This document discusses several concepts related to objects in Java including: 1. It is possible to instantiate an object without assigning it to a variable. This creates an "anonymous" object. 2. Objects can be instantiated in one line by declaring and initializing the variable together, or in two lines by declaring the variable separately and then initializing it. 3. Assigning one object variable to another makes them references to the same object, not copies. 4. The equals() method compares object references by default, but can be overridden to compare object contents instead. 5. String literals with the same value are stored in a constant pool and multiple variables can reference the same string object.

1. Anonymous objects
It is possible to instantiate an object without a
name.
public void melloJello(Circle cirA)
osborne.melloJello(new Circle(5) );
The code, new Circle(5), instantiates the
object; however, in the region of the calling
code it doesn’t have a name.

2. Lesson 16

3. Private methods can only be accessed from
within the class itself.

4. Declaring and instantiating an object
Normally when we instantiate an object, we do
it in one line of code:
Circle cir1 = new Circle(3.0);
However, it can be done in two lines:
Circle cir1; //Here, cir1 is merely declared to
be of type Circle
cir1 = new Circle(3.0); //Here, it is finally
instantiated.

5. Setting two objects equal
Circle cir1 = new Circle(5.3); //cir1 has a radius of 5.3
We will now demonstrate how to declare a cir2 object,
but not to instantiate it.
Then in another line of code, set it equal to cir1:
Circle cir2; //cir2 has only been declared to be of type
Circle cir2 = cir1; //cir2 and cir1 now refer to the same
object. There is only one object. It simply has two
references to it.
Thus, cir2.area( ) returns exactly the same as cir1.area( )
….and cir1.radius is exactly the same as cir2.radius,…
etc.

6. Determining if two objects are equal
System.out.println(cir1 = = cir2);
Circle cir1 = new Circle(11);
Circle cir2 = new Circle(11);
System.out.println(cir1 = = cir2);
//false, in spite of the fact they both have a
radius of 11

7. System.out.println( cir1.equals(cir2) ); //false.
(cir1.equals(cir2) ) is equivalent to (cir1 = = cir2).
Circle class inherits the equals method from a superclass
Object and simply compares to see if we are referring to
the same object.
If the programmer who created the Circle class created an
equals method for it, then that overrides the inherited
method and compares the contents of the two objects
(likely the radii). In this case, the println above would
print a true since the contents of the two objects are the
same (they both have a radius of 11).

8. Strings
String s1 = “Hello”;
String s2 = “Hello”; //s1 and s2 are String
constants
System.out.println(s1 = = s2); // prints true

9. The String constant pool
String literals are stored as String constants in a separate
memory area called theString constant pool.
When object s1 is created “Hello” it is placed in the String
constant pool with the reference s1 pointing to it. Then, for
efficiency, when the reference (variable) s2 is created, Java
checks the pool to see If the String constant being specified
for s2 is already there. Since it is in this case, s2 also points to
“Hello” stored in the String constant pool.
Physically, s1 and s2 are two separate String object
references, but logically they are pointing to the same object
in theString constant pool. So, in (s1 = = s2) from the code
above we see that both s1 and s2 are referencing the same
object, and a true is returned.

10. Exercises for Lesson 16
Problems 1 – 5 refer to the following code (assume
that equals is not an explicit, method of this class):
MoonRock myRock = new MoonRock(3, “Xeon”);
MoonRock yourRock = new MoonRock(2,
“Kryptonite”);
MoonRock ourRock = new MoonRock(3, “Xeon”);
MoonRock theRock;theRock = ourRock;
1. Does theRock.equals(ourRock) return a true or
false?

11. 1.true

12. MoonRock myRock = new MoonRock(3, “Xeon”);
MoonRock yourRock = new MoonRock(2, “Kryptonite”);
MoonRock ourRock = new MoonRock(3, “Xeon”);
MoonRock theRock;theRock = ourRock;
2. Does theRock.equals(yourRock) return a true or
false?
3. Does theRock.equals(myRock) return a true or false?
4. Does myRock = = ourRock return a true or false?
5. Does myRock.equals(yourRock) return a true or
false?

13. 2-5. All False

14. Problems 6 – 11 refer to the following code:
public class Weenie{public Weenie( )
{ . . . }
public String method1(int jj)
{ . . . }
private void method2(String b)
{ . . . }
public int method3( )
{ . . . }
public double x;
public int y;
private String z;
}
Now suppose from within a different class we instantiate a Weenie
object,oscarMayer. All of the code in questions 6 – 11 is assumed
to be in this otherclass.
6. Is int zz = oscarMayer.method1(4);
legal? If not, why?

15. 6. Is int zz = oscarMayer.method1(4);
legal? If not, why?
No, method1 returns a String and we are
trying to store it in xx, an int.

16. 6. Is int zz = oscarMayer.method1(4);
legal? If not, why?
7. Is oscarMayer.method2(“Hello”);
legal? If not, why?
8. Is int cv = oscarMayer.method3( );
legal? If not, why?
9. Is int cv = oscarMayer.method3(14);
legal? If not, why?
10. Is oscarMayer.z = “hotdog”; legal? If not, why?
11. Assume the following code is inside method1:method2(“BarBQ”);
Is this legal? If not, why?

17. 6. Is int zz = oscarMayer.method1(4);
legal? If not, why?
No, method1 returns a String and we are trying to store it in zz, an
int.Answers
7. Is oscarMayer.method2(“Hello”);
legal? If not, why?
No, method2 is private
8. Is int cv = oscarMayer.method3( );
legal? If not, why? Yes
9. Is int cv = oscarMayer.method3(14);
legal? If not, why?
No, method3 is not expecting to receive a parameter…yet we’resending a
14.
10. Is oscarMayer.z = “hotdog”; legal? If not, why?
No, z is private.
11. Assume the following code is inside method1:method2(“BarBQ”);
Is this legal? If not, why?
Yes, we can access a private method from within its class.

18. 12. Instantiate an object called surferDude from
the Surfer class using two separate lines of
code. One line should declare the object and
the other line should instantiate it. (Assume no
parameters are sent to the constructor.)

19. 12. Instantiate an object called surferDude from
the Surfer class using two separate lines of
code. One line should declare the object and
the other line should instantiate it. (Assume no
parameters are sent to the constructor.)
Surfer surferDude;
surferDude = new Surfer( );

20. 13. Which of the following is correct? (Assume
beco is an object with a method (method33)
that receives a Circle paramater.)
a.Circle cir5 = new Circle(10);
beco.method33(cir5);
b. beco.method33( new Circle(10) );
c. Both a and b

21. 13. Which of the following is correct? (Assume
beco is an object with a method (method33)
that receives a Circle paramater.)
a.Circle cir5 = new Circle(10);
beco.method33(cir5);
b. beco.method33( new Circle(10) );
c. Both a and b

22. 14. What is the value of balance after the following
transactions?//refer to the BankAccount class you
created on p. 15-7
BankAccount acc = new BankAccount(10, “Sally”);
acc.deposit(5000);
acc.withdraw(acc.balance / 2);

23. 14. What is the value of balance after the following
transactions?//refer to the BankAccount class you
created on p 15-7
BankAccount acc = new BankAccount(10, “Sally”);
acc.deposit(5000);
acc.withdraw(acc.balance / 2);
2505

24. 15. What’s wrong with the following code?
BankAccount b;
b.deposit(1000);

25. 15. What’s wrong with the following code?
BankAccount b;b.deposit(1000);
b was never instantiated

26. 16. What’s wrong with the following code?
BankAccount b new BankAccount(32.75,
“Melvin”);
b = new BankAccount(1000,”Bob”);
//ok to assign a new object to b
b.deposit(“A thousand dollars”);
//Wrong, deposit receives a double, not a String

27. 16. What’s wrong with the following code
BankAccount b new BankAccount(32.75, “Melvin”);
//= sign missing between b and new
b = new BankAccount(1000,”Bob”);
//ok to assign a new object to b
b.deposit(“A thousand dollars”);
//Wrong, deposit receives a double, not a String

28. 17. What is printed in the following?
String myString = “Yellow”;
String yourString = “Yellow”;
String hisString = new String(“Yellow”);
String ourString = myString;
System.out.println(myString = = yourString);
System.out.println(myString = = ourString);
System.out.println(myString.equals(yourString));
System.out.println(myString.equals(ourString));
System.out.println( myString = = hisString );

29. 17. What is printed in the following?
String myString = “Yellow”;
String yourString = “Yellow”;
String hisString = new String(“Yellow”);
String ourString = myString;
System.out.println(myString = = yourString); // true…both
references to same object
System.out.println(myString = = ourString); //true…both
references to same object
System.out.println(myString.equals(yourString)); //true…
contents are same
System.out.println(myString.equals(ourString)); //true…
contents are same
System.out.println( myString = = hisString ); //false… different
objects