181 - 210 Flashcards
is null an object?
no. null is not an object
any method invocation on a null results in a NullPointerException
output?
----------
public class TestClass {
public static void main(String[] args){
TestClass arr[] = new TestClass[4];
System.out.println(arr[0]);
}
}null
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]);
}
}0
valid?
int i = ‘8’;
yes. It is valid
System.out.println(i); –> 56
while(int k = 5; k >0){} –> valid?
invalid
should be
int k = 5
while(k >0){}
- when is a class defined in a default package?
2. can a class from a default package be used in a named packaged class?
- when no package name is defined
- 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
package statement:
(if present)
- where?
- how many in a class?
- can use the import statement to access multiple classes or interfaces with the same names from different packages?
- first statement in a class
- exactly once in a class
- 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
difference between:
- import
- import static
- import is for class
- import static is for static members of a class
which give NullPointerException?
String[] a = new String[2]; a[0] = null; System.out.println(a[0]); a[0].length();
a[0].length() –> NullPointerException
System.out.println(a[0]) –> print null
main method:
- must Java application have a main method?
- must public class?
- Every Java application must have a main method. It’s the starting point for the execution of the code in the application
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
//1 –> false
//2 –> true
is constructor inherited?
no. Never
default constructor of derived class calls default constructor of base class?
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
what is checked exception?
- 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.
what happens if a method uses another method that may throw a checked exception?
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
Error:
- what is?
- should be a part of a method signature?
- should be caught?
- is a serious exception thrown by the JVM as a result of an error in the environment state that processes your code
- need not be a part of a method signature
- can be caught by an exception handler, but it should not be
are IOException and FileNotFoundException a RuntimeException?
which one is the subclass of the other?
no
FileNotFoundException is a subclass of IOException
can a well written application recover from checked exception?
checked exception is exceptional conditions EXTERNAL to an application that a well written application should anticipate and from which it CAN RECOVER
how to make a good encapsulation?
- variables
- functionality that is to be exposed outside
- setter methods
- 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
double h = 9.0/5 - 1; System.out.println(“h = “ + h);
double k = 9/5 - 1; System.out.println(“k = “ + k);
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
default value of primitive:
- integer types
- decimal types
- boolean
- char
- 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
abstract method
- can be set static?
- ## what kind of access modifiers can be set to the method k?abstract class abc{
abstract void k();
}
- cannot be set to static
2. only public or protected
can variables (instance, static, local, and method parameters) be defined as abstract?
no
none of the different types of variables (instance, static, local, and method parameters) can be defined as abstract
works?
public int calculation(int z){return 0;}
public static void main(String[] args){
int h = calculation(10);
}
no. static method cannot use non-static method, variables,…
fix:
public static int calculation(int z){return 0;}
works?
public static int calculation(int z){return 0;}
public int recher(int m){
return calculation(10);
}
yes. non-static method can use static-method, variables but not vice-versa
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
}
}//1. class B --> getClass() gets class of the actual object
//2. ClassCastException: A cannot be cast to B
works?
-----
try{
int i = 0;
i++;
}
finally{
System.out.println(i);
}no
local variables:
1. specify visibility?
- initialized with their default values?
- cannot apply any modifier except final to a local variable –> no static, no public,…
- local variables are not initialized with their default values –> print before initializng = compilation error
Throwable and any subclass of Throwable that is not also a subclass of either RuntimeException or Error are regarded as checked exceptions?
yes
Throwable and any subclass of Throwable that is not also a subclass of either RuntimeException or Error are regarded as checked exceptions
only Throwable and its subclasses can be the argument type in a catch clause?
yes
