Java.Core.What is the order of calling constructors and initialization blocks, taking into account the class hierarchy?

Order of Constructor and Initialization Block Execution in Java

When a class hierarchy is involved, the execution order of constructors and initialization blocks follows a top-down approach.

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Order of Execution:

1. Static Blocks (from parent to child)
2. Instance Initialization Blocks (from parent to child)
3. Constructors (from parent to child)

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Step-by-Step Execution Order

Step	Element	Executed From
1️⃣	Static Blocks	Parent → Child
2️⃣	Instance Initialization Blocks	Parent → Child
3️⃣	Constructor Execution	Parent → Child

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Example to Demonstrate Execution Order

class Parent {
    // Static block
    static {
        System.out.println("Parent Static Block");
    }

    // Instance initialization block
    {
        System.out.println("Parent Instance Block");
    }

    // Constructor
    Parent() {
        System.out.println("Parent Constructor");
    }
}

class Child extends Parent {
    // Static block
    static {
        System.out.println("Child Static Block");
    }

    // Instance initialization block
    {
        System.out.println("Child Instance Block");
    }

    // Constructor
    Child() {
        System.out.println("Child Constructor");
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println("Creating First Child Object...");
        Child c1 = new Child();

        System.out.println("\nCreating Second Child Object...");
        Child c2 = new Child();
    }
}

Expected Output

Parent Static Block
Child Static Block
Creating First Child Object...
Parent Instance Block
Parent Constructor
Child Instance Block
Child Constructor

Creating Second Child Object...
Parent Instance Block
Parent Constructor
Child Instance Block
Child Constructor

Analysis of Execution Order

First Object Creation (new Child())

1️⃣ Static Blocks (Executed Only Once)

• Parent Static Block
• Child Static Block

2️⃣ Parent’s Instance Block

• Parent Instance Block

3️⃣ Parent’s Constructor

• Parent Constructor

4️⃣ Child’s Instance Block

• Child Instance Block

5️⃣ Child’s Constructor

• Child Constructor

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Second Object Creation (new Child())

1️⃣ Static blocks are NOT executed again (they run only once per class). 2️⃣ Parent’s Instance Block → Parent Instance Block 3️⃣ Parent’s Constructor → Parent Constructor 4️⃣ Child’s Instance Block → Child Instance Block 5️⃣ Child’s Constructor → Child Constructor

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Key Takeaways

1️⃣ Static blocks execute only once per class, when the class is loaded (top-down in hierarchy).
2️⃣ Instance blocks execute every time an object is created, before the constructor (top-down in hierarchy).
3️⃣ Constructors execute after all instance blocks (top-down in hierarchy).