61 - 90

Question 1

Passed by?

• primitive
• object

Answer 1

o primitive –> value
o object –> object

Java passes all primitive data types by value. This means that a copy is made, so that it cannot be modified. When passing Java objects, you’re passing an object reference, which makes it possible to modify the object’s member variables. If you want to pass a primitive data type by reference, you need to wrap it in an object

Question 2

String is immutable –> what does this mean?

Answer 2

1. created –> content not modified but can call method
2. methods that return a modified String value
   - return a new String object
   - original String value remains the same

Question 3

access modifiers for local variables

Answer 3

only final can be used

Question 4

ArrayList - 2:

• indexOf(Object o)
• lastIndexOf(Object o)
• .clone()
• contains(Object o)
• get(int index)

Answer 4

o indexOf(Object o) –> returns the index of the first occurrence of the specified element in this list + return –1 if this list does not contain the element

o lastIndexOf(Object o) –> returns the index of the last occurrence of the specified element in this list + return –1 if this list does not contain the element

o .clone() –> returns a value of the type Object

o contains(Object o) –> returns true if this list contains the specified element

o get(int index) –> returns the element at the specified position in this list

Question 5

ArrayList - 1:

• remove(int index) & remove(Object o)
• addAll(Collection extends E> c)
• addAll(int index, Collection extends E> c)
• clear()

Answer 5

o remove(int index) –> removes the element at the specified position

o remove(Object o) –> removes the first occurrence of the specified element if it’s present

o addAll(Collection extends E> c) –> appends all of the elements in the specified collection to the end of this list in the order in which they are returned by the specified collection’s Iterator

o addAll(int index, Collection extends E> c) –> inserts all of the elements in the specified collection into this list, starting at the specified position

o clear() –> remove all the ArrayList elements –> after removing all elements –> print out [ ] not null –> # set to null

Question 6

try-catch-finally –> what does the calling method receive when?

1. both catch & finally define return statements
2. finally modifies value returned from catch
3. 1. catch return primitive
4. 3. catch return object

Answer 6

1. finally
2. 1. finally cannot modify
3. 2. finally can modify

Question 7

what does getClass() return?

Answer 7

always returns the Class object for the actual object on which the method is called irrespective of the type of the reference

Question 8

setLength()

o method of String or StringBuilder?
o newLength argument = current length

Answer 8

o StringBuilder
o newLength argument length is changed to the specified length
o newLength argument >= current length –> sufficient null characters (‘\u0000’) are appended so that length becomes the newLength argument

Question 9

A || B khác gì A | B

A && B khác gì A & B

false && false = ?
false || false = ?
true || false = ?

Answer 9

‘&’ performs both tests, while ‘&&’ only performs the 2nd test if the first is also true

A || B khác gì A | B

A && B khác gì A & B

false && false = false
false || false = false
true || false = true

Question 10

access reference variable - object –> when depends on:

• reference variable
• object

how must variables, methods be declared?

Answer 10

static + khác –> reference variable

non-static method –> object

variables, methods must be declared in both superclass and derived class

Question 11

what happens when derived classes extend abstract class?

Answer 11

o implement all abstract method from base class

o derived classes have to be defined as abstract class

Question 12

3 categories of exception. Which ones do not have to be thrown?

Answer 12

only checked is required to be thrown. The two others should not (but can) be thrown

Question 13

variables defined in the body of interface are implicitly?

Method in interface? (3)

Answer 13

variables - abstract and public, static, final

method
+ must be public
+ no final + no static

Question 14

interface implement interface and extend class?

Answer 14

no. Interface cannot implement class but can extends many other interfaces with keyword extends

Question 15

constructors:

o name?
o static, final, abstract?
o how many constructors per class?
o return type?
o what is default constructor? Is it important?

Answer 15

o same name as the name of the class in which they’re defined
o can many but should not be
o no return type, even void --> return type = method
o cannot be static, final, abstract
o in the absence of a user-defined constructor --> Java inserts automatically + no argument. Without a constructor, a class won't run

Question 16

initializer block:

• what is? A part of method?
• parameter method?
• execute for what?
• what happen when define both initializer blocks and constructor in a class?

Answer 16

• an initializer block is defined within a class, not as a part of a method
• cannot accept method parameters
• executes for every object that’s created for a class
• call both + initializer blocks execute prior to the constructors

Question 17

byte a;
byte b;

s = a * b –> s thuộc type gì?

Answer 17

operands of mathematical operators are ALWAYS promoted to AT LEAST int and the return value will be AT LEAST int

Question 18

without casting, can something bigger than int be assigned to int and something smaller than int?

Answer 18

no

Question 19

default type of floating number?

When may float be written without f in suffix and when must?

Answer 19

• double
• should add suffic F or f if have decimal
  right:
  float i = 123;
  float j = 123.5f;

wrong:
float m = 123.5;

Question 20

output

double m = 50 / 100;
double i = 50.0 / 100;
double j = 50 / 100.0;
double k = 50.0 / 100.0;

Answer 20

output khi in

double m = 50 / 100; –> 0.0 –> default code number is int
double i = 50.0 / 100; –> 0.5
double j = 50 / 100.0; –> 0.5
double k = 50.0 / 100.0; –> 0.5

Question 21

for loop –> 3 fields = blank?

• initialization statement –> can define many variables? Type?
• Condition blank? Value?
• update statement –> many? Order of execution? Call to other methods?

Answer 21

3 fields can be blank

• initialization statement –> can define many variables + must have the same type?
• Condition blank –> true
• update statement –> can have many + execute in the order in which they appear + can include call to other methods

Question 22

equals & == khác gì nhau?

Answer 22

refer to the same object? ==

Question 23

what is instance method & class method?

Answer 23

instance method = non-static method

class method = static method

Question 24

o can size of an array be expanded or reduced once it is allocated?

o declaring array = create variable referring to ?

o can array be data type of null?

o can a char array be assigned to an int array?

Answer 24

o no

o null

o no

o no

Question 25

String String = “a”;

String char = “a”;

Answer 25

String String = “a”; –> ok

String char = “a”; –> not ok

Question 26

binary number –> properties (3)

Answer 26

• gồm ký tự 0 và 1
• 0b + ….
• cannot use suffix L, F,…

Question 27

can call both call super(…) and this(…) from a constructor?

Answer 27

can either call super(…) or this(…) but not both from a constructor

Question 28

main method:

• can args array be null?
• program runs without any arguments –> what does args point to?

Answer 28

o can never be null

o if the program is run without any arguments, args points to a String array of length 0

Question 29

chaining to use a value of a variable at the time of declaration --> allowed?
------------
int a = b = c = 100;
------------
int a, b, c;
a = b = c = 100;
----------
int b = 0, c = 0;
int a = b = c = 100;

Answer 29

no
-----
int a = b = c = 100; ==> wrong
-------
the other two = okay

Question 30

variable arguments:

• int… args or int args…
• meaning + no parameter –> length? Null?

Answer 30

o int… args
o can take any number of arguments (even 0)
o no parameter –> length = 0 + NOT null