Why does using 'this' in a Java instance initializer block resolve forward reference errors?

Question

Problem Description

I've encountered an interesting behavior in Java regarding forward references in instance initializer blocks. When I try to access an instance variable directly, I get a forward reference error. However, when I use the this keyword to access the same variable, the error disappears. I'm trying to understand why this happens.

Code Example

class Thing {
    // Instance Initializer
    {
        // firstString = secondString; // Compilation error: forward reference
        firstString = this.secondString; // No error, compiles successfully
    }

    // Instance variables
    String firstString;
    String secondString;

    public String toString() {
        return firstString + secondString;
    }
}

public class ForwardReference {
    public static void main(String[] args) {
        Thing one = new Thing();
        System.out.println(one);
    }
}

Specific Questions

1. Why does firstString = secondString; cause a forward reference error, but firstString = this.secondString; doesn't?
2. What is the underlying mechanism in Java that causes this difference in behavior?
3. What is the actual value of this.secondString when it's accessed in the initializer block, given that secondString hasn't been explicitly initialized yet?
4. Is this behavior specific to instance initializer blocks, or does it apply in other contexts as well?

What I've Observed

• Using this to access an instance variable in an initializer block seems to bypass the forward reference check.
• The code compiles without errors when using this, but I'm unsure about the implications or potential pitfalls of this approach.

Expected vs Actual Behavior

• Expected: Both firstString = secondString; and firstString = this.secondString; would either cause errors or be allowed.
• Actual: Only firstString = this.secondString; is allowed and compiles without errors.

Additional Context

I understand that instance variables are initialized to their default values (null for reference types) before any initializer blocks are executed. However, I'm curious about why the this keyword seems to change how the compiler treats these references.

As fas as I research: initialization occurs in these steps:

1. JVM checks whether class has been initialized.If not, JVM loads and initializes class
2. JVM allocates memory to house data for the new instance
3. JVM initializes instance variables to their default values
4. JVM executes custom initialization code found in assignment declarations of instance variables, initializer blocks and constructor

Environment

• Java Version: 11.0.9