EN
Java - what is java.lang.NullPointerException and how to prevent it?
14 points
NullPoiterException occurs when progremmer try to acces to object type variable that has been assigned with null. It is caused because of programmer mistake. There are few solution how to prevent it:
If some variable can be null then should be checked before usage by if instruction.
xxxxxxxxxx1
// some code...2
3
if(myVariable != null) {4
// do some operation with myVariable...5
}6
7
// some code...If programmer predicted null value can happen but he wanted to tell others it is not his mistake but mistake of the person who used souce code (not read documentation where is information acout not null argument/variable).
xxxxxxxxxx1
// some code...2
3
assert myVariable != null : "Variable can not be null";4
5
// some code...Notes:
assertallows for code execution only ifa != null.- assertions make source code more clear,
- read this article to enable assertions in your program.
xxxxxxxxxx1
public class Program {2
3
public static void main(String[] args) {4
String a = "text"; // or = null by default5
6
// some code...7
8
a = null; // for example: some operation caused null value9
10
// some code...11
12
System.out.println(a.length());13
}14
}Output:
xxxxxxxxxx1
Exception in thread "main" java.lang.NullPointerException2
at Program.main(Program.java:12)3
4
Process finished with exit code 1xxxxxxxxxx1
public class Program {2
3
public static void main(String[] args) {4
String a = "text"; // or = null by default5
6
// some code...7
8
a = null; // for example: some operation caused null value9
10
// some code...11
12
if(a != null) {13
System.out.println(a.length());14
} else {15
// other scenario...16
}17
}18
}Output (empty because nothing happened):
xxxxxxxxxx1
xxxxxxxxxx1
public class Program {2
3
public static void main(String[] args) {4
String a = "text"; // or = null by default5
6
// some code...7
8
a = null; // for example: some operation caused null value9
10
// some code...11
12
assert a != null : "Variable can not be null";13
14
System.out.println(a.length());15
}16
}Output:
xxxxxxxxxx1
Exception in thread "main" java.lang.AssertionError: Variable can not be null2
at Program.main(Program.java:12)3
4
Process finished with exit code 1Note:
AssertionErroroccured becausea != nullcondition retunredfalse. Programmer who used source code knows he made mistake.
