Why Does Constructor Invocation Order Matter in Java: `this()` and `super()`?

Constructor Invocation Order: Why Does this() and super() Matter?

Java mandates that calls to this() and super() within a constructor be the initial statements. This requirement stems from the sequential execution of constructors: the parent class' constructor must be invoked before the subclass' constructor.

By ensuring this order, Java guarantees that the parent class is properly initialized before accessing any of its members in the subclass' constructor. Consider the following example:

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

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

The compiler rejects this code with an error, as the call to super is not the first statement in MySubClass' constructor. Rearranging the code to ensure this order resolves the issue:

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

This order is crucial because attempting to access the parent class' methods or fields before initializing the parent class via super() can lead to undefined behavior.

Similarly, invoking this() must be the first statement in a constructor to ensure that the current object is fully initialized before calling another constructor in the same class. By imposing these restrictions, Java maintains the proper execution order of constructors and prevents potential errors.

The above is the detailed content of Why Does Constructor Invocation Order Matter in Java: `this()` and `super()`?. For more information, please follow other related articles on the PHP Chinese website!