Why Must `this()` and `super()` Be the First Statements in a Java Constructor?

Why Must this() and super() Be Invoked Before All Other Statements in a Constructor?

Java enforces the strict rule that this() and super() calls must be the first statements within a constructor. This restriction exists to ensure the proper initialization of parent classes before the execution of child class constructors.

For example, the following code will trigger a compile-time error because super(c) is not the first statement in MySubClass's constructor:

public class MySubClass extends MyClass {
    public MySubClass(int a, int b) {
        int c = a + b;
        super(c);  // COMPILE ERROR
    }
}

However, this can be circumvented by modifying the code to:

public class MySubClass extends MyClass {
    public MySubClass(int a, int b) {
        super(a + b);  // OK
    }
}

Why the Ordering Matters

The super() call within a subclass constructor initializes the parent class's instance variables and invokes the parent constructor. This ensures that the parent class is fully initialized before the subclass's constructor proceeds.

If super() were not required to be the first statement, it would be possible to perform operations that rely on the parent class being initialized before it has actually been initialized, potentially leading to undefined behavior.

Exceptions and Workarounds

Although the compiler requires this() and super() to be invoked first, it does not restrict the execution of logic within the constructor. Logic can be placed in a single expression or passed to the super() or this() calls.

For example:

public class MySubClass extends MyClass {
    public MySubClass(int a, int b) {
        super(Arrays.asList(new Object[] { a, b }));  // OK
    }
}

Conclusion

The restriction that this() and super() must be the first statements within a constructor serves to ensure proper object initialization and prevent potential errors caused by premature access to parent class members before they are properly initialized. This enforces a predictable order of constructor execution for classes, ensuring object integrity and consistency.

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