public class ExceptionalClass {
public void method1() throws CheckedException {
        // ... some code
        throw new CheckedException( "something bad 
happened" );
}
    public void method2( String arg ) {
        if( arg == null ) {
            throw new NullPointerException( "arg passed to 
method2 is null" );
}   
}
public void method3() throws CheckedException {
    method1();
}
}  

            Contrast method1() with method2(). When you 
make a call to method2(), you don't have to do so within a 
try/catch block. Since you do not have to catch the 
exception, the exception is said to be unchecked; it is a 
runtime exception. A method that throws a runtime exception 
need not declare the exception in its signature.

Runtime exceptions represent problems that are detected by 
the runtime system. This includes arithmetic exceptions 
(such as when dividing by zero), pointer exceptions (such 
as trying to access an object through a null reference), 
and indexing exceptions (such as attempting to access an 
array element through an index that is too large or too 
small). 

Runtime exceptions can occur anywhere in a program and in a 
typical program can be very numerous. Typically, the cost 
of checking for runtime exceptions exceeds the benefit of 
catching or specifying them. Thus the compiler does not 
require that you catch or specify runtime exceptions, 
although you can.