Garbage collection overview in Java - Mini Project: Build & Apply

Garbage Collection Overview in Java

📖 Scenario: You are managing a simple Java program that creates objects representing books in a library system. Over time, some books are no longer needed and should be cleaned up by Java's garbage collector to free memory.

🎯 Goal: Build a small Java program that creates a dictionary (HashMap) of books with their IDs, then removes some entries, and finally prints the remaining books to observe how unused objects can be discarded.

📋 What You'll Learn

Create a HashMap called books with these exact entries: 1: "Java Basics", 2: "Data Structures", 3: "Algorithms"

Create an integer variable called removeId and set it to 2

Remove the book with the key removeId from the books HashMap

Print the books HashMap to show the remaining entries

💡 Why This Matters

🌍 Real World

Managing collections of objects like books, users, or products in software applications often requires adding, removing, and cleaning up data efficiently.

💼 Career

Understanding how to manage data collections and how garbage collection works is essential for Java developers to write efficient and memory-safe applications.

1

Create the initial books HashMap

Create a HashMap called books with these exact entries: 1: "Java Basics", 2: "Data Structures", 3: "Algorithms"

Java

// Your code here

Hint

Use HashMap books = new HashMap<>(); and books.put(key, value); to add entries.

2

Add a variable to select which book to remove

Create an integer variable called removeId and set it to 2

Java

import java.util.HashMap;

public class GarbageCollectionDemo {
    public static void main(String[] args) {
        HashMap books = new HashMap();
        books.put(1, "Java Basics");
        books.put(2, "Data Structures");
        books.put(3, "Algorithms");
        // Your code here
    }
}

Hint

Use int removeId = 2; to create the variable.

3

Remove the selected book from the HashMap

Use the removeId variable to remove the corresponding entry from the books HashMap using books.remove(removeId);

Java

import java.util.HashMap;

public class GarbageCollectionDemo {
    public static void main(String[] args) {
        HashMap books = new HashMap();
        books.put(1, "Java Basics");
        books.put(2, "Data Structures");
        books.put(3, "Algorithms");
        int removeId = 2;
        // Remove the book with key removeId
        // Your code here
    }
}

Hint

Use books.remove(removeId); to remove the entry.

4

Print the remaining books

Print the books HashMap using System.out.println(books); to show the remaining entries

Java

import java.util.HashMap;

public class GarbageCollectionDemo {
    public static void main(String[] args) {
        HashMap books = new HashMap();
        books.put(1, "Java Basics");
        books.put(2, "Data Structures");
        books.put(3, "Algorithms");
        int removeId = 2;
        books.remove(removeId);
        // Print the books HashMap
        // Your code here
    }
}

Hint

Use System.out.println(books); to print the HashMap.

Practice

1. What is the main purpose of garbage collection in Java?

easy

A. Increase the size of the heap memory

B. Automatically free memory used by objects no longer needed

C. Manually delete objects from memory

D. Compile Java code into bytecode

Solution

1. Step 1: Recall the purpose of garbage collection and evaluate options

   Garbage collection automatically frees memory used by objects that are no longer referenced in the program. Only "Automatically free memory used by objects no longer needed" matches this definition.

2. Final Answer:

   Automatically free memory used by objects no longer needed -> Option B

3. Quick Check:

   Garbage collection = automatic memory freeing [OK]

Hint: Garbage collection frees unused memory automatically [OK]

Common Mistakes:

• Thinking garbage collection deletes objects manually
• Confusing garbage collection with compiling code
• Believing it increases heap size

2. Which of the following is the correct way to suggest garbage collection in Java?

easy

A. GarbageCollector.run()

B. System.collect()

C. Runtime.gcCollect()

D. System.gc()

Solution

1. Step 1: Recall and verify the method to suggest garbage collection

   Java provides System.gc() to suggest that the JVM runs garbage collection. Only System.gc() is a valid Java method; the others are incorrect or do not exist.

2. Final Answer:

   System.gc() -> Option D

3. Quick Check:

   Suggest GC = System.gc() [OK]

Hint: Use System.gc() to suggest garbage collection [OK]

Common Mistakes:

• Using System.collect() which does not exist
• Confusing Runtime methods with System methods
• Assuming garbage collection runs immediately after calling System.gc()

3. Consider the following Java code snippet:

public class TestGC {
  public static void main(String[] args) {
    String s = new String("hello");
    s = null;
    System.gc();
    System.out.println("Done");
  }
}

What is the expected output when this code runs?

medium

A. Done

B. hello

C. No output, program crashes

D. Done followed by garbage collection logs

Solution

1. Step 1: Trace code execution and determine output

   The string object "hello" is created, then the reference s is set to null, making it eligible for GC. System.gc() suggests collection but does not guarantee immediate action or logs. The only guaranteed output is "Done" from the print statement.

2. Final Answer:

   Done -> Option A

3. Quick Check:

   Print statement output = Done [OK]

Hint: System.gc() does not print anything; only explicit prints show [OK]

Common Mistakes:

• Expecting garbage collection logs to print automatically
• Thinking the string "hello" prints after null assignment
• Assuming program crashes due to null reference

4. Identify the error in the following Java code related to garbage collection:

public class GCExample {
  public static void main(String[] args) {
    Object obj = new Object();
    obj = null;
    System.gc;
  }
}

medium

A. Cannot assign null to an Object

B. Object cannot be garbage collected

C. Missing parentheses in System.gc call

D. System.gc is not a valid method

Solution

1. Step 1: Locate the syntax error in the code

   System.gc is a method call requiring parentheses (System.gc()); the code has System.gc without them, causing a compilation error. Assigning null to Object and the method's existence are valid.

2. Final Answer:

   Missing parentheses in System.gc call -> Option C

3. Quick Check:

   Method calls need parentheses [OK]

Hint: Always use parentheses when calling methods like System.gc() [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Thinking null assignment is invalid
• Believing System.gc is not a method

5. You have a Java program that creates many temporary objects inside a loop. Which approach best helps ensure efficient memory use with garbage collection?

hard

A. Set object references to null when no longer needed inside the loop

B. Call System.gc() inside every loop iteration

C. Increase heap size to avoid garbage collection

D. Avoid creating any objects inside the loop

Solution

1. Step 1: Identify best practice for efficient memory use in loops

   Many temporary objects in a loop can pressure memory; explicitly setting references to null when no longer needed helps the GC identify and free them sooner. Frequent System.gc() is inefficient and not guaranteed; larger heap delays issues; avoiding all objects may not be feasible.

2. Final Answer:

   Set object references to null when no longer needed inside the loop -> Option A

3. Quick Check:

   Null references help GC free memory efficiently [OK]

Hint: Nullify unused references to help garbage collection [OK]

Common Mistakes:

• Calling System.gc() too often expecting instant cleanup
• Thinking increasing heap size solves memory leaks
• Believing you must avoid all object creation in loops