Java: does Object class have a constructor?

Question

Javadoc mentions that Object class has a public no-arg constructor. But Object's source code doesn't have any explicit constructor in it. So obviously the compiler has generated one for it. However, if I see the call stack trace when a constructor is about to return (as shown below), I do not see any call to Object.<init> in that trace.

So the question is, does Object class have a default constructor as the doc says? If yes, why do I not see it in the call stack trace?

public ConTest()
{
    new Throwable().printStackTrace();
}

Result:

java.lang.Throwable
    at ConTest.<init>(ConTest.java:8)
    at ConTest.main(ConTest.java:16)

hmm from docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default the Object constructor if the class has no other parent. — MimiEAM
– MimiEAM, Commented Aug 22, 2012 at 5:56
Is it possible to accept more than one answer? Because I'd like to accept all 4 answers given below. — shrini1000
– shrini1000, Commented Aug 22, 2012 at 6:38
Strange, @shrini1000, you are right, it's not in the source code, i checked Java 1.8.0_241. But it's in the online java documentation, i checked from Java 13 all the way to 8, the Object() constructor is defined. — jumping_monkey
– jumping_monkey, Commented Feb 9, 2020 at 10:42

Aleksander Blomskøld · Accepted Answer · 2012-08-22 06:05:48Z

25

Super constructors are run before sub/base constructors. In your example Object's constructor has already been run when new Throwable().printStackTrace() is executed.

A more explicit version of your code:

public ConTest()
{
    super();
    new Throwable().printStackTrace(); // you will not see super() (Object.<init>) in this stack trace.
}

edited Aug 22, 2012 at 6:05

answered Aug 22, 2012 at 5:57

Aleksander Blomskøld

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Comments

Strelok · Accepted Answer · 2012-08-22 05:59:39Z

6

You do not see it in the stack trace, because it was already called. The exception is thrown in your code.

Your code is equivalent to writing:

public ConTest() {
  super(); // this will call the Object constructor
  new Throwable().printStackTrace();
}

answered Aug 22, 2012 at 5:59

Strelok

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Comments

Sumit Singh · Accepted Answer · 2012-08-22 06:05:46Z

6

You do not see it in the stack trace because the constructor of the super class is called before your new Throwable().printStackTace() call. What the compiler actually creates is following .

public ConTest()
{
    super();   // This is the call to the base class constructor
    new Throwable().printStackTrace();   // already back from the base class constructor
}

edited Aug 22, 2012 at 6:05

Sumit Singh

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answered Aug 22, 2012 at 6:00

Andreas Fester

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Comments

Kuldeep S. Mahajan · Accepted Answer · 2012-10-05 12:00:44Z

Yes,Object class have a default constructor as the doc says.

As you know insted of doing that you can check it by using javap -c ConTest in command prompt you can see it's calling the object class default constructor() in below's code's Line No:1

C:\stackdemo>javap -c ConTest
Compiled from "ConTest.java"
public class ConTest extends java.lang.Object{
public ConTest();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   new     #2; //class java/lang/Throwable
   7:   dup
   8:   invokespecial   #3; //Method java/lang/Throwable."<init>":()V
   11:  invokevirtual   #4; //Method java/lang/Throwable.printStackTrace:()V
   14:  return

public static void main(java.lang.String[]);
  Code:
   0:   new     #5; //class ConTest
   3:   dup
   4:   invokespecial   #6; //Method "<init>":()V
   7:   astore_1
   8:   return

}

Thank you

Amit Deshpande · Accepted Answer · 2012-08-22 06:16:51Z

2

As Suggested above super() is the first call in the constructor and for method More Information here

When you compile a class, the Java compiler creates an instance initialization method for each constructor you declare in the source code of the class. Although the constructor is not a method, the instance initialization method is. It has a name, <init>, a return type, void, and a set of parameters that match the parameters of the constructor from which it was generated

edited Aug 22, 2012 at 6:16

answered Aug 22, 2012 at 6:04

Amit Deshpande

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