Flexible Constructor Bodies (JEP 513)

For decades, Java developers have lived by a strict rule: super() or this() must be the very first statement in a constructor.

This restriction often led to awkward workarounds when we needed to validate arguments or prepare data before invoking the parent class constructor. Java 25 solves this with Flexible Constructor Bodies (JEP 513).

The Problem

Suppose you want to valid an argument before passing it to the superclass constructor. In older Java versions, you couldn’t simply do this:

public class PositiveBigInteger extends BigInteger {
    public PositiveBigInteger(long value) {
        if (value <= 0) throw new IllegalArgumentException("non-positive value");
        super(String.valueOf(value)); // ERROR in older Java!
    }
}

Because the validation check happens before super(), this code would fail to compile. We often had to resort to static helper methods:

public class PositiveBigInteger extends BigInteger {
    public PositiveBigInteger(long value) {
        super(validate(value));
    }
    
    private static String validate(long value) {
        if (value <= 0) throw new IllegalArgumentException("non-positive value");
        return String.valueOf(value);
    }
}

This works, but it’s verbose and less readable.

The Java 25 Solution

With Flexible Constructor Bodies, JDK 25 relaxes this rule. You can now place statements beforesuper() or this(), as long as they don’t access the instance being created (i.e., you can’t use this to access fields or methods yet).

Now, the first example is valid Java code!

public class PositiveBigInteger extends BigInteger {
    public PositiveBigInteger(long value) {
        if (value <= 0) throw new IllegalArgumentException("non-positive value");
        System.out.println("Validating value: " + value);
        super(String.valueOf(value)); // Perfectly legal in Java 25
    }
}

Key Rules

1. No instance access: You cannot access this (fields, instance methods) before the super() call completes. The object is not initialized yet.
2. Assignments allowed: You can assign to local variables.
3. Exception throwing: You can throw exceptions (fail fast!).

Why is this better?

• Readability: Validation logic stays inside the constructor where it belongs.
• Safety: Failing fast before the super constructor (which might be expensive) prevents unnecessary work.
• Cleanliness: Eliminates the need for artificial static helper methods just to satisfy compiler rules.

This small change makes writing robust constructors much more natural and intuitive.