31 - 60

Question 1

NumberFormatException

o base class
o function?

Answer 1

o thrown to indicate that the application has tried to convert a string (with an inappropriate format) to one of the numeric types

o subclass of IllegalArgumentException

Question 2

ClassCastException –> what is?

Answer 2

is thrown when an object fails an IS-A test with the class type to which it’s being cast

Question 3

if not overload toString() –> what will be printed?

Answer 3

object’s name + hashCode()

Question 4

can use “this” within static Method?

Answer 4

no

Question 5

order of execute (2)

Answer 5

o direct superclass must be initialized

o SIC way

Question 6

what is for-each loop?

How to read: for(String day : days)

Answer 6

enhanced for loop

Question 7

which kind of access modifiers can be set to top class?

Answer 7

public, default

no protected & private

Question 8

short b = 3;

b += 4.6;

output? Nguyên tắc cast của operand

Answer 8

o implicit cast to the type in left

o result = 7

Question 9

cách viết main method

Answer 9

public static void main(String[] args){

public static void main(String args[]){

static public void main(String args[]){

Question 10

how many public class / interface can be in a Java file? Name?

Answer 10

only 1 + same name as this file

Question 11

limit of enhanced for loop with: array, collection

Answer 11

• cannot be used to initialize an array + modify its elements
• cannot be used to delete or remove the elements of a collection
• can’t iterate over multiple collections or arrays in the same for-each loop

Question 12

exception handling –> order of exception

Answer 12

derived class –> base class

Question 13

where in the constructor can super, this be used?

Answer 13

first statement

Question 14

exception

o overriding - overridden method
o constructor - base constructor

Answer 14

overriding method / constructor can

1. only throw
   1. 1. exceptions that are declared in the throws clause of the overridden method
   1. 2.exceptions that are subclasses of the declared exceptions
2. choose NOT to throw any exception

Question 15

if a method uses another method that may throw a checked exception –> what does this method need? (2)

Answer 15

• the method should be enclosed within a try-catch block

- the method should specify this exception to be thrown in its method signature

Question 16

throw keyword:

1. base class does not throw exception –> derived class?
2. which kinds of exception can base class throw freely?

Answer 16

1. cannot throw an exception only applying to checked exception
2. Errors and Runtime Exception

Question 17

what should a class do when it implements an interface? (2)

Answer 17

• when a class implements an interface, it should implement all of the methods defined by this interface
• if it doesn’t, it must be defined as an abstract derived class

Question 18

a call needs to be wrapped up in try / catch block or with throws clause –> depend on class of reference or class of actual object?

Answer 18

class of reference

Question 19

overridden method has throw –> does overriding have to have?

Answer 19

overriding method may choose to have no throws clause even if the overridden method has a throws clause

Question 20

can use “this” within static Method? Use “this” to access static variable?

Can write: this.i = 4?

Answer 20

not within static method. Can use to access static variable

this.i = 4 –> wrong –> cannot use like a local variable

Question 21

IllegalStateException & IllegalArgumentException

Answer 21

• IllegalArgumentException: specify that a method has passed illegal or inappropriate arguments
• IllegalStateException: signals that a method has been invoked at an illegal or inappropriate time

Question 22

class A implements interface I
class B expends class A

–> can class B use interface I?

Answer 22

can

when base class implements an interface, derived class can use this interface without implementing it

Question 23

members of an interfaces are implicitly?

Answer 23

all members of an interface are implicitly abstract and public, static, final

Question 24

what happens when a constructor in a subclass is invoked?

Answer 24

• when a constructor in a subclass is invoked, some constructors of its superclass have to be invoked first

+ explicitly call it

+ or the compiler will add super() as the first line of the subclass constructor

• baseclass has to have a no args constructor

Question 25

many overloaded methods with many “appropriate” parameters –> choose

Answer 25

1. compiler always tries to choose the most specific method available with least number of modifications to the arguments
2. Java designers have decided that old code should work exactly as it used to work before boxing-unboxing functionality became available
3. Widening is preferred to boxing/unboxing (because of rule 2), which in turn, is preferred over var-args ===> widening > boxing / unboxing > var-args

Question 26

• trim() is method of?

- trimToSize() is method of?

Answer 26

• trim() is method of String

- trimToSize() is method of ArrayList

Question 27

ExceptionInInitializerError –> when?

Answer 27

is typically thrown by the JVM when a static initializer in the code throws any type of RuntimeException

Question 28

how to write octal & binary number?

Answer 28

binary number begins with 0b…. + not L, D, F in suffix

Octal –> 0… (không có b hoặc B) + not L, D, F in suffix

Question 29

abstract method –> body? What does a method body look like? What does that mean?

Access modifier

static?

Answer 29

no. Body { }
no block. Only semicolon
no private
cannot static

Question 30

precedence: == và =

Answer 30

precedence: == > =