01 - 30

Question 1

Narrowing conversion

• what is it?
• how to do?
  o bigger than int
  o equals / smaller than int

Answer 1

• bigger –> smaller
• bigger than int –> need casting
• equals / smaller than int –> do not need casting

Question 2

implicit promotion:

• operand
• int argument –> accept char?

Answer 2

• operands of mathematical operators are ALWAYS promoted to AT LEAST int and the return value will be AT LEAST int
• method takes int as argument –> can also takes char
• -> char will be implicitly promoted to int

Question 3

casting requirements (2)

Answer 3

• IS_A relationship
  (left hand side = super class)
• compatible data type

Question 4

hierarchy Exception

Answer 4

-

Question 5

what must be imported if want to use ArrayList?

Answer 5

import java.util.ArrayList;

Question 6

o how to know the number of elements in Array and ArrayList

o length of String & StringBuilder

Answer 6

o array –> .length

o ArrayList –> .size()

o String & StringBuilder –> .length()

Question 7

array delaration:

• new is required?
• new is not required?

Answer 7

o 1 line –> not required

o 2 line –> required

Question 8

operator + when use with String (concatenation) –> vị trí của “ “ ảnh hưởng ntn?

Answer 8

-

Question 9

how do derived classes use the variables and methods defined in the superclass?

Answer 9

directly as if they were defined in their own classes

Question 10

properties of final:

o class
o method
o variable + how must it be initialized?
o interface

Answer 10

• final class cannot be extended by another class
• final method cannot be overridden
• final variables cannot be reassigned a value but can call methods on this variable
  o final variable must be explicitly initialized in the same line of code or initialization block
• kein final interface

Question 11

when is a variable marked as eligible to be garbage collected? (2)

Answer 11

• can know which are eligible but not sure when
• when this object can no longer be accessed
  o object’s reference variable is assigned an explicit null value or is reinitialized
  o this object goes out of scope

Question 12

4 loại access modifiers + range of accessability

Answer 12

public > protected > default > private

Question 13

can static method access non-static variable?

Answer 13

no

Question 14

can use this and super in a static method?

Answer 14

no

Question 15

super.method # BaseClassName.method?

Answer 15

BaseClassName.method only applies to static method

super –> no limit

Question 16

can switch work with Array?

Answer 16

no

Question 17

what type of methods cannot be overridden? (2)

Answer 17

static, final

Question 18

define variables with the same name

Answer 18

local variables - instance variables

local variables - class / static variables

Question 19

• operator precedence: highest and second heighest
• && và ||
• == và =

Answer 19

• postfix - unary
• && > ||
• == > =

Question 20

overriding - overridden methods:
- return type of
o primitive
o object

• restrictivity of access modifiers
• what kind of method cannot be overridden?
• overriding: static and non-static method

Answer 20

• primitive = same
• object = same or subclass of overridden
• access modifiers of the overriding –> same or less
• cannot override final and static method
• cannot override/hide a static method with a non-static method and vice versa

Question 21

initial size:

• ArrayList
• StringBuilder

Answer 21

ArrayList –> 10

StringBuilder –> 16

Question 22

o method parameter is variable arguments –> what must it be? (2)
o meaning of variable arguments
o how to write?

Answer 22

o only 1 + last in parameter list

o (int… days)

Question 23

WrapperClass:

o mutable? How to compare?
o provide what mechanism? (2)
o constructor accept what? (2)
o can 2 primitive wrapper be compatible?
o PrimitiveWrapper.MIN_VALUE + 1 =?

Answer 23

• immutable
• == operator –> compare with compatible primitive
• 2 primitive wrappers are never compatible
• primitive wrapper constructor accepts
  + compatible primitive value
  + the String representation
• PrimitiveWrapper.MIN_VALUE + 1 == PrimitiveWrapper.MAX_VALUE

Question 24

startsWith, endsWith

• what is?
• accepted type?

Answer 24

• determines whether a String starts / ends with a specified prefix, specified as a String
• only String with “ “

Question 25

when instance variables and methods bind?

Answer 25

instance variables bind at compile time

methods bind at runtime

Question 26

package:

o if present, how many time can a package statement appear?
o can a class from default package be used in named packaged class?
o how is default package imported in classes and interfaces defined in the same directory?

Answer 26

• if present, the package statement must appear exactly once in a class
• a class from a default package can’t be used in any named packaged class, regardless of whether they are defined within the same directory or not
• can define multiple classes and interfaces in the same Java source code file
• classes and interfaces defined in the same Java source code file cannot be defined in separate packages

Question 27

import static or static import?

Answer 27

import static