Transcript quiz answers

Quiz 1
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Quiz 1
What is this? (explain the use of
the reserved word “this” in a
class method).
Answer each question briefly.
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– A method invoked on object
creation.
– Typically, a Constructor is
executed when an object is
created by use of the “new”
operator.
– Yes, as long as they have
different parameters.
– What is a Constructor?
– Under what circumstances
would a Constructor method for
a class execute?
– Can a class have more than one
Constructor?
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Given
String s = "potato";
String v =
s.substring(0,3);
String w =
s.substring(0,3);
Is v==w true or false?
This is an object reference that
refers to the current object for
which a method was invoked.
Brief answers
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v == w is false. The substring
method will create new objects
each time it is invoked.
Quiz 2
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The following code creates a 2x2 block of buttons.
Add/modify the code to create an applet with 6 buttons
arranged in 3 rows and 2 columns..
import javax.swing.*;
import java.awt.*;
public class ButtonJComponent extends JApplet
{
public void init()
{
Container c = getContentPane();
c.setLayout(new GridLayout(2,2);
JButton button1 = new JButton("one");
JButton button2 = new JButton("two");
JButton button3 = new JButton("three");
JButton button4 = new JButton("four");
}
}
2. Discuss the changes necessary to make the
program in 1 a stand-alone java app instead of an applet.
You may discuss this in words…exact code is not
necessary in all cases, but try to cover all the steps.
For example:
“extends JApplet” changes to “???”
The init method changes to….
Must add a main method that does
what?
Anything else?
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Change body to:
c.setLayout(new GridLayout(3,2);
JButton button1 = new JButton("one");
JButton button2 = new JButton("two");
JButton button3 = new JButton("three");
JButton button4 = new JButton("four");
JButton button3 = new JButton("five");
JButton button4 = new JButton("six");
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The following two lines are sufficient:
for(int i=0; i < b.length; i++)
System.out.println(b[i]);
-extends JApplet changes to extends JFrame
-the init method becomes a constructor ButtonJComponent()
-A main method must be added which creates a ButtonJComponent
object, and calls the setSize() and show() methods
-a windowclosing handler should be added
Quiz 3-1
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1. What error will Java report from the
following code? (That is…what’s
wrong with this sequence? You do not
need the actual error message.)
int[] a;
a[3] = 5;
2. Correct any errors in the following
declaration.
char[] vowel = {a, e, i, o, u};
3. Consider
int v = {23, 44, 78};
int w = v;
w[2] = 100;
What is the value of v[2]?
4. Write Java code to swap integer
array elements a[i] and a[j].
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a is a reference to an array, but the
array has not been created (with the
new operator for instance). The actual
error reported is: variable a might not
have been initialized
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char[] vowel = {‘a’,’b’,’c’,’d’,’e’};
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v[2] has the value of 100. (w is an alias
of v so the statement w[2] = 100 also
changes v.
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Something like this:
int I,J,temp;
temp = a[I];
a[I] = a[J];
a[I] = temp;
Quiz 4
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Show the contents of a stack (of Strings) after the every 5
operations and at the end.Assume the stack is initially empty.
Display the stack from Top to bottom. (Commands are listed left
to right to conserve space.
Push (“Example”)
Push (“Bogus”)
Push (“Sham”)
Pop()
Pop()
Push(“Fine”)
Push(“A”)
Push(“Of”)
Pop()
Push(“Heck”)
Push(“One”)
Pop()
Pop()
Push(“Is”)
Push(“List”)
Pop()
Push(“This”)
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At the end the stack will have (from top to bottom)
This
Is
A
Fine
Example
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The list of operations is intended to be one continuous list, with line
breaks. So the final answer is (from left to right):
it is a Fine Example Anyway
Quiz 5
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Which of the following correctly describes the
relation between an abstract class and an
interface?
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An interface is another name for an abstract class.
An interface is not a class, but is otherwise identical
to an abstract class.
Every abstract class implements an interface.
An abstract class must be totally abstract, but an
interface may be partially implemented.
None of the above
The code
public class B extends A { }
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None of the above. All of the statements are false.
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None of the above. B inherits public and protected
methods and if they are in the same package, B will
inherit methods created with no modifier.
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B is a subclass of A
defines a class that inherits all the methods of A
defines a class that inherits the public methods of A
only
will not compile because the body in empty
none of the above
The code
public class B extends A { }
defines a class B that
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implements A
is a superclass of A
is a subclass of A
none of the above
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Quiz 6
What is displayed on the console when the following code is run?
class Test
{
public static void main(String[] args)
{
try
{
System.out.println("Welcome to Java");
int i=0;
int y=2/i;
System.out.println("Exceptions are fun");
}
catch(RuntimeException ex)
{
System.out.println("I take Exception to that");
}
finally
{
System.out.println("End of the block");
}
}
}
Welcome to Java
I take Exception to that
End of the block
What is displayed on the console when the following code is run?
class Test1
{
public static void main(String[] args)
{
try
{
System.out.println("Welcome to Java");
int i=0;
System.out.println("Exceptions are fun");
}
catch(RuntimeException ex)
{
System.out.println("I take Exception to that");
}
finally
{
System.out.println("End of the block");
}
}
}
Welcome to Java
Exceptions are fun
End of the block