java - When do instance variables get initialized and values assigned?

Question

When does the instance variable get initialized? Is it after the constructor block is done or before it?

Consider this example:

public abstract class Parent {

 public Parent(){
   System.out.println("Parent Constructor");
   init();
 }

 public void init(){
   System.out.println("parent Init()");
 }
}

---

public class Child extends Parent {

private Integer attribute1;
private Integer attribute2 = null;

public Child(){
    super();
    System.out.println("Child Constructor");
}

public void init(){
    System.out.println("Child init()");
    super.init();
    attribute1 = new Integer(100);
    attribute2 = new Integer(200);
}

public void print(){
    System.out.println("attribute 1 : " +attribute1);
    System.out.println("attribute 2 : " +attribute2);
}
}

---

public class Tester {

public static void main(String[] args) {
    Parent c = new Child();
    ((Child)c).print();
    
}
}

---

OUTPUT:

Parent Constructor

Child init()

parent Init()

Child Constructor

attribute 1 : 100

attribute 2 : null

---

1. When are the memory for the atribute 1 & 2 allocated in the heap ?

2. Curious to know why is attribute 2 is NULL ?

3. Are there any design flaws?

Answer 1

When the memory for the atribute 1 & 2 are allocated in the heap ?

The memory for the object as a whole is allocated when the new operator is invoked, before the java.lang.Object constructor is entered. Memory is allocated for individual Integer instances in init, but there is no point when memory is allocated for individual properties -- only whole objects.

Curious to know why is attribute 2 is NULL ?

The init method is called in the super constructor, so attribute2 is assigned new Integer(200), and then the subclass constructor is invoked which applies property initializers in the order they appear in the source code. This line

private Integer attribute2 = null;

overwrites the value assigned by init() to null.

If you add a call to

 System.out.println("attribute 2 : " +attribute2);

right after your call to super(); then this will become apparent.

Are there any design flaws?

Calling sub-class methods before the base class has finished initializing is dangerous. The sub-class might rely on its base-class's invariants to protect its own invariants, and if the base-class constructor has not completed, then its invariants may not hold.

This is also likely to confuse C++ programmers and the like who would expect a call to init from the base class to invoke the base class's version since C++ rewrites the vtable pointer as constructors are entered.

See The Java Language Specification for all the gory details.