Top Most Notes on Flow control, Assertions, and Exception Handling

1. To enable assertions on non – system classes use –ea or –enableassertions
2. To disable assertions on non – system classes use –da or –disableassertions
3. To enable assertions on system classes use –esa or –enablesystemassertions
4. To disable assertions on system classes use –dsa or –disablesystemassertions
5. Assertions can be selectively enabled for the unnamed package in the current working directory.
6. In any method with a non void return type we can’t place an assert statement like

“assert false;” ,it results in compiler error but we can place a

“throw new AssertionError();” in place of a return statement.

7. A instance variable can be redeclared within a method ,like for example

class A{

int i=1;

void m(){int i=2;}

}

8. A method local variable cannot be redeclared within another block inside a method after declaring it inside the method ,like for example

class A{

void m(){

int i = 10;

for(int i = 1;i<20;i++){//this line causes compiler error
System.out.println(i);

}}}

9. If a  method local variable can be declared inside a method after declaring it inside a block of code then no compiler sets in ,like for example

class A{

void m(){

for(int i=0;i<10;i++){

System.out.println(i);

}

int i=100;

}}

the above class when compiled doesn’t cause error because the variable ‘i’ is declared inside the method after it is declared inside the for loop.

10. A constructor can have the following 4 access levels,
• public
• private
• protected
• default or package access level( no modifier)

→If no constructor is supplied by the programmer then the compiler itself  creates a “default constructor” for the class. The default has the following properties

·         it takes no argument

·         it contains a call to the no-arg constructor of the super class as its first statement.

·         it will have the same access level as that of the given class.

11. A constructor

·         cannot be declared final , static , native , synchronized.

·         It can declare any exception in it’s throws clause.

·         if any other class extends the given class then that class must include the exception in its throws clause or must include a wider exception in its throws clause.

→ it cannot declare the call to super class constructor in try catch clause.

12. An anonymous class doesnot have any constructor.
13. initializer blocks cannot throw exceptions.