
64
33
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(names)
• public void method1(String people)
• // do something here

names
Moe
null
null
null
null
null
null
null
34
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(names)
• public void method1(String people)
• // do something here

names
Moe
null
people
null
null
null
null
null
null
35
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(names)
• public void method1(String people)
• // do something here

names
Moe
null
people
null
null
null
null
null
people1.equals(names1) people1
names1 people names
null
36
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(names)
• public void method1(String people)
• // do something here

names
Moe
null
people
null
null
null
null
null
people1.equals(names1) ? true people1
names1 people names
null
37
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(names)
• public void method1(String people)
• // do something here

names
Moe
null
people
null
null
null
null
null
people1.equals(names1) ? true people1
names1 ? true people names
null
38
Arrays

• Just like regular objects, you can pass and
  return arrays to and from methods
• example.method1(names)
• public void method1(String people)
• // do something here

names
Moe
null
people
null
null
null
null
null
people1.equals(names1) ? true people1
names1 ? true people names ? true
null
39
Arrays

• Questions????