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Calling the noarg constructor
The code in the makeObj method of the Worker class attempted to instantiate a new object using the following code:
NewClass obj = new NewClass(); Since the class definition didn't contain a definition for a noarg constructor, the following compiler error was produced by JDK 1.3.
Ap093.java:18: cannot resolve symbol
symbol : constructor NewClass()
location: class NewClassNewClass obj = new NewClass(); C. An Object
We finally got it right!
Did you identify the errors in the previous two programs before looking at the answers?
This program declares the class named NewClass correctly and uses the new operator correctly in conjunction with the default noarg constructor for the NewClass class to create a new instance of the class (an object) .
Making the class public
One of the things that I could do differently would be to make the declaration for the NewClass class public (as shown in the following code fragment) .
public class NewClass{
public String toString(){return "An Object";
}//end toString()}//end NewClass I am a little lazy
The reason that I didn't declare this class public (and haven't done so throughout this series of modules) is because the source code for all public classes and interfaces must be in separate files. While that is probably a good requirement for large programming projects, it isoverkill for simple little programs like I am presenting in this group of self-assessment modules.
Dealing with multiple files
Therefore, in order to avoid the hassle of having to deal with multiple source code files for every program, I have been using package-private access for class definitions other than the controlling class (the controlling class is declared public) . Although I won't get into the details at this point, when a class is not declared public, it is common to saythat it has package-private access instead of public access.
A. Compiler Error
Java is a case-sensitive language
Java keywords must be written exactly as specified. The keyword class cannot be written as Class , which is the problem with this program.
The inappropriate use of the upper-case C in the word Class caused the following compiler error.
Ap091.java:25: 'class' or 'interface' expected
Class NewClass{ The solution to the problem
This problem can be resolved by causing the first character in the keyword class to be a lower-case character as shown in the following code fragment.
class NewClass{
public String toString(){return "An Object";
}//end toString()}//end NewClass A. Compiler Error
Instantiating an object
There are several ways to instantiate an object in Java:
Of all of these, the last two are by far the most common.
What you cannot do!
You cannot instantiate a new object using code like the following code fragment that was extracted from this program.
NewClass obj = NewClass(); This program produces the following compiler error:
Ap090.java:18: cannot resolve symbol
symbol : method NewClass ()location: class Worker
NewClass obj = NewClass(); The solution to the problem
This problem can be solved by inserting the new operator to the left of the constructor as shown in the following code fragment.
NewClass obj = new NewClass(); -end-
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