Java Programming and Garbage Collection

Question

This is a question I have from Lecture about the discussion of Garbage Collection. I cannot figure which line cause the garbage collection.

I have tried using the compiler to run the programming and the outputs are

#1: [C001, C002]
#2: [C001, C002, C001]
#3: [C001, C002, C001]

I know that there is a line that cause Garbage Collection but I cannot figure it out. At the same time, professor required us to draw a diagram showing how the objects and data, lists go.

import java.util.ArrayList;

public class Main {

    public static void main(String[] args) {

        Customer a = new Customer("C001");
        Customer b = new Customer("C002");

        ArrayList<Customer> list1 = new ArrayList<Customer>();
        list1.add(a);
        list1.add(b);
        System.out.println("#1: " + list1.toString()); Icdddd

        ArrayList<Customer> list2 = new ArrayList<Customer>();
        list2 = list1;
        list2.add(a);
        System.out.println("#2: " + list1.toString()); 
        System.out.println("#3: " + list2.toString()); 
    }

}

you can not trigger garbage collection. you can only kindly ask the garbe colletion to run (System.gc()). — Philipp Sander
– Philipp Sander, Commented Oct 28, 2019 at 8:45

Elliott Frisch · Accepted Answer · 2019-10-28 08:45:02Z

3

There is no line that causes Garbage Collection, because garbage collection is not something that you as a programmer can trigger. However, after list2 = list1; the ArrayList reference created by ArrayList<Customer> list2 = new ArrayList<Customer>(); is eligible for garbage collection.

answered Oct 28, 2019 at 8:45

Elliott Frisch

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6 Comments

CodingKingggg Over a year ago

I understand and agree to your point! However, the lecturer asked us to tell her which is the line cause garbage collection.

Curiosa Globunznik Over a year ago

Maybe debugging the lecturer might give an explanation

Elliott Frisch Over a year ago

I'm afraid this is the only answer in Java, perhaps follow-up with her. It might also be worth asking your lecturer why they chose to use ArrayList<Customer> list1 = new ArrayList<Customer>(); instead of using the diamond operator ArrayList<Customer> list1 = new ArrayList<>(); (Java 7 came out in 2011).

www.hybriscx.com Over a year ago

ArrayList<Customer> list2 = new ArrayList<Customer>(); As mentioned by Elliott, line above creates a new object which becomes eligible for garbage collection as soon as you do list2=list1

Elliott Frisch Over a year ago

@another-dave That is true, but this code will almost certainly complete and terminate before garbage collection runs.

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CodingKingggg · Accepted Answer · 2019-10-29 15:10:01Z

0

so finally the professor said the answer is list2 = list1; which cause the garbage collection, whereas line 7 is redundant

answered Oct 29, 2019 at 15:10

CodingKingggg

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1 Comment

Elliott Frisch Over a year ago

You might want to ask your professor to explain the purpose of Runtime.gc(); and also what the word "suggests" means in Calling this method suggests that the Java virtual machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. The reality is that such a short program with no obvious memory constraints is highly unlikely to trigger garbage collection during its' runtime. Computers with 32mb of RAM ran garbage collection a lot, you have more.

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