151 - 180 Flashcards
which one is correct
int a[ ][ ] = new int [ ][4] ;
int[ ] a[ ] = new int[4][ ] ;
only int[ ] a[ ] = new int[4][ ] ; is correct
static block of the base class / class
non-static block of the base class / class
constructor of the base class / class
- Static blocks of the base class (only once, in the order they appear in the class).
- Static blocks of the class.
- Non-static blocks of the base class.
- Constructor of the base class.
- Non-static blocks of the class.
- Constructor of the class.
- Derived class’s static or non-static blocks are not executed if that class is not being used.
instance variable
static variable
local variable
–> which one is not automatically given default value?
local variable
char[] b = { 'j', 'a', 'v', 'a' };
String c = new String(b);
System.out.println(c); –> output?
java
can a class be imported statically?
no
a class cannot be imported statically
class B {B(){ } }
does B have a default constructor?
no
What will be the output of the following program ?
class CorbaComponent{
String ior;
CorbaComponent(){ startUp(“IOR”); }
void startUp(String s){ ior = s; }
void print(){ System.out.println(ior); }
}
class OrderManager extends CorbaComponent{
OrderManager(){ }
void startUp(String s){ ior = getIORFromURL(s); }
String getIORFromURL(String s){ return "URL://"+s; }
}
public class Application{
public static void main(String args[]){ start(new OrderManager()); }
static void start(CorbaComponent cc){ cc.print(); }
}
It will print URL://IOR
The method selection is done on the basis of the actual class of the object (which is OrderManager here). So OrderManager’s startUp is called, which sets the ior variable to URL://IOR
while(int k=5;k valid?
not. Not valid
In Java, a while or do/while construct takes an expression that returns a boolean. But unlike a for loop, you cannot put instantiation and increment sections in the while condition. Therefore, for(int k=5;k
- primitives are object?
- array of primitives and of objects is object?
- An array of objects can store Objects of any class.
- Primitives (i.e. int, byte, char, short, boolean, long, double, and float) are NOT objects.
- An array (of primitives as well as of objects) is an Object.
meaning of using an asterisk (*) in import statement?
by using an asterisk (*), you can import all of the public members, classes, and interfaces of a package
can use import statement to access multiple classes or interfaces with the same names from different packages?
No
cannot use the import statement to access multiple classes or interfaces with the same names from different packages
int i = 5; float m = 5.0f; System.out.println(i == m); float n = 5.5f; System.out.println(i == n);
–> output
i == m –> true
i == n –> false
value of i will be promoted to a float i.e. 5.0, and so it returns false
….? method1() : method2()
- method1() and method2() are void
No. It does not compile
- it is not permitted for either the second or the third operand expression of the ? operator to be an invocation of a VOID method
- -> non-VOID method = okay
class B {}
class B1 extends B {}
class B2 extends B {}
public class ExtendsTest{
public static void main(String args[]){
B b = new B();
B1 b1 = new B1();
B2 b2 = new B2();
// insert statement here
}
}
which one is correct?
- b1 = (B1) b;
- b1 = (B) b1;
correct: b1 = (B1) b;
b1 = (B) b1; –> won’t compile. By casting b1 to B, you are telling the compiler that b1 points to an object of class B. But you are then trying to assign this reference to b1, which is of class B1. Compiler will complain against this assignment because there is no guarantee that an object of class B will also be of class B1.
is ArrayList a subclass of AbstractList?
yes
ArrayList is a subclass of AbstractList
what can be stored in an ArrayList?
only Objects can be stored
no Primitive
limit of for-each loop (3)
- cannot be used to initialize an array + modify its elements
- cannot be used to delete or remove the elements of a collection
- cannot iterate over multiple collections or arrays in the same for-each loop –> cannot define multiple looping variables in a for-each loop
output?
class TestClass{
void probe(Object x) { System.out.println("In Object"); }
void probe(Number x) { System.out.println("In Number"); }
void probe(Float x) { System.out.println("In Float"); }
void probe(Integer x) { System.out.println("In Integer"); }
void probe(Long x) { System.out.println("In Long"); }
public static void main(String[] args){
double a = 10;
new TestClass().probe(a);
}
}
In Number
Here, we have four overloaded probe methods but there is no probe method that takes a double parameter. However, a double will be boxed into a Double and class Double extends Number. Therefore, a Double can be passed to the method that takes Number. A Double can also be passed to a method that takes Object, but Number is more specific than Object therefore probe(Number ) will be called.
what is the problem here?
float f = 0.0f;
try{
f = Float.valueOf(s).floatValue();
return f ;
}
catch(NumberFormatException nfe){
System.out.println("Invalid input " + s);
f = Float.NaN ;
return f;
}
finally { System.out.println("finally"); return f ;}
return f;
}
the last return f; (after finally) is unreachable
how to solve?
- remove return in finally
- remove either return in try or in catch
which one is wrong?
String abc = “abc”;
String 2abc = “2abc”;
String _abc = “_abc”;
only String 2abc = “2abc”; is wrong
an identifier must not start with a digit
What will be the result of attempting to compile and run the following program?
class TestClass{
public static void main(String args[]){
boolean b = false;
int i = 1;
do{
i++ ;
} while (b = !b);
System.out.println( i );
}
}
3
does String has methods:
- reverse( )
- hashCode( )
- trim( )
have:
- hashCode( )
- trim( )
don’t have
- reverse( ) –> StringBuilder
- can an interface extend from multiple interfaces?
- can a class extend from multiple classes?
- an interface can extend from multiple interfaces
- a class cannot extend from multiple classes
Consider :
class A { public void perform_work(){} }
class B extends A { public void perform_work(){} }
class C extends B { public void perform_work(){} }
How can you let perform_work() method of A to be called from an instance method in C?
impossible
There is no way to go more than one level up for methods
–> this problem doesn’t occur for instance variables because variable are never overridden. They are shadowed. So to access any of the super class’s variable, you can unshadow it using a cast. For example, ((A) c).data; This will give you the data variable defined in A even if it is shadowed in B as well as in C
what is overload method?
- define a method argument list with either a different number or type of method parameters
- only share the same name
what does default constructor in the derived class call? (if no other contructors are defined)
calls the no-args constructor of the super class.
constructor:
- have empty bodies?
- can be static, final, synchronized, native and abstract?
- can be applied access modifiers?
- constructors cannot have empty bodies (i.e. they cannot be abstract)
- cannot be static, final, synchronized, native and abstract
- can apply public, private, protected to a constructor
_ –> how to use it?
_
only occur in between two digits
long y = 123_456_L; --> wrong long z = _123_456L; --> wrong float f1 = 123_.345_667F; --> wrong float f2 = 123_345_667F; --> valid float f2 = 123\_\_345_667F; --> valid
precedence: comparison operators - mathematical operators
Comparison operators have lower precedence than mathematical operators
valid declaration?
int a = ‘a’;
yes
System.out.println(a); –> 97
