Ch 4 Objects and References

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Ch 4 Objects and References
Qi ZhangOct 6
COMP 110 Introduction to Programming
2
Questions

• What is the output?
• s1 Hello!
• s2 Hey!
• s1 s2
• s1 Sure.
• s2 Okey Dokey.
• System.out.print(s1)

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3
Questions

• Which one(s) of a,b,c,d is(are) false after
  the following statements are executed?
• s1 Hello!
• s2 Hey!
• s3 Hey!
• s1 s2
• s1 s2
• s1.equals(s2)
• s2 s3
• s2.equals(s3)

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4
Questions

• True or False
• The return type of constructor is void
• The return type of constructor is the same type
  pf the class.

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Announcement

• Assignment 3 today
• Lab 5 Fri

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Today in COMP 110

• More on Strings, Classes, and Memory
• Objects, references, passing parameters

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Strings

• Declaring and initializing strings
• String s1 a string
• String s2 new String(a string)
• What is the difference?
• String s3 new String(s2)
• s3 s2

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Defining A Class

• Typical Order in the Java Source File
• data members
• constructor(s)
• other methods

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class Rectangle
class name
data members
methods
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Constructor
public class Rectangle private int
length private int width
public Rectangle (int l, int w) length
l width w
// definition of other classes
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Constructor
Rectangle r2 new Rectangle (5, 10)
public class Rectangle private int
length private int width
public Rectangle (int l, int w) length
l width w
// definition of other classes
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Rectangles in Memory
Rectangle r1 new Rectangle (5, 10)

• Rectangle r2 new Rectangle (20, 30)

3800
r1
10
3800
5
4500
r2
4500
20
30
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Using the Rectangle Class
r

• Create an object
• Rectangle r
• r new Rectangle(2, 3)
• OR
• Rectangle r new Rectangle(2, 3)
• Use the object and the dot operator to access
  methods
• r.setLength(5)
• r.setWidth(10)

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Using the Rectangle Class
r
2500

• Create an object
• Rectangle r
• r new Rectangle(2, 3)
• OR
• Rectangle r new Rectangle(2, 3)
• Change the values of instance variables using
  accessors
• r.setLength(5)
• r.setWidth(10)

2500
3
2
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Using the Rectangle Class
r
2500

• Create an object
• Rectangle r
• r new Rectangle(2, 3)
• OR
• Rectangle r new Rectangle(2, 3)
• Change the values of instance variables using
  accessors
• r.setLength(5)
• r.setWidth(10)

2500
3
2
3
2
10
5
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Calling methods

• int answer math.add(4, 6)
• answer math.add(answer, 7)

• public int add(int a, int b)
• return (a b)

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Global vs. Local

• Variables are local to methods
• Instance variables are global for all methods in
  a class

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public int computeArea()
int area width length return
area
public boolean ifBigRectangle()
if(area gt 100) return area
else return false
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Class Parameters

• When a parameter in a method invocation is a
  primitive type, the value of the parameter is
  copied.
• When a parameter in a method invocation is a
  reference to a named object, the addess is
  copied, not the value of the object.
• public void countBigRec(int count, Rectangle
  rec)
• if (rec.ifBigRectangle() true)
• rec.length rec.length/2
• count

count4 countBigRec(count, rec)
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Class Parameters
2500
50 10
4
count
2500
rec

• public void countBigRec(int count, Rectangle
  rec)
• if (rec.ifBigRectangle() true)
• rec.length rec.length/2
• count

count4 countBigRec(count, rec)
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Class Parameters
2500
50 10
4
count
2500
rec

• public void countBigRec(int count, Rectangle
  rec)
• if (rec.ifBigRectangle() true)
• rec.length rec.length/2
• count

2500
rec(local)
4
count(local)
count4 countBigRec(count, rec)
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Class Parameters
2500
25 10
4
count
2500
rec

• public void countBigRec(int count, Rectangle
  rec)
• if (rec.ifBigRectangle() true)
• rec.length rec.length/2
• count

2500
rec(local)
4
count(local)
count4 countBigRec(count, rec)
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Class Parameters
2500
25 10
4
count
2500
rec

• public void countBigRec(int count, Rectangle
  rec)
• if (rec.ifBigRectangle() true)
• rec.length rec.length/2
• count

2500
rec(local)
5
count(local)
count4 countBigRec(count, rec)
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Class Parameters
2500
3500
50 10
5 15
4
count
2500
rec
3500
rec2

• public void countBigRec(int count, Rectangle
  rec, Rectangle rec2)
• if (rec.ifBigRectangle() true)
• rec rec2
• count

2500
rec(local)
5
count(local)
3500
rec2(local)
count4 countBigRec(count, rec)
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Class Parameters
2500
3500
50 10
5 15
4
count
2500
rec
3500
rec2

• public void countBigRec(int count, Rectangle
  rec, Rectangle rec2)
• if (rec.ifBigRectangle() true)
• rec rec2
• count

3500
rec(local)
5
count(local)
3500
rec2(local)
count4 countBigRec(count, rec)
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Assignment 3
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In-class Exercises-1

• Add a method getName() to Student class
• Test it add s1.getName after s1.name
  "Jim" in StudentStats.java, compile and run
  StudentStats.java
• Add a private data member secondMajor to Student
  class
• Add a method setSecondMajor to Student class,
  with one input parameter (the 2nd major) of type
  String
• Test it add s1.setSecondMajor(Biology)
  after s1.major "Computer Science" in
  StudentStats.java. Compile and run
  StudentStats.java.
• Modily printData() to also output the second
  major, compile and run StudentStats.java again.

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In-class Exercises-2
Write the class Rectangle with empty methods