181 - 210

Question 1

is null an object?

Answer 1

no. null is not an object

any method invocation on a null results in a NullPointerException

Question 2

output?
----------
public class TestClass {
    public static void main(String[] args){
    	TestClass arr[] = new TestClass[4];
    	System.out.println(arr[0]);
    }
}

Answer 2

null

Question 3

output?
--------
public class TestClass {
    public static void main(String[] args){
    	int arr[] = new int[4];
    	arr[1] = 1;
    	arr[2] = 2;
    	arr[3] = 3;
    	System.out.println(arr[0]);
    }
}

Answer 3

0

Question 4

valid?

int i = ‘8’;

Answer 4

yes. It is valid

System.out.println(i); –> 56

Question 5

while(int k = 5; k >0){} –> valid?

Answer 5

invalid

should be
int k = 5
while(k >0){}

Question 6

1. when is a class defined in a default package?

2. can a class from a default package be used in a named packaged class?

Answer 6

1. when no package name is defined
2. a class from a default package cannot be used in any named packaged class, regardless of whether they are defined within the same directory or not –> can never access a class that is defined in the default package from a class in any other package

Question 7

package statement:
(if present)

1. where?
2. how many in a class?
3. can use the import statement to access multiple classes or interfaces with the same names from different packages?

Answer 7

1. first statement in a class
2. exactly once in a class
3. cannot

• but can duplicate import
  –> example:
  import java.util.name
  import java.util.name
• example of a wrong case:
  import java.util.name
  import java.util.xyz.name

Question 8

difference between:

• import
• import static

Answer 8

• import is for class

- import static is for static members of a class

Question 9

which give NullPointerException?

	String[] a = new String[2];
	a[0] = null;
	System.out.println(a[0]);
	a[0].length();

Answer 9

a[0].length() –> NullPointerException

System.out.println(a[0]) –> print null

Question 10

main method:

1. must Java application have a main method?
2. must public class?

Answer 10

1. Every Java application must have a main method. It’s the starting point for the execution of the code in the application

Question 11

output:

String str1 = “java”;
char[] char1 = {‘j’, ‘a’, ‘v’, ‘a’};
String str2 = “”;
for(char i:char1){
str2 +=i;
}
System.out.println(str2);
System.out.println(str1 == str2); //1
System.out.println(str1.equals(str2)); //2

Answer 11

//1 –> false

//2 –> true

Question 12

is constructor inherited?

Answer 12

no. Never

Question 13

default constructor of derived class calls default constructor of base class?

Answer 13

unless you use super(…) a constructor calls the empty constructor of its parent. Note: It does this on all you classes, even the ones which extend Object

Question 14

what is checked exception?

Answer 14

• a subclass of class java.lang.Exception, but it is not a subclass of java.lang.RuntimeException

o the class Exception and any subclasses that are not also subclasses of RuntimeException are checked exceptions
o Throwable and any subclass of Throwable that is not also a subclass of either RuntimeException or Error are regarded as checked exceptions

• unacceptable condition FORSEEN by the author of a method at the time of writing this method but outside the immediate control of the code
• exceptional conditions external to an application that a well written application should anticipate and from which it can recover.

Question 15

what happens if a method uses another method that may throw a checked exception?

Answer 15

if a method uses another method that may throw a checked exception, one of the two following things should be true

• the method should be enclosed within a try-catch block
• the method should specify this exception to be thrown in its method signature

–> only applied to checked exception

Question 16

Error:

1. what is?
2. should be a part of a method signature?
3. should be caught?

Answer 16

1. is a serious exception thrown by the JVM as a result of an error in the environment state that processes your code
2. need not be a part of a method signature
3. can be caught by an exception handler, but it should not be

Question 17

are IOException and FileNotFoundException a RuntimeException?

which one is the subclass of the other?

Answer 17

no

FileNotFoundException is a subclass of IOException

Question 18

can a well written application recover from checked exception?

Answer 18

checked exception is exceptional conditions EXTERNAL to an application that a well written application should anticipate and from which it CAN RECOVER

Question 19

how to make a good encapsulation?

• variables
• functionality that is to be exposed outside
• setter methods

Answer 19

• data variable should be either private or protected
• functionality that is to be exposed outside should be public
• setter methods should be coded such that they don’t leave the data members inconsistent with each other

Question 20

double h = 9.0/5 - 1; System.out.println(“h = “ + h);

double k = 9/5 - 1; System.out.println(“k = “ + k);

Answer 20

h = 0.8
k = 0.0

lưu ý:

• double –> muốn hiện kết quả số thập phân thì phải khai báo có .
• float –> muốn hiện kết quả số thập phân thì phải khai báo có f ở suffix

Question 21

default value of primitive:

• integer types
• decimal types
• boolean
• char

Answer 21

• integer types (byte, short, int, long) –> 0
• decimal types (float and double) –> 0.0 –> not 0.0f nor 0.0d
• boolean –> false
• char –> /u0000 –> decimal value –> 0

Question 22

abstract method

1. can be set static?
2. ## what kind of access modifiers can be set to the method k?abstract class abc{
   abstract void k();
   }

Answer 22

1. cannot be set to static

2. only public or protected

Question 23

can variables (instance, static, local, and method parameters) be defined as abstract?

Answer 23

no

none of the different types of variables (instance, static, local, and method parameters) can be defined as abstract

Question 24

works?

public int calculation(int z){return 0;}
public static void main(String[] args){
int h = calculation(10);
}

Answer 24

no. static method cannot use non-static method, variables,…

fix:

public static int calculation(int z){return 0;}

Question 25

works?

public static int calculation(int z){return 0;}
	public int recher(int m){
		return calculation(10);
	}

Answer 25

yes. non-static method can use static-method, variables but not vice-versa

Question 26

output
---------
class A{}
class B extends A{}
public class TestClass {
    public static void main(String[] args){
    	A a = new B();
    	System.out.println(a.getClass()); //1
    	B b = (B) new A();
    	System.out.println(b.getClass()); //2
    }
}

Answer 26

//1. class B
--> getClass() gets class of the actual object

//2. ClassCastException: A cannot be cast to B

Question 27

works?
-----
try{
    		int i = 0;
    		i++;
    	}
    	finally{
    	System.out.println(i);
    	}

Answer 27

no

Question 28

local variables:
1. specify visibility?

2. initialized with their default values?

Answer 28

1. cannot apply any modifier except final to a local variable –> no static, no public,…
2. local variables are not initialized with their default values –> print before initializng = compilation error

Question 29

Throwable and any subclass of Throwable that is not also a subclass of either RuntimeException or Error are regarded as checked exceptions?

Answer 29

yes

Throwable and any subclass of Throwable that is not also a subclass of either RuntimeException or Error are regarded as checked exceptions

Question 30

only Throwable and its subclasses can be the argument type in a catch clause?

Answer 30

yes