Java Programmer Certification

Front 1

Instance variables can have which access controls?
public, private, protected, or default?

Back 1

All of the above

Hint 1

Instance variables can have any access control and can be marked final or transient

Front 2

Instance variables can be marked transient or _______. (non-access modifier)

Back 2

Final

Hint 2

Instance variables can be marked final or transient

Front 3

Instance variables can NOT be synchronized, native, strictfp, or _______

extra credit: why?

Back 3

abstract

Hint 3

abstract keyword can only be applied to methods and classes

Front 4

Shadowing is when the name of a local variable matches the name of an instance variable. Is it legal to do that?

Back 4

Yes

Hint 4

but it's very bad and can create some hard to find bugs

Front 5

Besides strictfp, what are the other non-access modifier keywords you can use for your classes?

extra: when would you use each?

Back 5

final and abstract

Hint 5

final when you don't want the class to be subclassed

abstract when you want to force the class to be subclassed by anyone who wants to use it. It also can't be instantiated.

Front 6

What is wrong with this:
public final abstract class Foo{ }

Back 6

A class can't be final. You would be saying that you have to subclass the class that can't be subclassed. compiler error ensues

Front 7

Which kind of class can't be subclassed?

Back 7

final

Front 8

which class can't be instantiated?

Back 8

abstract

Front 9

What is the effect of a class having at least one final method?

Back 9

nothing, this is a trick question. if you said that the whole class must be marked final, shame on you

Hint 9

seriously, this is the kind of tricky stuff that will get you on the exam

Front 10

what is the effect of a class that has at least one abstract method?

Back 10

it must be marked abstract or you will get a compiler error

Hint 10

make sure you know how to spot a real abstract method:
public foo();
and one that only looks abstract because it seems like it doesn't have a body:
public foo(){}
curly braces = method body despite its emptiness

Front 11

Why is this legal:
abstract class Foo(){
void bar();
void baz(){}
}

Back 11

Because abstract classes can have both abstract and non-abstract methods

Hint 11

if you said both methods were abstract, note the curly braces on method baz, that means it has a body and is not abstract.

Front 12

if class Bar extends abstract class Foo, Bar must do what to be considered a concrete class?

Back 12

It must implement all of Foos abstract methods or else Bar is a de facto abstract class

Hint 12

If Bar doesn't implement the methods or doesn't get marked abstract you will get a compiler error.

Front 13

Interfaces are contracts for what a class CAN do, but doesn't tell you what?

Back 13

Interfaces don't say HOW a class must do something

Hint 13

example: an interface with an addUser method only says that a class that implements the interface will addUser but doesn't say how it will do it. Will it add it to a list and return the list, or will it add it to a string builder and print the string? you'll just have to find out.

Front 14

Interface Bouncy has methods for a class to bounce.

So in your code you see class Ball (which implements Bouncy) that is a subclass of class Toy which implements Bouncy.

you also see class Tire (which also implements Bouncy) but is a subclass of class AutoPart

Is this Madness???

Back 14

No! This. IS. JAVA!!!!!!! *kick*
Even though ball and tire are from different inheritance trees, they can still implement the interface

Hint 14

interfaces can be implemented by any class from any inheritance tree

Front 15

Interfaces are really just purely abstract classes. What does that even mean? Also can you use the interface and abstract keyword together?

Back 15

It means that interfaces are 100% abtract classes since none of their methods are allowed to be concrete. Of course you can use interface and abstract keyword together.

Hint 15

Not only is it legal to use the abstract keyword when declaring an interface, but if you don't do it, the compiler will

Front 16

Interfaces can only have one kind of method. For this reason they have two keywords declared by default (implicit, you know added by compiler if you don't)

Back 16

They can only be abstract. So they are always declared public and abstract.

Hint 16

whenever you see an interface method imagine (public abstract) in front of it, so if you see:
final void foo();
or
private void foo();

you will see them as
(public abstract) final void foo();
and
(public abstract) private void foo();

That makes the error more obvious when you see the confilcting keywords.

Front 17

Interface constants are always (read: implicitly)
1)
2)
3)
declarations are option in any order

Back 17

1)public
2)static
3)final

extra credit: any programmer using constants in an interface should be considered bad at software design

Hint 17

static imports should always be considered as a replacement for interface constants.

Placing constants in an interface was a popular technique in the early days of Java, but now many consider it a distasteful use of interfaces, since interfaces should deal with the services provided by an object, not its data. As well, the constants used by a class are typically an implementation detail, but placing them in an interface promotes them to the public API of the class.

Front 18

When a concrete class implements an interface there are about 5 things to look for to ensure it is legal. Name as many as you can and imagine how each of those will look in code.

Back 18

1) it must have concrete implementations of ALL abstract methods

2)follow legal override rules for those methods

3)Avoid declaring any new checked exceptions for those methods (non-interface methods are fine though)

4)Avoid declaring exceptions that are broader than the exceptions in the interface

5)Maintain the signature and return type of the methods it implements, any other combo is an overloaded method and doesn't have any of the above restrictions.

Hint 18

exam gotchas:
1) the code will have the class implement a method without code between the braces so you think its abstract.

2) watch for any code that violates override rules for arg list, return type, access level, final and static keywords, unchecked exceptions, and constructors

3)code will either have a concrete method with unchecked exceptions (which is fine) or have an overloaded method (meaning this isn't an interface method) with checked exceptions.

4)the code will declare exceptions that are more narrow to trick you into thinking it would generate an error

5) the code will have a huge list of methods, some are implementations and some are overloaded so you have to keep track of slight variations in the return type or a type in the argument list.

Also, a different variable name in the argument list does not count as a different argument list.

public void foo(String u){} is the same as
public void foo(String s){} is the same as
public void foo(String a){} mkay?

Front 19

Local variables may have wihich access modifiers?

Back 19

none. ANY local variable declared public, private, or protected is illegal

Hint 19

Seriously, think about the life span of a local variable and think about what access modifiers do. Now you see why it would be silly to try to make a local variable public.

Front 20

What's the only modifier that a local variable can have.

extra credit: why is it good?

Back 20

final

Hint 20

extra credit: why is it good to mark a local variable final? it expresses intent and can prevent some sly bugs in your code. google "java final local variables" to find a lively discussion on the subject.

Front 21

What is wrong with this?
public int foo(int x){
int i;
i = x*2;
return i;
}

Back 21

Local variable was not initialized.

not to mention very bad method design.

Hint 21

Local variables don't get default values when declared, so declaration and initialization must be inline.

Front 22

The "this" keyword (as is this.name or this.whatnot) refers to what?

Back 22

The current object.

Hint 22

class OneNumber {
int n;
void setValue (int n) {
this.n=n;
};
}

Front 23

The only code that can see or access private members is...

Back 23

the code in its class

Front 24

Are private members invisible to subclasses?

Back 24

Yes, so they can't be inherited.

Front 25

What's the difference between default and protected keyword?

Back 25

default = package level
protected = package level plus subclasses regardless of package

Hint 25

NOTE: subclasses can NOT see protected members of its super class and can NOT see protected members of other subclasses

Front 26

Encapsulation helps hide _______ behind an interface or API

Back 26

implementation

Front 27

Encapsulated code has two features:
1) Instance variable are ______
2)_____ and _____ methods provide access to instance variables

Back 27

1) protected
2) Getter and Setter

note:
Although, if your getter is public, how protected is your class from exposing its implementation from other classes?

Front 28

IS-A refers to _________ or __________

Back 28

inheritance or implementation

note:
This means that if a class extends another class or implements an interface it has an IS-A relationship

Front 29

IS-A is expressed with the keyword _____

Back 29

extends

Front 30

True/False

"inherits from" and "is a subtype of " are all equal expressions

Back 30

True!

Front 31

HAS-A means that an instance of one class "has a" ______ to an instance of another class or the same class

Back 31

reference

Front 32

Inheritance allows a class to be a subclass of a superclass. What does it inherit and what are their access modifiers?

Back 32

public and protected
variables and methods

Front 33

________ is a key concept that underlies IS-A, polymorphism, overriding, overloading, and casting.

Back 33

Inheritance

Front 34

ALL classes (except class Object), are a subclass of type Object. So EVERY class inherits what from Object?

Back 34

Object's methods.

note:
extra credit if you can name them. Not now, though...later.

Front 35

Polymorphism means _____ ______

(hint: not the concept, but what is the literal Greek translation)

Back 35

πολλές μορφές = polus morphē =
many forms

Front 36

A reference variable is ALWAYS of a single, unchangeable type, but it CAN refer to what?

(what type of object)

Back 36

a subtype object

Front 37

A single object can be referred to by reference variables of many different types as long as they are the same what?

(types)

Back 37

type or supertype

Front 38

Does the reference variable's type determine which methods can be called or is it the object's type?

Back 38

The reference variables

example:
class X{void doXStuff(){}}
class Y extends X {void doYStuff}}

X x1 = new Y();
x1.doYStuff();

won't compile. x1 is a Y object but the reference type is an X.

You'd have to cast it.
And THIS is NOT a proper cast: (Y)x1.doYStuff(); //generates compiler error

PROPER CASTING:
((Y)x1).doYStuff();

Front 39

Polymorphic method invocations apply ONLY to overridden ______ methods.

Back 39

instance

Front 40

Methods can be overridden or overloaded, but constructors can only be...

Back 40

overloaded.


note:
why not overridden?

constructors aren't inherited, so they have to provide their own constructors. Even the default, no-arg constructor the compiler provides still has to call the superclass's constructor manually.

so:
class A{
public A(){}
}
class B extends A{
public B(){
}
}

B's constructor isn't overriding A's constructor, since

Front 41

edit later

Back 41

okay