This is the fourth day of my participation in the August More text Challenge. For details, see: August More Text Challenge
You can start with this article I wrote yesterday – garbage collection in Java
Difficulty level: Intermediate In Java, object destruction is the responsibility of the garbage collector module, and objects without any reference are eligible for garbage collection. Here are some important output questions about garbage collection.
The program
Program a
Predict the output of the following Java program:
public class Test
{
public static void main(String[] args) throws InterruptedException
{
String str = new String("Haiyong");
// Make STR conform to GC conditions
str = null;
// Call the garbage collector
System.gc();
// Wait for the GC to complete
Thread.sleep(1000);
System.out.println("end of main");
}
@Override
protected void finalize(a)
{
System.out.println("Finalize method called"); }}Copy the codeClick here to skip to the answer
Program 2
public class Test
{
public static void main(String[] args) throws InterruptedException
{
Test t = new Test();
// Make t eligible for garbage collection
t = null;
// Call the garbage collector
System.gc();
// Wait for the GC to complete
Thread.sleep(1000);
System.out.println("end main");
}
@Override
protected void finalize(a)
{
System.out.println("Finalize method called");
System.out.println(10/0); }}Copy the codeClick here to skip to the answer
Application of three
public class Test
{
static Test t ;
static int count =0;
public static void main(String[] args) throws InterruptedException
{
Test t1 = new Test();
// Make T1 eligible for garbage collection
t1 = null; // line 12
// Call the garbage collector
System.gc(); // line 15
// Wait for the GC to complete
Thread.sleep(1000);
// Make t eligible for garbage collection
t = null; // line 21
// Call the garbage collector
System.gc(); // line 24
// Wait for the GC to complete
Thread.sleep(1000);
System.out.println(Call finalize method+count+"Time");
}
@Override
protected void finalize(a)
{
count++;
t = this; // line 38 }}Copy the codeClick here to skip to the answer
Application of four
public class Test
{
public static void main(String[] args)
{
// How many objects are eligible for garbage collection after this row?
m1(); // Line 5
}
static void m1(a)
{
Test t1 = new Test();
Test t2 = newTest(); }}Copy the codeClick here to skip to the answer
Application of five
public class Test
{
public static void main(String [] args)
{
Test t1 = new Test();
Test t2 = m1(t1); // line 6
Test t3 = new Test();
t2 = t3; // line 8
}
static Test m1(Test temp)
{
temp = new Test();
returntemp; }}Copy the codeClick here to skip to the answer
The second half of the article is the output and parsing of the program
Output and parsing
Program one output
Output:
end of main
Copy the codeExplanation:
We know that the garbage collector calls the Finalize () method on an object before destroying it. But here, the trick is that STR is a String object, not a Test. Therefore, call the Finalize () method of the String class on STR (if overridden in the String class). If a class does not override the Finalize method, then the Object class’s Finalize () method is called by default.
Program two output
Output:
Call finalize method end mainCopy the codeDescription:
When the garbage collector calls the Finalize () method on an object, it ignores all exceptions raised in that method, and the program terminates normally.
Program three output
Output:
Call the Finalize method1 次
Copy the codeDescription:
After line 12, T1 can be garbage collected. So when we call the garbage collector in line 15, the garbage collector calls the Finalize () method on T1 before destroying it. But in the Finalize method, at line 38, we refer to the same object again by t, so after line 38, the object no longer meets the garbage collection criteria. Therefore, the garbage collector does not destroy objects.
Now, again in line 21, we make the same object eligible for garbage collection again. Here, we want to be clear about the fact about the garbage collector that it will call the Finalize () method on a particular object at exactly one time. Since the Finalize () method has already been called on this object, the garbage collector will now destroy it without having to call the Finalize () method again.
4.
Question: After executing line 5, how many objects qualify for garbage collection?
Answer:
2
Copy the codeDescription:
Because T1 and T2 are local objects to the M1 () method, they are eligible for garbage collection after the method completes unless either is returned.
5.
Question: How many objects qualify for garbage collection after line 8 is executed?
Answer:
1
Copy the codeDescription:
By line 8, the only unreferenced object is the one generated on line 6. Remember that “Java passes strictly by value,” so the reference variable T1 is not subject to the M1 () method. We can check it using the Finalize () method. The statement “system.out.println (this.hashCode ())” in the Finalize () method prints the hashcode value of the object that called the Finalize () method, and then compares that value to the hashcode values of other objects created in the main method.
That’s all for this article
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