Oca Java Se 7 Hard Questions

Front 1

Class modifiers (access and nonaccess)

Back 1

-public, default
-final, abstract, strictfp

Front 2

What does class access mean when the class access modifier is not stated?

Back 2

Default Access

Front 3

When would you consider using the modifier 'final' on a class?

Back 3

methods in that class will ever be overridden

Front 4

What is the purpose of the non-access modifier 'abstract' for a class?

Back 4

Abstract Classes An abstract class can never be instantiated. Its sole purpose, mission in life is to be extended (subclassed).

Hint 4

Note, however, that you can compile and execute an abstract class, as long as you don't try to make an instance of it.

Front 5

What non-access modifier is required of a class that has a abstract method?

Back 5

if even a single method is abstract, the whole class must be declared abstract.

Front 6

When can a class be marked with the non-access modifiers 'final' and 'abstract' ?

Back 6

you can't mark a class or a method as both abstract and final. They have nearly opposite meanings. An abstract class must be subclassed, whereas a final class must not be subclassed.

Hint 6

The code will not compile.

Front 7

What is the difference between an abstract class and an interface?

Back 7

Abstract class may have at least one abstract method. An interface has nothing but abstract methods. An interface will not have method implementations .

Front 8

What modifiers can be declared in a variable in an interface?

Back 8

public static final (constants)

Front 9

What modifiers can be used for interface methods?

Back 9

public, abstract

Front 10

What two access modifiers can be used with an interface?

Back 10

public access and default access (package)

Front 11

When can the value of a constant declared in an interface be changed?

Back 11

You can't change the value of a constant! Once the value has been assigned, the value can never be modified.

Hint 11

This is due to the use of the 'final' non-access modifier.

Front 12

What are considered class members?

Back 12

instance variables and methods

Front 13

What access-levels can be used for a method?

Back 13

Whereas a class can use just two of the four access control levels (default or public), members can use all four: public protected default private

Front 14

What is the difference between the default access control and protected access control?

Back 14

Default protection is what you get when you don't type an access modifier in the member declaration. The default and protected access control types have almost identical behavior, except for one difference:

Front 15

What are the three ways to access a method?

Back 15

1. Put the method in the same class.
2. Invoke the method using a reference to the class the method is in.
3. Invoke an inherited method.

Front 16

Can a subclass override a superclass member that is marked private?

Back 16

No. You can, however, declare a matching method in the subclass. But regardless of how it looks, it is not an overriding method.

Front 17

What access modifiers can apply to local variables?

Back 17

only final

Front 18

When would you consider using 'final' on a method that is in a superclass?

Back 18

final keyword prevents a method from being overridden in a subclass, and is often used to enforce the API functionality of a method.

Front 19

Can you have an abstract class that does not contain an abstract method?

Back 19

An abstract class can be valid if it does not have any abstract methods.

Front 20

What class members are able to use the synchronized modifier?

Back 20

synchronized modifier can be applied only to methods—not variables, not classes, just methods.

Front 21

What is the purpose of the modifier strictfp?

Back 21

strictfp forces floating points (and any floating-point operations) to adhere to the IEEE 754 standard.

Front 22

What class members can strictfp be used with?

Back 22

Only classes and methods

Front 23

What are declaration rules for using var-args?

Back 23

1. Can only use one var-arg per method.
2. To declare a method using a var-arg parameter, you follow the type with an ellipsis (…), a space, and then the name of the array that will hold the parameters received.
3. The var-arg must be the last parameter in the method's signature, and you can have only one var-arg in a method.
Parameter type can be primitive or an object.

Hint 23

Example:
doStuff(Animal... animal)

Front 24

What are the differences between a constructor and a method?

Back 24

The first thing to notice is that constructors look an awful lot like methods. A key difference is that a constructor can't ever, ever, ever, have a return type…ever! Constructor declarations can however have all of the normal access modifiers, and they can take arguments (including var-args), just like methods. The other BIG RULE, to understand about constructors is that they must have the same name as the class in which they are declared. Constructors can't be marked static (they are after all associated with object instantiation), they can't be marked final or abstract

Front 25

What are the two types of variables in Java

Back 25

Primitives and Object References.

Front 26

What are the eight types of primitive variables?

Back 26

A primitive can be one of eight types: char, boolean, byte, short, int, long, double, or float.

Front 27

What is the range of floating point types such as float and double?

Back 27

float is 32 bits long
double is 64 bits long.

Front 28

What modifiers can a local variable be associated with?

Back 28

final

Front 29

What are rules relating to initializaing local variables?

Back 29

must be initialized before you try to use it

Front 30

Describe what an array is including the types an array can hold? Can an array hold primitives and object references?

Back 30

In Java, arrays are objects that store multiple variables of the same type, or variables that are all subclasses of the same type. Arrays can hold either primitives or object references,

Front 31

How do you declare an array of primitives or an array of objects?

Back 31

int [ ] myNumbers;
Customer[ ] customers;
alternate syntax is also valid:
int myNumbers[ ] ;
Customer customers [ ];

Front 32

Why would you want to use one of the Collection types in java.util instead of using an array?

Back 32

Collections can dynamically adjust their size where an array can not.

Front 33

How would you declare a 2 dimensional array?

Back 33

int [ ] [ ] myNumbers;

Front 34

How do you declare an array using a fixed size?

Back 34

You don't. You never specify the size in declaration. Size matters when the array is constructed.

Front 35

What does final mean when a primitive variable is declared as final?

Back 35

Once the variable is initialized the value will not be able to be changed for the life of the variable.

Front 36

What does final mean when a object reference variable is declared as final?

Back 36

Once the variable is initialized the object reference can not be used to refer to another object. You can modify the state of the variable but not the object that is being referenced.

Hint 36

There are no final objects, just final object references.

Front 37

What can you declare as static in a class?

Back 37

methods, instance variables, initialization block, nested classes.

Front 38

Where can you define an enum?

Back 38

An enum can be declared inside or outside of a class, but not inside of a method!

Front 39

interface modifiers

Back 39

abstract, public

Front 40

method modifiers

Back 40

-public, private, protected, default
- final, abstract, static, synchronized, strictfp, native

Front 41

instance variable modifiers

Back 41

-public, private, protected, de
fault
-final, static, transient, volatile

Front 42

what's another name for an interface instance variable?

Back 42

constant

Front 43

how do you initialize an array

Back 43

int[] myArray = new int[3]

Front 44

method signature of the main

Back 44

public static void main (String[] args)

Front 45

can main be overloaded?

Back 45

Yes!

Front 46

T/F determine class visibility before determining member visibility

Back 46

True. If a class cannot be accessed it's members cannot be accessed.

Front 47

5 Overriding method rules

Back 47

1. Same argument list

2. Same return type

3. Less restrictive access

4. Throw narrower exception

5. No override final method

6. No override static

Front 48

Overloading method rules

Back 48

1. Must different argument list
2. May have diff return type
3. May have diff access mod
4. May throw diff exception

Front 49

T/F Object type not reference type determines which overridden method is used at runtime

Back 49

True

Front 50

Which is safer, downcasting or upcasting?

Back 50

Upcasting, becuase assignment restrict the access capabilities of the new variable

Front 51

What type of casting must be done explicitly and why?

Back 51

Downcasting, because the reference variable referes to a subtype object and downcasting assigns the variable to a reference of the subtype (an animal becomes a horse)

Front 52

What is the first statement of every constructor call?

Back 52

this() - for overloaded constructor
or
super() - for no arg contructor

Front 53

can constructors be inherited?

Back 53

No. They can never be overriden either.

Front 54

Can you have calls to this() and super() in the same constructor

Back 54

No, never calls to both in the same constructor

Front 55

When would you use super(param1, param2)

Back 55

calls a superclass constructor that requires parameters

Front 56

static init block vs init block

Back 56

static { code here }
runs once when class is loaded
{ code here }
runs for every new instance

Front 57

can static method access instance variables?

Back 57

No! Unless the variable is static

Front 58

What's the call order of init blocks and constructors

Back 58

1. Static init blocks run when class is first loaded
2. Instance init blocks run after the constructor calls super()
3. instance init block runs everytime a class instance is created
4. init blocks execute in the order in which they appear

Front 59

What are the 4 basic scopes

Back 59

1. Static vars (class)
2. Instance vars (object)
3. Local vars (method)
4. Block vars (for, if, while...)

Front 60

Give an example of this: reference variables can refer to subclasses of the declared type, but not to superclasses.

Back 60

A boxer variable can refer to a dog object. But not an Animal object.

Front 61

What is the finalize() method

Back 61

It comes from the object class and it runs once before the garbage collector deletes an object/but it's not guaranteed

Front 62

When does an object become eligible for garbage collection

Back 62

when no live thread can reach it

Front 63

The XOR ( "^" hat operand)

Back 63

Evaluates to true if exactly one operand is true

Front 64

In a switch statement what are the rules for the variable

Back 64

It must be a literal or final variable. Declared and initialized I same line. Can use int, enum or string

Front 65

What must you do with checked exceptions?

Back 65

Must declare using throwS it must catch using try/catch

Front 66

Which exceptions are unchecked?

Back 66

Error and RuntimeException

Front 67

When is the finally block executed?

Back 67

Always, whether or not an exception is caught

Front 68

In what order should catch blocks be used? why? give an example.

Back 68

From most specific to most generic. IOException first then Exception last. If Exception was caught first then IOException would never be reached. This will cause a compiler error