What's the Correct Execution Order of Instance Variable Initialization and Constructor Execution in Java?

Instance Variable Initialization vs. Constructor Execution in Java

One common misconception about Java object initialization is the order in which fields and constructors are executed. Let's clarify this concept with an example.

In the given code snippet:

class X {
    Y b = new Y();

    X() {
        System.out.print("X");
    }
}

class Y {
    Y() {
        System.out.print("Y");
    }
}

public class Z extends X {
    Y y = new Y();

    Z() {
        System.out.print("Z");
    }

    public static void main(String[] args) {
        new Z();
    }
}

Contrary to the assumption that fields are initialized before constructors, the output of this program is "YZX". To understand why, we need to delve into the Java initialization process.

Initialization Order:

Java initializes classes in a specific sequence:

1. Static Members: Static variables and initializers in textual order.
2. Constructor Invocation: Super constructor call, either explicit or implicit.
3. Instance Variable and Initializer Blocks: Fields and instance initialization blocks in textual order.
4. Constructor Body: Remaining code in the constructor after the super() call.

In the example above, the sequence of events is:

• Y's constructor is called (Y).
• Z's constructor calls X's constructor (YZ).
• X's constructor is called (XYZ).
• X's instance variable b is initialized (YZXb).
• Z's instance variable y is initialized (YZXby).

Therefore, the correct order of execution is "YZX". This highlights the importance of understanding the Java initialization order to avoid unexpected behavior in object construction.

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