Flexible Constructor Bodies In Java 25

Java 25 introduces several improvements, but one of the most developer-friendly updates is JEP 513: Flexible Constructor Bodies. This feature simplifies how constructors are written, making Java code cleaner, and easier to read. If you’ve ever found yourself writing awkward constructor boilerplate just to forward arguments or handle initialization/validation logic, this update is for you.

By the end, you’ll have a clear understanding of how to use Flexible Constructor Bodies effectively in your code.

Previously, we explained JEP 512: Instance Main Methods and Compact Source Files — you can check it out!

Prior To Java 25

In Java versions prior to 25, constructors were rigid:

• The call to another constructor (this(...)) or a superclass constructor (super(...)) had to be the very first statement in the constructor body.
• This meant you couldn’t write even a single line of logic (like validating parameters or assigning temporary variables) before calling this() or super().

This restriction often led to clunky workarounds like Creating unnecessary helper methods (Often logic had to be moved into private static methods to avoid violating constructor rules).

Example: using static helper method to validate arguments before passing them!

class Parent {
    private int age;
    public Parent(int age) {
        this.age = age;
    }
}

class Child extends Parent {

    public Child(int age) {
        super(validateAge(age));
    }

    public static int validateAge(int age) {
        if (age < 0 || age > 100) {
            throw new IllegalArgumentException("not valid age");
        }
        return age;
    }
}

The result was code that worked but didn’t read naturally. Constructors, which are supposed to express object initialization clearly, became harder to follow.

JEP 513 fixes this by introducing flexible constructor bodies.

With this enhancement, Java developers finally gain:

• The ability to validate arguments inline.
• Clearer expression of intent in constructor chaining.

Flexible Constructor Bodies In Java 25

With JEP 513, constructors can now:

• Contain statements before a this(...) or super(...) call.
• Use these statements to initialize local variables, validate arguments, or prepare values.
• Continue to follow the existing model where an implicit call to super() is added if no explicit constructor invocation is present.

Rules

• Implicit Invocation Still Applies: If no this(...) or super(...) call is written, the compiler still inserts an implicit call to super() as before.
• Execution Order: Statements before a this(...) or super(...) call are executed in order, just like any other block of code.
• Restriction on this: you can’t explicitly/implicitly refers to the current object (this ) cause it’s still in construction, but you can initialize it’s fields this.field = value is allowed (if the field is initialized inline this.field = value will. give a compilation issue as well!); assignment to an uninitialized declared field is fine! also note that calling a field of a super class using super is not allowed (even if the field is public and you tried this.field = value will give a compilation issue).

Practical Examples

Example 1: Argument Validation Before Superclass Call

Example: Without using flexible constructor bodies

class Person {
    int age;

    Person(int age) {
        System.out.println("Calling Person Constructor");
        this.age = age;
    }

}


class Employee extends Person {

    Employee(int age) {
        super(age);
        if (age < 23 || age > 60)
            throw new IllegalArgumentException("not allowed age");
    }

}


public class Main {
    public static void main(String[] args) {
       Employee employee = new Employee(80);
    }
}

Result : 
Calling Person Constructor
Exception in thread "main" java.lang.IllegalArgumentException: not allowed age

As you might think, the call to super(age) is potentially unnecessary work, because if the Employee being created has an age greater than 60, an IllegalArgumentException will be thrown — meaning we’ve already wasted time invoking the parent constructor.

Here is the most accurate approach using Java 25 syntax!

class Person {
    int age;

    Person(int age) {
        System.out.println("Calling Person Constructor");
        this.age = age;
    }

}


class Employee extends Person {

    Employee(int age) {
        if (age < 23 || age > 60)
            throw new IllegalArgumentException("not allowed age");
        super(age);
    }

}





public class Main {
    public static void main(String[] args) {
       Employee employee = new Employee(80);
    }
}

Result: No call was made to the parent constructor.

Exception in thread "main" java.lang.IllegalArgumentException: not allowed age

Example 2: Temporary Variable Before Constructor Chaining

class Rectangle {
    private final int width;
    private final int height;

    Rectangle(int size) {
        int doubled = size * 2;
        this(doubled, size); // Chaining after calculation
    }

    Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }
}

Previously, the calculation would have to be done inline in the call, making the code harder to read:

this(size * 2, size);

Now, it’s possible to use a named variable, improving clarity.

Restrictions in Flexible Constructor Bodies

Example 1: Reading a field in a flexible constructor body

class C {
    int i;  // no initializer

    C(int v) {
        int t = i + v;  // **error**: implicitly referencing field `i`
        this.i = v;
        super();
    }
}

This fails because i on the right of i + v is implicitly this.i, and you’re accessing a field before the superclass constructor. That’s disallowed.

Example 2: Assigning to a Field with an Inline Initializer

class D {
    int a = 5;    // inline initializer

    D(int v) {
        this.a = v;  // **error**: `a` is already initialized, cannot reassign
        super();
    }
}

Here this.a = v is disallowed because a already has an initializer in its declaration.

Example3: calling a super method or accessing super field

class Super {
    public int s;
    void foo() { System.out.println("super foo"); }
}

class Sub extends Super {
    Sub() {
        super.s = 10;     // **error**
        foo();            // **error**, implicit `this.foo()`
        super.foo();      // **error**
        super();
    }
}

You cannot use super to access superclass fields or methods before the superclass constructor runs.

Conclusion

JEP 513 makes Java constructors more flexible, concise, and developer-friendly. With Flexible Constructor Bodies, you can:

• Validate inputs before constructor delegation.
• Simplify logic without external helpers.
• Improve readability and maintainability.

This change does not alter Java’s safety guarantees. It’s purely a syntactic improvement, but one that has real impact on how Java code looks and feels. If you’re exploring Java 25 new features, JEP 513 is one of the most practical improvements to adopt right away.