What's the Exact Initialization Order of Fields and Constructors in Java?

Initialization Order of Fields and Constructor Code in Java

In a Java program, the order of initialization of fields and execution of constructor code can be confusing, especially regarding the precedence between the two. However, the Java Virtual Machine Specification outlines a clear sequence of events that determines the proper order:

• Static Variables and Blocks: First, static variables and static initialization blocks are initialized in textual order, provided that their class has not been initialized previously.
• Super Constructor Invocation: Next, the super() call is executed within the constructor, either explicitly or implicitly. This call invokes the constructor of the parent class.
• Field Initialization and Blocks: Following the super() call, instance variables are initialized in textual order, along with any associated instance initialization blocks.
• Constructor Body: Lastly, the remaining body of the constructor is executed after the super() call has completed.

In the provided code snippet, the output "YZXZ" is generated because of the specified initialization order. Let's break it down step by step:

1. Static initializers and static variables are not present in the code.
2. The constructor call Z() is executed, invoking Z's superclass constructor X() implicitly.
3. The superclass constructor X() is executed, printing "X" to the console.
4. The instance variable y in the Z class is initialized, printing "Y" to the console.
5. The instance variable b in the X class is initialized, printing "Z" to the console (as Z is the current subclass).

Therefore, the output "YZXZ" correctly reflects the order of initialization and execution defined by the Java Virtual Machine Specification.

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