In Java constructors, how are this() and super() used, and what important restriction applies to their placement inside a constructor body?

Introduction / Context:
Java provides two special constructor related calls: this() and super(). They help chain constructors within a class hierarchy so that object initialisation is performed correctly and code duplication is reduced. Understanding how and where these calls can be used inside a constructor is an important part of object oriented programming in Java and is a frequent topic in certification and interview questions.

Given Data / Assumptions:

• We are working with Java classes that have constructors.
• this() and super() are being considered inside constructor bodies.
• We are interested in their meaning and the rules governing their placement.
• We assume simple single inheritance hierarchies.

Concept / Approach:
In a constructor, this() is a special call that invokes another constructor in the same class, allowing one constructor to reuse the initialisation logic of another. super() is a call to the constructor of the superclass, ensuring that the base portion of the object is initialised before the subclass adds its own state. Java enforces a key rule: if this() or super() is used, it must appear as the very first statement in the constructor body. You cannot call both this() and super() in the same constructor, because only one constructor can be invoked directly as the first step. If you do not explicitly call either, Java implicitly inserts a call to super() with no arguments.

Step-by-Step Solution:
Step 1: Recognise that this() is used to chain constructors within the same class, for example public MyClass(int x) { this(x, 0); }. Step 2: Recognise that super() is used to call a constructor of the direct superclass, for example public SubClass() { super(); }. Step 3: Recall the language rule that any explicit call to this() or super() in a constructor must be the first statement; no other code can come before it. Step 4: Understand that a constructor cannot call both this() and super() directly; if you call this(), the other constructor you delegate to may call super(), but the current constructor can use only one of them as its first statement. Step 5: Match this behaviour with the option that correctly describes the roles of this() and super() and the first statement restriction.

Verification / Alternative check:
If you write a constructor that calls super() after some other statement, such as printing a message, the Java compiler will report an error indicating that the call to super must be the first statement. Similarly, attempting to call both this() and super() directly in the same constructor will result in a compilation error, because Java cannot satisfy both first statement requirements. By contrast, if one constructor calls this(), and the delegated constructor calls super(), the compiler accepts this pattern, since each constructor individually has exactly one such call as its first statement. These observations confirm the rules described.

Why Other Options Are Wrong:
Option this() calls a static method, super() calls a private method, and both can appear anywhere in the constructor: this() and super() do not call ordinary methods; they invoke constructors and are constrained to be the first statement if present. Option this() creates a new object, super() destroys the current object, and they must always appear together: Object creation happens before the constructor body runs, and super() does not destroy objects. This description is incorrect. Option this() can only be used in static methods, while super() can only be used in abstract classes: Both this() and super() are constructor specific and not related to static methods or abstract class restrictions in this manner.

Common Pitfalls:
Developers sometimes forget the first statement rule and attempt to perform logging or calculations before calling super(), which is not allowed. Another common mistake is misunderstanding that this() and super() are not ordinary method calls; they have special syntax and semantics. Additionally, some programmers try to use this() in methods, which is not possible; only constructors can use this() in this special way. For exam questions, clearly stating that this() and super() must be the first statement if used and that only one of them can appear in a constructor is often enough to earn full credit.

Final Answer:
In Java constructors, this() calls another constructor in the same class, super() calls the superclass constructor, and either one (but not both) must appear as the first statement if used.