Cmsc131 Exam 2

Front 1

What package is the Scanner class located in? What is the fully qualified name of the Scanner class?

Back 1

java.util

java.util.Scanner

Front 2

What is accomplished when you type “import java.awt.Color;” at the top of a file?

Back 2

Whenever you type “Color”, the compiler knows that you are talking about the Color class that resides in the package called “java.awt”.

Front 3

What is accomplished when you type “import java.awt.*;” at the top of a file?

Back 3

You are importing EVERYTHING that is in the package java.awt.

Front 4

Which java package is automatically imported in it’s entirety into every Java program you write?

Back 4

java.lang

Front 5

What method of the String class can be used to pick out one particular character in the string?

Back 5

charAt

Front 6

What method of the String class can tell you how many characters are in the String?

Back 6

length

Front 7

What method of the String class can be used to compare two Strings for alphabetical order?

Back 7

compareTo

Front 8

What method of the String class can select a portion of an existing String?

Back 8

substring

Front 9

What does API stand for?

Back 9

Application Programming Interface

Front 10

If someone showed you a Java class, how can you quickly identify which members were part of the API for that class?

Back 10

everything that is public

Front 11

If a member is declared as “public”, can it be accessed from inside the same class?

Back 11

Yes

Front 12

If a member is declared as “public”, can it be accessed from another class?

Back 12

Yes

Front 13

If a member is declared as “private”, can it be accessed from inside the same class?

Back 13

Yes

Front 14

If a member is declared as “private”, can it be accessed from another class?

Back 14

No

Front 15

What is a “getter”?

Back 15

A method that returns the value of an instance variable of the object.

Front 16

What is a “setter”?

Back 16

A method that modifies the value of an instance variable of the object.

Front 17

Explain why it is important to limit the number of “public” members.

Back 17

Encapsulation of the fields (instance variables) of a class is very important. We frequently need to make changes to a class. If these changes do not modify the class API, then the modified class will work perfectly well within an existing project. However, if you modify the class in such a way that the API is changed, then you will have to re-program other components of the project so that they will work with the new class. By limiting the API we are free to make more extensive changes to the class without having to re-program other parts of the project.

Front 18

Name and describe the two visibility specifiers that you should know at this point.

Back 18

public – these members are visible everywhere

private – these members are visible only within the class

Front 19

True/False – if you change a class in such a way that the API changes, then other classes which depend on this one will have to be re-coded.

Back 19

True

Front 20

True/False – if you change a class without modifying the API, then other classes which depend on this one will have to be re-coded.

Back 20

False

Front 21

True/False – in java, when you pass a reference variable as an argument to a method, it is possible for the method to modify the object to which the variable refers.

Back 21

True

Front 22

Assume that you have a class called “Stuff”, which contains the following method:

public static void eat(Cat c)

Show the syntax for a statement which will make a call to this method, passing in a brand new Cat object (without using a variable).

Back 22

Stuff.eat(new Cat());

Front 23

Is it better to write many small tests, or a few long ones?

Back 23

Lots of small ones.

Front 24

What is JUnit?

Back 24

A useful feature of Java that allows us to quickly write “unit tests” for classes we are writing.

Front 25

Give an example of the correct syntax for “assertEquals” and “assertTrue” in a JUnit test.

Back 25

assertEquals(5, 2 + 3);

assertTrue(!“cat”.equals(“dog”));

Front 26

If a JUnit test has several separate assertions in it, will the test continue after one of the assertions fails?

Back 26

NO, but other tests in the entire suite will still run.

Front 27

If a JUnit test suite has several different JUnit tests in it (separate methods), will the rest of the tests run after one of them fails?

Back 27

YES.

Front 28

What is a Stack (in general, not just in Java)?

Back 28

A stack is a simple “linear” data structure in which elements are both inserted (“pushed”) and removed (“popped”) from the same end. (We usually picture the stack vertically, and say that we “push” and “pop” items from the top.)

Front 29

When you push an entry into the stack does it go on the top or bottom?

Back 29

Top

Front 30

When you pop an entry from the stack, does it come off the top or bottom?

Back 30

Top

Front 31

If you do not write any constructors, what values will instance variables of the following primitive types be assigned: int, double, boolean, char ?

Back 31

int 0
double 0.0
boolean false
char ‘\0’ (character with ascII code)

Front 32

If you do not write any constructors, what values will instance variables that are references by assigned?

Back 32

null

Front 33

Under what circumstances will Java provide a default constructor for you automatically?

Back 33

If you don’t write ANY constructors, Java will give you a default constructor automatically.

Front 34

What is a copy constructor? Give an example.

Back 34

The copy constructor accepts an argument that is the same type as the object being constructed. It initializes the fields of the current object to match that of the parameter. Example:

public Cat(Cat x) {

numWhiskers = x.numWhiskers;

name = x.name;

}

Front 35

In what kind of methods does it make sense to use “this”?

Back 35

Instance methods only.

Front 36

What does “this” refer to?

Back 36

“this” is a reference to the current object

Front 37

How many ints are created by the statement: int[] a = new int[5];

Back 37

5

Front 38

How many Strings are created by the statement: String[] a = new String[5];

(Hint: The answer to this question and the previous question are different!)

Back 38

None. What is created is an array of null-references.

Front 39

Are the elements of an array of primitives automatically initialized? If so, to what values?

Back 39

Yes, to 0 or false.

Front 40

Are the elements of an array of references to objects initialized? If so, to what values?

Back 40

Yes, to null.

Front 41

Define the terms mutable and immutable.

Back 41

A mutable class has a public method (not a constructor) that can be used to change the State of the current object.

Any class that is not mutable is called immutable.

Front 42

Are String objects mutable?

Back 42

No

Front 43

Are Integer objects mutable? (If you’re not sure, inspect the online API documentation and find out!)

Back 43

No

Front 44

Suppose you are passing (or returning) an array of primitives to/from a method. Is it safe to make a reference copy only?

Back 44

No

Front 45

Suppose you are passing (or returning) an array of references to immutable objects to/from a method. Is it safe to make a reference copy only? Is it safe to make a shallow copy?

Back 45

Reference copy is unsafe. Shallow is safe.

Front 46

Suppose you are passing or returning an array of references to mutable objects to/from a method. Is it safe to make a reference copy only? Is it safe to make a shallow copy?

Back 46

Neither one is safe. Only a deep copy is safe in this case.

Front 47

What is a Java interface?

Back 47

Typically, it is just a collection of method prototypes that are implemented by several related classes.

Set of public methods through which we can interact with an object

Front 48

What is polymorphism?

Back 48

Sometimes a single variable can refer to objects of different types. For example, if you have an interface called “CanFly”, then you can create a variable of type “CanFly” which can refer to any kind of object that knows how to fly. (In other words, it can refer to an object of any class which implements the “CanFly” interface.) This kind of variable is “polymorphic”.

Front 49

What is meant by the term “algorithm”?

Back 49

A sequence of instructions or steps that can be followed in order to solve a relatively simple well-defined problem. Frequently there are many different known algorithms for solving a problem.

Front 50

Explain the relationship between exception handling and the call stack. What happens if an exception is thrown but not caught anywhere in your program?

Back 50

When an exception is thrown, Java looks for an appropriate “catch” block in the place where the problem occurred. If a catch is not found there, then Java discards the current frame on the call stack, and looks for an appropriate catch block in the previous frame. If none is found, that frame is discarded, etc. If the exception is not caught anywhere, then the program terminates, and the exception propagates to the “outside world”, which for us is the Eclipse IDE. Eclipse displays that red and blue error notification with stack trace in the console.

Front 51

Under what circumstances will the finally block run?

Back 51

The finally block ALWAYS runs.

Front 52

What does “overloading” mean?

Back 52

Writing more than one method in the same class with the same name. (This only works if the signatures of the

methods are different – i.e. the parameter lists have different types.)

Front 53

Where can you use a “continue” statement?

Back 53

Inside of any loop.

Front 54

Describe how a continue statement behaves in a while loop.

Back 54

Goes to top of loop immediately and checks the boolean condition to see if looping should continue.

Front 55

Describe how a continue statement works in a for-loop.

Back 55

It will execute the statement that is usually processed at the end of an iteration through the loop, and then check the boolean condition to see if looping should continue.

Front 56

Where can you use a “break” statement?

Back 56

Either in a switch statement or in a loop.

Front 57

Describe how a break statement behaves.

Back 57

Causes the inner-most loop or switch statement to be exited immediately.

Front 58

Which kinds of variables are given default values if you do not initialize them?
(Local/ Instance/ Static)

Back 58

-Instance and static variable are given default values
-Local variables must be initialized by the programmer explicitly

Front 59

What is a reference variable?

Back 59

Reference variables are variables that refer a class name..
Demo d1;
d1 = new Demo();
//d1 would be the reference variable

Front 60

What are the default values used for Local, instance, and static variables?

Back 60

primitive values are set to 0
(Primitive values are ints, booleans, chars, etc...)
reference variables are set to "null"
(Reference variables are variables that refer a class name..)

Front 61

CanDance x = new CanDance(); ?

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 61

No,
Interface can declare itself

Front 62

CanDance x = new Student; ?

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 62

Yes

The reference variable x refers to the object of the class 'Student'

Front 63

Student z = new CanDance();

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 63

No

Class can not reference another method

Front 64

Student z = new Penguin();

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 64

No

Class can not reference another class

Front 65

CanDance a;
a = new Student();
a = new Penguin();
a = new Cow();

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 65

Yes, yes, and yes

The interface name, CanDance, declared 'a' as an object reference variable

The reference variable 'a' can be used to refer to any object of any class that implements CanDance interface

Student, penguin, and cow all reference Speaker

Front 66

Penguin b = new Penguin();
doSquareDance(b);

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 66

Yes

The penguin class calls a new penguin class and assigns doSquareDance to that class

Front 67

Student c = new Student();
doSquareDance(c);

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 67

Yes
The student class calls a new penguin class and assigns doSquareDance to that class

Front 68

Cow d = new Cow();
doSquareDance(d);

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 68

Yes
The cow class calls a new penguin class and assigns doSquareDance to that class

Front 69

CanDance e = new Student();
doSquareDance(e);

Suppose you have an interface called CanDance &
three classes (Student, Penguin, and Cow), all which implement the CanDance interface.
Also asume that there is a method available with the following prototype:
public static doSquareDance (CanDance a).

Back 69

Yes

The reference variable e refers to the object of the class 'Student' and then assigns the doSquareDance method

Front 70

Write a public static count method that takes one parameter (a reference to a string)

Back 70

public static int count (String s) {
}

Front 71

Write a copy constructor

Back 71

public Cat (Cat x) {
numWhiskers = x.numWhiskers;
name = x.name;

Front 72

Example of mutability

Back 72

public class StringsAndMutability {

public static void main(String [] args) {
StringHolder s1 = new StringHolder("One");

s1.addOn("Two");
StringHolder s2 = new StringHolder(s1);
s2.addOn("Three");

System.out.println("S1: " + s1);
System.out.println("S2: " + s2);
}

}

Front 73

A java source-code file can contain more than one class.

Back 73

True

Front 74

A java source-code file can contain more than one public class.

Back 74

False

Front 75

Can you use a public class outside of its package? Do you need to use a fully qualified name?

Back 75

Yes, but then you must use the fully qualified name

Front 76

When you use a public class inside of its own package, do you need to use a fully qualified name?

Back 76

No

Front 77

When you use a non-public class (with no visibility specified) inside of its own package, do you need to use a fully qualified name?

Back 77

No

Front 78

Can you use a non-public class outside of its package?

Back 78

No

Front 79

If you write a class without specifying a particular package, which package will it become a part of?

Back 79

default package

Front 80

What syntax would you use at the top of a source-code file to specify that the class belongs in a subpackage called "stuff" ths is part of a larger package called "junk"

Back 80

package junk.stuff;

Front 81

What do you accomplish by using "extends" (What does it mean to write: public class A extends B)

Back 81

"Extends" allows the class A to "inherit" all of the elements from B (variables and methods). A can also have other features of its own, and it can "over-ride" some of B's features if you want.

Front 82

What does it mean to say a class is derived from another class?

Back 82

"a is derived from b" means the same thing as "A extends B"

Front 83

What does it mean to say a class inherits members from another class?

Back 83

The derived class automatically shares those features