Flexible Constructor Bodies: A Preview Feature in Java 24 (JEP 492)

    With the introduction of JEP 492, Java brings a new way to handle constructors, making them more flexible and intuitive. This enhancement allows developers to include statements before an explicit constructor invocation (super() or this()) — something that was previously forbidden. Let’s explore how this improves constructor behaviour, making code simpler and more maintainable.

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Old Behaviour

In Java, constructors followed a rigid rule: the first statement in a constructor must be an explicit constructor invocation (super() or this()). If omitted, the compiler automatically added super(). This ensured that superclass constructors executed first, guaranteeing safe object initialisation.

take an example where we want to validate arguments -

Example: Argument Validation in Old Java

public class PositiveBigInteger extends BigInteger {     
    public PositiveBigInteger(long value) {         
        super(value); // Must call super first         
        if (value <= 0) throw new IllegalArgumentException("Value must be positive");     
    } 
}

This caused unnecessary computation (super(value)) even if the input was invalid. To avoid this, developers had to resort to static helper methods:

private static long validatePositive(long value) {     
    if (value <= 0) throw new IllegalArgumentException("Value must be positive");     
    return value; 
}  

public PositiveBigInteger(long value) {     
    super(validatePositive(value)); 
}

Such workarounds added complexity, making code less readable and harder to maintain.

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New Behavior

The new model divides constructor execution into two phases:

1. Prologue: Code before super() or this() that runs without accessing the instance under construction.
2. Epilogue: Code that runs after the superclass constructor completes.

Pseudocode with new Syntax

public MyClass(arg1, arg2){
    //prolouge part
    super()
    //epilouge part
}

With this change, developers can now validate or compute arguments directly in the constructor body before calling the superclass constructor.

Example: New Constructor Syntax

public class PositiveBigInteger extends BigInteger {     
    public PositiveBigInteger(long value) {         
        if (value <= 0) throw new IllegalArgumentException("Value must be positive");         
        super(value); // Explicit call after validation     
    } 
}

This simplifies the code by removing the need for auxiliary methods.

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Syntax and Compiler Grammar

The constructor grammar has been updated as follows:

Old Syntax:

ConstructorBody:     
    { [ExplicitConstructorInvocation] [BlockStatements] }

New Syntax:

ConstructorBody:     
    { [BlockStatements] ExplicitConstructorInvocation [BlockStatements] }     
    { [BlockStatements] }

In simpler terms, statements can now appear before and after the explicit constructor invocation.

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Key Rules for Prologue

• Allowed:
  • Argument validation.
  • Assigning fields of the instance (only those without initializers).
• Disallowed:
  • Using the instance (this) or calling its methods.
  • Accessing fields of the instance directly or indirectly.

Example: Assigning Fields in Prologue

class Sub extends Super {     
    final int x;      

    Sub(int value) {         
        this.x = value; // Initialise fields in the prologue         
        super();        // Call superclass constructor     
    } 
}

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Key Benefits

1. Improved Readability: Simplifies code by eliminating auxiliary methods or constructors.
2. Safer Initialisation: Fields can be initialised early, avoiding bugs when superclass constructors invoke overridable methods.
3. Flexible Validation: Fail-fast logic can now be embedded naturally in constructors.

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Conclusion

This preview feature in JDK 24 introduces an important improvement for cleaner and more maintainable code. By enabling statements to appear before constructor invocations, Java is evolving its approach to object initialization, offering more flexibility while still ensuring the safety of the process.

To try this feature, enable it as a preview:

javac --release 24 --enable-preview Main.java 
java --enable-preview Main

Happy coding!