Using constructor within another in Java

The use of instance variables in an explicit constructor invocation is prohibited by the JLS, Section 8.8.7.1.

An explicit constructor invocation statement in a constructor body may not refer to any instance variables or instance methods or inner classes declared in this class or any superclass, or use this or super in any expression; otherwise, a compile-time error occurs.

This prohibition on using the current instance explains why an explicit constructor invocation statement is deemed to occur in a static context (§8.1.3).

You referenced the instance variable b. The compiler didn't raise this error on a because it's a local variable, which shadows the instance variable a.

The "static context" may be why your IDE is suggesting to make b static, so it can be referenced. It makes sense; the ClassName part of the object isn't constructed yet. Replace that usage of b with something else, such as a static constant, or a literal int value.

To answer your other question, typing new ClassName(a, b) isn't an error because at this point, this instance has been constructed, and you're creating a separate, unrelated ClassName object.

First question: I get an error saying "b should be static". Why can't I use the default value (3) for b in this way?

The correct way to supply a default value for b to the other constructor is this(a, 3);. You cannot refer to instance variables until after this(...). That's just one of the rules of the language.

Second question: In the first constructor if I use the comment outed part I do not get an error. Is it an acceptable usage?

new ClassName(a, b); does something different. It creates a separate instance of the class. In my humble opinion it is probably not the best thing to do. People expect new to create one instance of a class, whereas using new ClassName in the constructor creates two.

When using variables in classes, it is important to note where that validity of scope is. You've instantiated new a,b variants of the variables there. You're tricking yourself into believing those are the same variables. Actually they're in another address space. If you want to use your class variables you'll have to take out the parameters to the functions. Then they'll sync with the class you're in, rather than isolating the arguments a, b to within the scope of your function,