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

Enforcing Constructor Execution Order: Why this() and super() Must Be the First Statement

Java strictly enforces that if this() or super() are invoked within a constructor, they must be the first statement. This restriction ensures that the constructor initialization sequence for parent and child classes occurs in the correct order.

Consider the following example:

public class MyClass {
    public MyClass(int x) {}
}

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

The call to super(c) in the MySubClass constructor will result in a compile-time error because it is not the first statement. The compiler requires that the parent class's constructor be called first, initializing the parent class's state before the subclass constructor can execute.

One way to resolve this error is to rearrange the code:

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

Here, the calculation of c is performed within the argument to super(), allowing the parent class's constructor to be called first.

Another example demonstrates how the restriction prevents erroneous code from executing:

public class MyClass {
    public MyClass(List list) {}
}

public class MySubClassA extends MyClass {
    public MySubClassA(Object item) {
        List list = new ArrayList();
        list.add(item);
        super(list); // Compile error
    }
}

The code attempts to create a list and pass it to the parent class's constructor, which is not allowed. If the restriction were not in place, the code could fail with an error when calling methods on the parent class before it is initialized.

Enforcing this() and super() to be the first statement ensures that the constructor invocation sequence is correct, guaranteeing the proper initialization of parent and child classes in Java.

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