While trying to compile my java class files with the 1.6 java compiler, I came across an error message that read :
“no unique maximal instance exists for type variable U with upper bounds U”
The weird thing was that the file compiled just fine in eclipse!
On further probing I realized it’s a javac bug (check the bug here)
Here’s the workaround -
Bug Scenario :
public class SomeObject {
<U extends SomeObject> U foo1() throws Exception {
SomeObject obj = new SomeObject();
return obj.foo1();
}
}
Solve this way :
public class SomeObject {
<U extends SomeObject> U foo1() throws Exception {
SomeObject obj = new SomeObject();
return obj.<U>foo1();
}
}
I know this may be trivial for many people but then it can be pretty confusing for new bees (pssst.. in the beginning me too used to be confused between overloading and overriding)
OVERRIDING – when you extend a class and write a method in the derived class which is exactly similar to the one present in the base class, it is termed as overriding.
Example:
public class BaseClass{
public void methodToOverride()
{
//Some code here
}
}
public class DerivedClass extends BaseClass{
public void methodToOverride()
{
//Some new code here
}
}
As you can see, in the class DerivedClass, we have overridden the method present in the BaseClass with a completely new piece of code in the DerivedClass.
What that effectively means is that if you create an object of DerivedClass and call the methodToOverride() method, the code in the derivedClass will be executed. If you hadn’t overridden the method in the DerivedClass then the method in the BaseClass would have been called.
OVERLOADING - when you have more than one method with the same name but different arguments, the methods are said to be overloaded.
Example:
public class OverLoadingExample{
public void add(int i, int j)
{
int k = i + j;
}
public void add(String s, String t)
{
int k = Integer.parseInt(s) + Integer.parseInt(t);
}
}
As you can see in the example above, we have the same method add() taking two parameters but with different data types. Due to overloading we can now call the add method by either passing it a String or int 
Time to explore the ‘final‘ keyword.
When you use the keyword ‘final’ with a variable declaration, the value stored inside that variable cannot be changed come what may!!
So suppose you do this :
final int i =10;
After this, nowhere in your code can you change the value of ‘i’ i.e. ‘i’ will always have a value ‘10′ .
So i = 11; //will give you a compile time error.
Now, suppose you did not initialize the final integer ‘i’ while declaring it.
final int i;
Can you assign any value to this variable now?
Yes, you definitely can. But only once!
So,
final int i;
i = 10; //This is fine
i = 11; //Compiler error
Now that was the core stuff about ‘final’ . Let’s learn a bit more in detail about it.
The ‘final’ keyword can be used for a class declaration or method declaration.
What happens when you use ‘final’ for a class declaration?
A: You cannot have a sub class of a class declared final i.e. No other class can extend this class.
What happens when you declare a method as final?
A: Simply put, the method cannot be overridden.
Can you override a method(Not declared as ‘final’) present inside a final class?
A: Obviously Not. Since there’s no way to extend a class declared final there’s no way of overriding its methods!
What happens when I declare a method inside a class as both ‘private’ and ‘final’ ?
A: Making a class both ‘private’ and ‘final’ makes the ‘final’ keyword redundant as a private method cannot be accessed in its sub class. But hey, you’ll be able to declare a method of the same name as in the base class if the method has been made private in the base class. But then that doesn’t mean you’re overriding the method. You’re simply declaring a new method in the sub class.
Note: You cannot make an ‘abstract’ class or method as ‘final’ because an ‘abstract’ class needs to be extended which will not be possible if you mark it as ‘final’!
I found this article on ‘final’ worth reading:
http://www.ibm.com/developerworks/java/library/j-jtp1029.html
