7.8 Ap0080: self-assessment, classes, constructors, and accessor  (Page 8/8)

Calling the noarg constructor

The code in the makeObj method of the Worker class attempted to instantiate a new object using the following code:

Since the class definition didn't contain a definition for a noarg constructor, the following compiler error was produced by JDK 1.3.

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Answer 3

C. An Object

Explanation 3

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) .

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.

Back to Question 3

Answer 2

A. Compiler Error

Explanation 2

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.

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.

Back to Question 2

Answer 1

A. Compiler Error

Explanation 1

Instantiating an object

There are several ways to instantiate an object in Java:

• Use the newInstance method of the class named Class .
• Reconstruct a serialized object using an I/O readObject method.
• Create an initialized array object such as {1,2,3}.
• Create a String object from a literal string such as "A String".
• Use the new operator.

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.

This program produces the following compiler error:

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.

Back to Question 1

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