Java Reference
In-Depth Information
About the null pointer . Suppose you declare a variable of some class-type, say
JFrame jframe;
but forget to store anything in it. Variable jframe contains null rather than the
name of a folder. If you then try to access a method, say with jframe.get-
Width() , Java will give this message: java.lang.NullPointerException .
By “null pointer”, they mean “ null folder name”.
A Java-console message describes the call stack: the stack of
methods that have been called but that have not yet completed.
You can use this stack of calls to help figure out how your program got to the
point of throwing an Exception .
Output of an Error
A runtime error can lead to throwing either an Exception or an Error .
Throwing an Exception may or may not cause immediate termination; as you
will see later, it depends on whether your program “handles” it. Throwing an
Error always causes immediate termination of a program. Error s are too severe
to consider continuing execution. Here are examples of Error s that you may see:
/** = the value r that satisfies x = q*y+r and 0<=r<y for some q.
Throw an IllegalArgumentException if y = 0.*/
public static int mod( int x, int y) {
if (y == 0)
{ throw new IllegalArgumentException("x mod 0 is illegal"); }
/* { Because q*y = (-q)*(-y) , we have: mod(x,y) = mod(x,-y). } */
y= Math.abs(y);
int r= x % y;
/* For x>=0,mod(x,y)=x%y */
if (x >= 0)
{ return r; }
/* For x<0,x%y is the value r' that satisfies
x=q*y+r' and -y<r'<-0
= < manipulate >
x = (q - 1) * y + (r' + y) and -0<r'+y<=y
Hence , x mod y is x%y+y */
return r+y;
Figure 10.1:
A throw-statement in function mod
Search WWH ::

Custom Search