10. Accessing non-static member variables from a static method.
public class StaticDemo
{
public String my_member_variable = "somedata";
public static void main (String args[])main is a static method. So, we must create an instance of the class if we want to access the non-static members of the class.
{
// Access a non-static member from static method
System.out.println ("This generates a compiler error" +
my_member_variable );
}
}
9. Mistyping the name of the method while overriding
public class MyWindowListener extends WindowAdapter {
// This should be WindowClosed
public void WindowClose(WindowEvent e) {
// Exit when user closes window
System.exit(0);
}
});
Compilers won't pick up on this one, and the problem can be quite frustrating to detect. In the past, I've looked at a method, believed that it was being called, and taken ages to spot the problem. The symptom of this error will be that your code isn't being called, or you think the method has skipped over its code. The only way to ever be certain is to add a println statement, to record a message in a log file, or to use good trace debugger (like Visual J++ or Borland JBuilder) and step through line by line. If your method still isn't being called, then it's likely you've mistyped the name.
8. Comparision assignment (= instead of ==)
Fortunately, even if you don't spot this one by looking at code on the screen, your compiler will. Most commonly, it will report an error message like this : "Can't convert xxx to boolean", where xxx is a Java type that you're assigning instead of comparing.
7. Comparing two objects ( == instead of .equals)
When we use the == operator, we are actually comparing two object references, to see if they point to the same object. We cannot compare, for example, two strings for equality, using the == operator. We must instead use the .equals method, which is a method inherited by all classes from java.lang.Object.
Here's the correct way to compare two strings.
String abc = "abc"; String def = "def";
// Bad way
if ( (abc + def) == "abcdef" )
{
......
}
// Good way
if ( (abc + def).equals("abcdef") )
{
.....
}
6. Confusion over pass by value and pass by reference
This can be a frustrating problem to diagnose, because when you look at the code, you might be sure that its passing by reference, but find that its actually being passed by value. Java uses both, so you need to understand when you're passing by value, and when you're passing by reference.
When you pass a primitive data type, such as a char, int, float, or double, to a function then you are passing by value.
When you pass a Java object, such as an array, a vector, or a string, to a function then you are passing by reference. Yes - a String is actually an object, not a primitive data type. So that means that if you pass an object to a function, you are passing a reference to it, not a duplicate. Any changes you make to the object's member variables will be permanent - which can be either good or bad, depending on whether this was what you intended.
On a side note, since String contains no methods to modify its contents, you might as well be passing by value.
5. Writing blank Exception handlers
I know it's very tempting to write blank exception handlers, and to just ignore errors. But if you run into problems, and haven't written any error messages, it becomes almost impossible to find out the cause of the error.
4. Forgetting that Java is 0-based index
3. Preventing concurrent access to shared variables by threads
When writing multi-threaded applications, many programmers (myself included) often cut corners, and leave their applications and applets vulnerable to thread conflicts. When two or more threads access the same data concurrently, there exists the possibility (and Murphy's law holding, the probability) that two threads will access or modify the same data at the same time. Don't be fooled into thinking that such problems won't occur on single-threaded processors. While accessing some data (performing a read), your thread may be suspended, and another thread scheduled. It writes its data, which is then overwritten when the first thread makes its changes.
Such problems are not just limited to multi-threaded applications or applets. If you write Java APIs, or JavaBeans, then your code may not be thread-safe. Even if you never write a single application that uses threads, people that use your code WILL. For the sanity of others, if not yourself, you should always take precautions to prevent concurrent access to shared data.
How can this problem be solved? The simplest method is to make your variables private (but you do that already, right?) and to use synchronized accessor methods. Accessor methods allow access to private member variables, but in a controlled manner. Take the following accessor methods, which provide a safe way to change the value of a counter.
public class MyCounter
{
private int count = 0; // count starts at zero
public synchronized void setCount(int amount)
{
count = amount;
}
public synchronized int getCount()
{
return count;
}
}
2. Capitalization errors
This is one of the most frequent errors that we all make. It's so simple to do, and sometimes one can look at an uncapitalized variable or method and still not spot the problem. I myself have often been puzzled by these errors, because I recognize that the method or variable does exist, but don't spot the lack of capitalization.
And.. last but not the least.....
1. NULL Pointers!!!!
Null pointers are one of the most common errors that Java programmers make. Compilers can't check this one for you - it will only surface at runtime, and if you don't discover it, your users certainly will.
When an attempt to access an object is made, and the reference to that object is null, a NullPointerException will be thrown. The cause of null pointers can be varied, but generally it means that either you haven't initialized an object, or you haven't checked the return value of a function.
Many functions return null to indicate an error condition - but unless you check your return values, you'll never know what's happening. Since the cause is an error condition, normal testing may not pick it up - which means that your users will end up discovering the problem for you. If the API function indicates that null may be returned, be sure to check this before using the object reference!
Another cause is where your initialization has been sloppy, or where it is conditional. For example, examine the following code, and see if you can spot the problem.
public static void main(String args[])
{
// Accept up to 3 parameters
String[] list = new String[3];
int index = 0;
while ( (index < i =" 0;">This code (while a contrived example), shows a common mistake. Under some circumstances, where the user enters three or more parameters, the code will run fine. If no parameters are entered, you'll get a NullPointerException at runtime. Sometimes your variables (the array of strings) will be initialized, and other times they won't. One easy solution is to check BEFORE you attempt to access a variable in an array that it is not equal to null.
Courtesy : http://www.javacoffeebreak.com/articles/toptenerrors.html
