I'm new to Java, reading Oracle tutorial. After each section, there are questions and answers, and I don't understand a sentence within one answer (see below bolded line).
source is https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/objects-answers.html
I'm referring to question 2, see the bolded words. As far as I understand, an array is eligible to garbage collection, if there is no reference to the array. It does not matter, whether there is a reference to the objects held by this array, as the inner objects (within the array) have their own reference counting. Is that right? Please explain the bolded sentence.
cite from oracle tutorial: https://docs.oracle.com/javase/tutorial/java/javaOO/QandE/objects-answers.html
Question: The following code creates one array and one string object. How many references to those objects exist after the code executes? Is either object eligible for garbage collection?
String[] students = new String[10]; String studentName = "Peter Smith"; students[0] = studentName; studentName = null;
Answer: There is one reference to the students array and that array has one reference to the string Peter Smith. Neither object is eligible for garbage collection. The array students is not eligible for garbage collection because it has one reference to the object studentName even though that object has been assigned the value null. The object
studentName
is not eligible either becausestudents[0]
still refers to it.
Answer: This handler catches exceptions of type Exception ; therefore, it catches any exception.
Question: How does a program destroy an object that it creates? Answer: A program does not explicitly destroy objects. A program can set all references to an object to null so that it becomes eligible for garbage collection.
Answer 1: Bytecode. Answer 2: c is an invalid comment.
A try statement does not have to have a catch statement if it has a finally statement. If the code in the try statement has multiple exit points and no associated catch clauses, the code in the finally statement is executed no matter how the try block is exited.
The array students is not eligible for garbage collection because it has one reference to the object studentName even though that object has been assigned the value null.
Yeah, that sentence is... odd. It makes no sense.
An array can be eligible for garbage collection, no matter what references it holds to other objects.
students
is a reference to the array, so it's not eligible for garbage collection as long as students
remains in scope.
Neither object is eligible for garbage collection.
It is right.
But the explanation is unclear :
The
array
students is not eligible for garbage collection because it has one reference to the objectstudentName
even though that object has been assigned the valuenull
.
studentName
is not an object, it is a variable.
Besides, null
elements in the array will not have influence on the array eligibility to be GC but it will have only on the GC eligibility of the array elements.
For example :
String[] students = new String[10];
// the object referenced by students is not eligible to be GC
Or :
String[] students = new String[10];
String studentName = "Peter Smith";
students[0] = studentName;
students[0] = null;
// no object is eligible to be GC
A correct sentence could be :
The String
object is not eligible for garbage collection because the object previously referenced by the studentName
variable is still referenced by the array and assigning a new object to a variable (as assigned studentName
to null
) changes only the reference of this variable, not these of variables that refer the same object.
Note that the array doesn't change nothing in the way which Java works with object assignment.
With a List
you could notice the same behavior.
For example :
String a = "Peter";
List<String> list = ...
list.add(a);
a = null;
No object is eligible to be GC for the same reason.
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