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- Question
When is the Float object, created in line 3, eligible for garbage collection?
public Object m() { Object o = new Float(3.14F); Object [] oa = new Object[l]; oa[0] = o; /* Line 5 */ o = null; /* Line 6 */ oa[0] = null; /* Line 7 */ return o; /* Line 8 */ }
Options- A. just after line 5
- B. just after line 6
- C. just after line 7
- D. just after line 8
- Correct Answer
- just after line 7
Explanation
Option A is wrong. This simply copies the object reference into the array.Option B is wrong. The reference o is set to null, but, oa[0] still maintains the reference to the Float object.
Option C is correct. The thread of execution will then not have access to the object.
- 1. Where will be the most chance of the garbage collector being invoked?
class HappyGarbage01 { public static void main(String args[]) { HappyGarbage01 h = new HappyGarbage01(); h.methodA(); /* Line 6 */ } Object methodA() { Object obj1 = new Object(); Object [] obj2 = new Object[1]; obj2[0] = obj1; obj1 = null; return obj2[0]; } }
Options- A. After line 9
- B. After line 10
- C. After line 11
- D. Garbage collector never invoked in methodA() Discuss
- 2. After line 11 runs, how many objects are eligible for garbage collection?
class X2 { public X2 x; public static void main(String [] args) { X2 x2 = new X2(); /* Line 6 */ X2 x3 = new X2(); /* Line 7 */ x2.x = x3; x3.x = x2; x2 = new X2(); x3 = x2; /* Line 11 */ doComplexStuff(); } }
Options- A. 0
- B. 1
- C. 2
- D. 3 Discuss
- 3. After line 8 runs. how many objects are eligible for garbage collection?
public class X { public static void main(String [] args) { X x = new X(); X x2 = m1(x); /* Line 6 */ X x4 = new X(); x2 = x4; /* Line 8 */ doComplexStuff(); } static X m1(X mx) { mx = new X(); return mx; } }
Options- A. 0
- B. 1
- C. 2
- D. 3 Discuss
- 4. When is the Demo object eligible for garbage collection?
class Test { private Demo d; void start() { d = new Demo(); this.takeDemo(d); /* Line 7 */ } /* Line 8 */ void takeDemo(Demo demo) { demo = null; demo = new Demo(); } }
Options- A. After line 7
- B. After line 8
- C. After the start() method completes
- D. When the instance running this code is made eligible for garbage collection. Discuss
- 5. What allows the programmer to destroy an object x?
Options- A. x.delete()
- B. x.finalize()
- C. Runtime.getRuntime().gc()
- D. Only the garbage collection system can destroy an object. Discuss
- delete() - Method in class java.io.File : Deletes the file or directory denoted by this abstract pathname.
- delete(int, int) - Method in class java.lang.StringBuffer : Removes the characters in a substring of this StringBuffer.
- delete(int, int) - Method in interface javax.accessibility.AccessibleEditableText : Deletes the text between two indices
- delete(int, int) - Method in class : javax.swing.text.JTextComponent.AccessibleJTextComponent; Deletes the text between two indices
- 6. Which of the following class level (nonlocal) variable declarations will not compile?
Options- A. protected int a;
- B. transient int b = 3;
- C. private synchronized int e;
- D. volatile int d; Discuss
- 7. Given a method in a protected class, what access modifier do you use to restrict access to that method to only the other members of the same class?
Options- A. final
- B. static
- C. private
- D. protected
- E. volatile Discuss
- 8. Which is a valid declaration within an interface?
Options- A. public static short stop = 23;
- B. protected short stop = 23;
- C. transient short stop = 23;
- D. final void madness(short stop); Discuss
- 9. Which of the following code fragments inserted, will allow to compile?
public class Outer { public void someOuterMethod() { //Line 5 } public class Inner { } public static void main(String[] argv) { Outer ot = new Outer(); //Line 10 } }
Options- A. new Inner(); //At line 5
- B. new Inner(); //At line 10
- C. new ot.Inner(); //At line 10
- D. new Outer.Inner(); //At line 10 Discuss
- 10. Which is valid in a class that extends class A?
class A { protected int method1(int a, int b) { return 0; } }
Options- A. public int method1(int a, int b) {return 0; }
- B. private int method1(int a, int b) { return 0; }
- C. public short method1(int a, int b) { return 0; }
- D. static protected int method1(int a, int b) { return 0; } Discuss
Garbage Collections problems
Search Results
Correct Answer: Garbage collector never invoked in methodA()
Explanation:
Option D is correct. Garbage collection takes place after the method has returned its reference to the object. The method returns to line 6, there is no reference to store the return value. so garbage collection takes place after line 6.
Option A is wrong. Because the reference to obj1 is stored in obj2[0]. The Object obj1 still exists on the heap and can be accessed by an active thread through the reference stored in obj2[0].
Option B is wrong. Because it is only one of the references to the object obj1, the other reference is maintained in obj2[0].
Option C is wrong. The garbage collector will not be called here because a reference to the object is being maintained and returned in obj2[0].
Correct Answer: 2
Explanation:
This is an example of the islands of isolated objects. By the time line 11 has run, the objects instantiated in lines 6 and 7 are referring to each other, but no live thread can reach either of them.
Correct Answer: 1
Explanation:
By the time line 8 has run, the only object without a reference is the one generated as a result of line 6. Remember that "Java is pass by value," so the reference variable x is not affected by the m1() method.
Correct Answer: When the instance running this code is made eligible for garbage collection.
Explanation:
Option D is correct. By a process of elimination.
Option A is wrong. The variable d is a member of the Test class and is never directly set to null.
Option B is wrong. A copy of the variable d is set to null and not the actual variable d.
Option C is wrong. The variable d exists outside the start() method (it is a class member). So, when the start() method finishes the variable d still holds a reference.
Correct Answer: Only the garbage collection system can destroy an object.
Explanation:
Option A is wrong. I found 4 delete() methods in all of the Java class structure. They are:
None of these destroy the object to which they belong.
Option B is wrong. I found 19 finalize() methods. The most interesting, from this questions point of view, was the finalize() method in class java.lang.Object which is called by the garbage collector on an object when garbage collection determines that there are no more references to the object. This method does not destroy the object to which it belongs.
Option C is wrong. But it is interesting. The Runtime class has many methods, two of which are:
Correct Answer: private synchronized int e;
Explanation:
Option A and B will compile because protected and transient are legal variable modifiers. Option D will compile because volatile is a proper variable modifier.
Correct Answer: private
Explanation:
Option A, B, D, and E are wrong because protected are the wrong access modifiers, and final, static, and volatile are modifiers but not access modifiers.
Correct Answer: public static short stop = 23;
Explanation:
(B) and (C) are incorrect because interface variables cannot be either protected or transient. (D) is incorrect because interface methods cannot be final or static.
Correct Answer: new Inner(); //At line 5
Explanation:
Option A compiles without problem.
Option B gives error - non-static variable cannot be referenced from a static context.
Option C package ot does not exist.
Option D gives error - non-static variable cannot be referenced from a static context.
Correct Answer: public int method1(int a, int b) {return 0; }
Explanation:
Option A is correct - because the class that extends A is just simply overriding method1.
Option B is wrong - because it can't override as there are less access privileges in the subclass method1.
Option C is wrong - because to override it, the return type needs to be an integer. The different return type means that the method is not overriding but the same argument list means that the method is not overloading. Conflict - compile time error.
Option D is wrong - because you can't override a method and make it a class method i.e. using static.
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