91 - 120

Question 1

what happen when do not catch an exception?

Answer 1

if do not catch an exception, it is propagated up the stack of method calls until it is handled

If nobody handles it, the JVM handles that exception and kills the thread. If that thread is the only user thread running, the program ends.

Question 2

StringBuilder:

• substring() –> modify value?
• append()
• insert()
• delete() & deleteCharAt()
• reverse() - reverse substring?
• replace() –> end position?
• subSequence()

Answer 2

o substring() = not modify
o append() --> adds the specified value at the end of the existing value of a StringBuilder object
o insert() --> insert the requested data at a particular position  removes the characters in a substring of the specified StringBuilder
o deleteCharAt() --> removes the char at the specified position
o reverse() --> reverse the sequence of characters of a StringBuilder + cannot reverse a substring of StringBuilder
o replace() --> replaces a sequence of characters / substring, identified by their positions (end position not included)
o subSequence() ~ substring()

Question 3

StringBuilder:

setLength()

newLength >= current length?

Answer 3

sets the length of the character sequence to a new character sequence whose length is specified by the argument

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

Question 4

Primitive wrappers + java.lang.System

• final?
• meaning?

Answer 4

yes. Final –> cannot be extended

Question 5

how do compound assignment operators cast?

Answer 5

all compound assignment operators internally do an explicit cast with the type of the variable on the left hand side
*=
\+=
-=
/=

Question 6

can catch block for exception catch an Error?

Answer 6

no

Question 7

String, StringBuilder, StringBuffer —> final? What does this mean? (2)

Answer 7

• cannot be extended

- all methods of these 3 classes are also final

Question 8

while-loop —> condition expression blank?

Answer 8

no —> cannot leave blank = must have a condition

Question 9

To construct an instance of a sub class, what needs to be done?

Answer 9

to construct an instance of a sub class, its super class needs to be constructed first.

Since an instance can only be created via a constructor, some constructor of the super class has to be invoked. Either you explicitly call it or the compiler will add super() (i.e. no args constructor) as the first line of the sub class constructor

Question 10

? constructor –> what should both side of : be?

Answer 10

both sides of : should return some value –> break and continue do not work

however, this works:

?true:false

Question 11

order of the exceptions caught in the catch blocks

1 . unrelated class
2. IS-A relationship

Answer 11

1. unrelated classes –> order does not matter but each catch must be of a different type
2. related classes sharing an IS-A relationship: catch an exception of the base class before an exception of the derived class –> Fail to compile

Question 12

try - catch - finally

• exist independently?
• 1 try block - how many catch and finally block?
• what follow try block?

Answer 12

• try, catch, and finally blocks cannot exist independently
• 1 try block - (can define) multiple catch blocks - only a single finally block
• try block may be followed by either a catch or a finally block or both

Question 13

throw exception in Method and Constructor –> difference?

Answer 13

Observe that the rule for overriding a method is opposite to the rule for constructors.

• An overriding method cannot throw a superclass exception –> must be same or sub
• A constructor of a subclass cannot throw subclass exception –> must be same or super

Question 14

how must checked exception be dealt with?

Answer 14

must be handled by either a try catch block or declared in the throws clause of the enclosing method

Question 15

overriding method - overridden method:

• return type
• access modifier

Answer 15

• an overriding method can change the return type of the overridden method to a subclass of the original return type

–> Covariant returns

• access modifier of the overriding method cannot be more restrictive

Question 16

what is?

• null array
• empty array

Answer 16

• a null array is a null Array Reference (since arrays are reference types in Java)

int[] notAnArray = null;

• an empty array is an array of length zero; it has no elements

int[] emptyArray = new int[0];

Question 17

class A calls method B, which throws exception X –> what should A do?

Answer 17

class A should either (or both):

• throws exception Y, which is the same or subclass of X
• wrap up the exception in a try - catch block

Question 18

what is instance variable? Its accessibility

Answer 18

• non-static variable + declared within a class, outside of all of the methods
• accessible to all the non-static methods defined in a class + only objects of this class will have this field

Question 19

are local variables initialized with default value?

Answer 19

local variables are not initialized with their default values

–> must be explicitly initialized

Question 20

without argument –> is array in main method assigned to null?

Answer 20

no –> the length is 0

Question 21

switch –> case statement: what should value be? (2)

Answer 21

compile-time constant value

• the value should be known at the time of code compilation
• cannot pass an expression if variable is not final

Question 22

What will be the result of attempting to compile and run the following program?

public class TestClass{
  public static void main(String args[ ] ){
    Object a, b, c ;
    a = new String("A");
    b = new String("B");
    c = a;
    a = b;
    System.out.println(" "+c);
  }
}

Answer 22

print A

Question 23

which declaration is correct?

A: double x = 10, double y;
B: double x = 10, y = 5;

Answer 23

A —> wrong

B —> correct

Question 24

• what is method signature?

- more than one method in a class with the same signature?

Answer 24

• includes method name and the argument list but does not include return type.
• cannot have more than one method in a class with the same signature.

Question 25

operator with least precedence?

Answer 25

=

Question 26

exception in overriding method

• Runtime
• Checked

Answer 26

• Runtime Exception / Unchecked Exception
  o can only throw
  + exceptions that are declared in the throws clause of the overridden method
  + exceptions that are subclasses of the declared exceptions
  o can choose NOT to throw any exception.
• Checked Exception: if overridden method does not have any throw clause, the overriding method cannot declare any checked exceptions

Question 27

can abstract method overrides non-abstract method?

Answer 27

yes

independent on abstract / non-abstract

Question 28

A:
int a = b = c = 100;

B:
int a, b, c;
a = b = c = 100;

C:
int b = 0, c = 0;
int a = b = c = 100;

which one is correct?

Answer 28

A —> wrong

B, C —> correct

Question 29

static variable

• can static variable be left without being initialized?
• no instances of the object of the class have been initialized —> can be accesses?

Answer 29

• yes —> default value

- yes. Can be accessed

Question 30

làm tròn số

0. 5
   - 0.5

Answer 30

0. 5 —> 1
   - 0.5 —> 0

Observe that rounding is a standard mathematical procedure where the number that lies exactly between two numbers always rounds up to the higher one

Question 31

&& - || –> precedence?

Answer 31

&& have more precedance than || operator

Question 32

package
import

–> which should come first?

Answer 32

package