About Question enthuware.ocajp.i.v7.2.1151 :

[Sweetpin2]

In below example

TestClass t1, t2, t3, t4;
t1 = t2 = new TestClass();
t3 = new TestClass();

why can't t2 be an Object? Is this becuase t2 is assigned to t1 & hence after assignment it's not pointing to TestClass object?

[admin]

I am not sure I understand your confusion. t2 is declared of class TestClass, so you cannot assign new Object() to t2. The t1 = t2 part has nothing to do with it.

[Sweetpin2]

I am not sure I understand your confusion. t2 is declared of class TestClass, so you cannot assign new Object() to t2. The t1 = t2 part has nothing to do with it.

I want to know why t2 is not an object of class TestClass as the answer says? Also i am not sure what you mean by "t2 is declared of class TestClass, so you cannot assign new Object() to t2" as we are able to assign new Object() to t1 which is also declared of class TestClass.

[admin]

Where do you see new Object() being assigned to t1?

The question asks you how many objects and references are being created in the given code. Whenever you do a "new" of any class, you create a new object, so in this case there are only two objects being created. Both the objects are objects of class TestClass.

t1, t2, t3, and t4 are not objects. They are references that point to objects. In this case, t4 is not pointing to any object.

[Sweetpin2]

Where do you see new Object() being assigned to t1?

The question asks you how many objects and references are being created in the given code. Whenever you do a "new" of any class, you create a new object, so in this case there are only two objects being created. Both the objects are objects of class TestClass.

t1, t2, t3, and t4 are not objects. They are references that point to objects. In this case, t4 is not pointing to any object.

Thanks a lot for this clarification. I got it clear now.

[mhasse]

hi all

for


Given that TestClass is a class, how many objects and reference variables are created by the following code? TestClass t1, t2, t3, t4; t1 = t2 = new TestClass(); t3 = new TestClass();

why when i print

System.out.println(t1 instanceof Object);
System.out.println(t2 instanceof Object);
System.out.println(t3 instanceof Object);

does it print true for all? therefore i presume 3 object created?

tsk

[admin]

mhasse, you have asked a very fundamental question. To answer this question, you need to understand the difference an object and a reference. Please go through this first:
http://www.hacktrix.com/difference-betw ... ce-in-java
and
http://www.dreamincode.net/forums/topic ... reference/

After going through the above links, you need to read about the instanceof operator. This link is good: http://stackoverflow.com/questions/7526 ... of-in-java

Once you go through the above, it will be clear to you why all three lines print true but there are not 3 objects.

HTH,
Paul.

[kirankumar]

System.out.println(t1); //enthuwaremocktest.test2.Ex67@3dccbdf7
System.out.println(t2); // enthuwaremocktest.test2.Ex67@3dccbdf7
System.out.println(t3); //enthuwaremocktest.test2.Ex67@4ccbc2d3


See above , only 2 objects are created
1) 3dccbdf7 (t1 and t2 referring same object)
2) 4ccbc2d3
and 3 references t.e., t1, t2, and t3.

Please let me know If I am wrong.

[admin]

You are right.

[aguywholikesjava]

4 reference variables (t1, t2, t3, t4)
3 references (t1, t2, t3)
2 objects.

nah?

[admin]

Not sure what are you asking.
Paul.

[pupmonster]

>>>> See above , only 2 objects are created
>>>> 1) 3dccbdf7 (t1 and t2 referring same object)
>>>> 2) 4ccbc2d3
>>>> and 3 references t.e., t1, t2, and t3.

>> You are right.

Could the answer possibly be corrected in the next version?

[admin]

Ok, I see the confusion now.

No distinction is made between a reference and reference variable in Java because unlike c/c++, which has the concept of pointers as well as references, Java has only one thing - references. Each reference variable in Java stores a value - either that value is the address to an object or that value represents null.

For this reason, the term reference and reference variable are interchangeable in Java.

Therefore, in this case, there are 4 references (not three). t4 is not pointing to the same object as t1, t2, and t3 are pointing to, but t4 is a reference nonetheless, pointing to null.

[igor.simecki]

Ok, I see the confusion now.

No distinction is made between a reference and reference variable in Java because unlike c/c++, which has the concept of pointers as well as references, Java has only one thing - references. Each reference variable in Java stores a value - either that value is the address to an object or that value represents null.

For this reason, the term reference and reference variable are interchangeable in Java.

Therefore, in this case, there are 4 references (not three). t4 is not pointing to the same object as t1, t2, and t3 are pointing to, but t4 is a reference nonetheless, pointing to null.

except reference variables in methods, in which case t4 doesn't store anything and will not compile if used.

[admin]

Ok, I see the confusion now.

No distinction is made between a reference and reference variable in Java because unlike c/c++, which has the concept of pointers as well as references, Java has only one thing - references. Each reference variable in Java stores a value - either that value is the address to an object or that value represents null.

For this reason, the term reference and reference variable are interchangeable in Java.

Therefore, in this case, there are 4 references (not three). t4 is not pointing to the same object as t1, t2, and t3 are pointing to, but t4 is a reference nonetheless, pointing to null.

except reference variables in methods, in which case t4 doesn't store anything and will not compile if used.

Not sure what you mean. t4 is a reference variable. No matter whether it is used or not or where is it used. Can you post some code to show what you mean exactly?

[igor.simecki]

void test(){
Object o1,o2;
o1= new Object();
o2.toString();
}

this will cause a compiler error because o2 references nothing.

[admin]

void test(){
Object o1,o2;
o1= new Object();
o2.toString();
}

this will cause a compiler error because o2 references nothing.

Did you read the error message that the compiler generated when you tried to compile the above code? The error is not because o2 points to nothing. The error is because you are trying to use o2 without initializing it. Try o2 = null; first and then see what happens.

Regardless, o2 is still a valid reference variable.