Flexible Constructor Bodies in Java 25

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With Java 25, constructors become more flexible and expressive. The new feature called Flexible Constructor Bodies (defined in JEP 513) lets developers write statements before calling a superclass constructor (super(...)).

This change simplifies how constructors are written, making it easier to perform calculations, validations, or prepare arguments before invoking the superclass constructor—something that wasn’t possible in earlier Java versions.

This article explains what flexible constructor bodies are, how they work, and how they affect initialization, early construction contexts, field assignments, nested classes, and records.

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Why Flexible Constructor Bodies Were Introduced

Before Java 25, Java enforced a strict rule: The first statement inside any constructor must be either super(...) or this(...).

That meant you could not perform any computation, validation, or data preparation before calling a superclass constructor. For example, if you needed to check an argument before passing it to super(), you had to do it outside the constructor or in a static helper method.

This restriction made some code unnecessarily complex.

Java 25 changes this rule. You can now add statements before the explicit call to super(...), as long as those statements do not reference the instance being created.

This gives you more control over initialization logic and allows you to perform pre-processing directly in the constructor body.

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Example: Validating Arguments Before Calling super()

Here’s a simple example that demonstrates the new flexibility:

public class PositiveBigInteger extends BigInteger {

    public PositiveBigInteger(long value) {
        if (value <= 0)
            throw new IllegalArgumentException("non-positive value");
        super(Long.toString(value));
    }
}

In this example:

• The constructor checks whether the incoming value is positive.
• Only if the check passes does it call the super() constructor of BigInteger with the validated argument.

Before Java 25, this code would not compile because the if statement appears before the super() call. With Flexible Constructor Bodies, this code is now valid.

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Understanding Constructor Prologue and Epilogue

The constructor body is now conceptually divided into two parts:

1. Prologue: Statements that appear before the super(...) or this(...) call.
2. Epilogue: Statements that appear after that call.

The prologue forms part of what’s known as the early construction context—a region of the constructor that executes before the superclass constructor has run.

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What Is the Early Construction Context?

The early construction context consists of:

• The arguments passed to an explicit constructor invocation (super(...) or this(...)).
• All statements that appear before that invocation.

During this phase, the object is not yet fully constructed, so you cannot access the instance itself (this), its fields, or its methods.

For example, the following is not allowed:

class A {
    int i;
    A() {
        this.i++;          // Error: Cannot reference 'this' before supertype constructor has been called
        i++;               // Error: Field access before super() is not allowed
        this.hashCode();   // Error: Method call on 'this' not permitted
        hashCode();        // Same error
        System.out.print(this); // Also invalid
        super();           // Must come after all valid pre-super logic
    }
}

In short: You can write logic before super() but cannot touch anything that belongs to the object being built.

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Restrictions on super References

Similarly, you cannot access the superclass’s members or methods before calling its constructor.

Example:

class D {
    int j;
}

class E extends D {
    E() {
        super.j++;   // Error: Cannot reference 'super' before supertype constructor has been called
        j++;         // Error: Same reason
        super();     // Must be invoked after any pre-super logic
    }
}

These restrictions ensure that superclass initialization happens in the correct order and that the object is in a valid state before you interact with it.

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Initializing Fields Before Calling super()

Although you cannot read the instance’s fields before calling super(), you can initialize them using assignments.

Let’s look at an example to understand why this matters.

Problem: Field Used Before Initialization

class Super {
    Super() { overriddenMethod(); }
    void overriddenMethod() { System.out.println("hello"); }
}

class Sub extends Super {
    final int x;

    Sub(int x) {
        // The Super constructor is implicitly invoked first,
        // which calls overriddenMethod() before x is initialized.
        this.x = x;
    }

    @Override
    void overriddenMethod() { System.out.println(x); }

    public static void main(String[] args) {
        Sub myApp = new Sub(42);
        myApp.overriddenMethod();
    }
}

Output:

0
42

Explanation:

• When the Sub constructor runs, Super() is called implicitly.
• The superclass constructor calls overriddenMethod().
• At that moment, x is not yet initialized, so it prints 0.

This behavior has long been a subtle source of bugs in Java constructors.

Solution: Initialize Before super()

With flexible constructor bodies, you can now safely initialize x before calling super():

class BetterSub extends Super {
    final int x;

    BetterSub(int x) {
        // Initialize the field before invoking Super constructor
        this.x = x;
        super();
    }

    @Override
    void overriddenMethod() { System.out.println(x); }

    public static void main(String[] args) {
        BetterSub myApp = new BetterSub(42);
        myApp.overriddenMethod();
    }
}

Output:

42
42

This new behavior ensures that the subclass’s fields can be properly initialized before the superclass constructor triggers any method calls that depend on them.

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Nested Classes and the Early Construction Context

Nested classes have a special relationship with their enclosing class, and flexible constructor rules apply here as well.

You cannot access a nested class from the early construction context of its enclosing class, because the enclosing class is not yet fully constructed.

Example:

class B {

    class C { }

    B() {
        new C();   // Error: Cannot reference this before supertype constructor has been called
        super();
    }
}

However, the reverse is allowed. A nested class can access its enclosing class instance within its early construction context, because the enclosing instance already exists when the nested class is being constructed.

class F {

    int f;

    void hello() {
        System.out.println("Hello!");
    }

    class G {
        G() {
            F.this.f++;  // Allowed
            hello();     // Allowed
            super();     // Invocation of superclass constructor
        }
    }
}

This distinction helps maintain consistent and safe construction ordering between outer and inner classes.

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Flexible Constructor Bodies and Records

Records follow slightly different rules. Record constructors cannot directly invoke super(...), but non-canonical record constructors can invoke another constructor in the same record using this(...).

Now, thanks to flexible constructor bodies, these non-canonical constructors can include statements before the this(...) call.

Example: Validating Arguments Before Calling this(...)

record Pair<T extends Number>(T x, T y) { }

record RectanglePair(float length, float width) {
    public RectanglePair(Pair<Float> corner) {
        float x = corner.x().floatValue();
        float y = corner.y().floatValue();
        if (x < 0 || y < 0) {
            throw new IllegalArgumentException("non-positive value");
        }
        this(corner.x().floatValue(), corner.y().floatValue());
    }
}

Here, the constructor performs validation before invoking the canonical constructor with this(...). This makes record constructors more expressive and prevents invalid state construction without requiring static helper methods.

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Key Takeaways

• Flexible Constructor Bodies (Java 25 / JEP 513) let you add logic before super(...) or this(...) in constructors.
• Code in this early phase cannot access this, instance fields, or superclass members.
• You can use this feature to validate inputs, compute arguments, or initialize fields before superclass invocation.
• Nested classes follow the same safety rules regarding early construction.
• Records benefit from the same flexibility for non-canonical constructors.

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Conclusion

The Flexible Constructor Bodies feature in Java 25 is a subtle yet powerful improvement that makes constructor design more natural. Developers can now write initialization and validation logic directly where it belongs—inside the constructor—without awkward workarounds or helper methods.

For educators and new Java learners, this update clarifies how object construction flows while reducing confusing compiler restrictions. For professional developers, it provides more expressive and safer initialization patterns in both traditional classes and modern record types.

By combining structure with flexibility, Java 25 continues its evolution toward a cleaner, more approachable language—one that’s powerful enough for enterprise development yet simple enough for learning and experimentation.

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Tags: Java 25, Flexible Constructor Bodies, JEP 513, Java constructor rules, early construction context, superclass invocation, Java records, Java new features, learn Java, Java for beginners