Object lifecycle in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new object of class Car.

Java

Car myCar = new [1]();

Drag options to blanks, or click blank then click option'

AObject

Bcar

CCar

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase class name like 'car' causes errors.

Using unrelated class names like 'Vehicle' or 'Object'.

In Java, to create a new object, you use the class name with new. Here, Car is the class name.

2fill in blank

medium

Complete the code to call the constructor of class Person with a name parameter.

Java

Person p = new Person([1]);

Drag options to blanks, or click blank then click option'

A"Alice"

B25

Ctrue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a number instead of a string.

Forgetting quotes around the name.

The constructor expects a name, which is a string. Strings in Java are written inside double quotes.

3fill in blank

hard

Fix the error in the code to properly destroy an object reference.

Java

myObject = [1];

Drag options to blanks, or click blank then click option'

Adelete

Bnull

Cremove

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'delete' which is not valid in Java.

Using 'remove' or 'void' which are invalid here.

In Java, to remove a reference to an object so it can be garbage collected, assign null to the variable.

4fill in blank

hard

Fill both blanks to create an object and call its method.

Java

Car [1] = new [2]();
[1].startEngine();

Drag options to blanks, or click blank then click option'

AmyCar

BCar

Cengine

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for variable and constructor.

Using invalid variable names.

First, we name the object myCar. Then, we create a new Car object. Finally, we call the method on myCar.

5fill in blank

hard

Fill all three blanks to create a Person object, assign it to a variable, and call a method.

Java

Person [1] = new [2]([3]);
[1].sayHello();

Drag options to blanks, or click blank then click option'

Aperson1

BPerson

C"Bob"

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching variable and constructor names.

Passing a parameter without quotes.

We name the variable person1, create a new Person with the name "Bob", then call sayHello() on person1.

Practice

1. Which statement best describes the lifecycle of an object in Java?

easy

A. Objects must be manually deleted to free memory.

B. An object is created automatically without new and never gets removed.

C. Objects live forever once created.

D. An object is created with new and exists as long as it has references.

Solution

1. Step 1: Understand object creation

   In Java, objects are created using the new keyword which allocates memory.

2. Step 2: Understand object lifetime

   An object remains alive as long as there is at least one reference pointing to it. When no references remain, it becomes eligible for garbage collection.

3. Final Answer:

   An object is created with new and exists as long as it has references. -> Option D

4. Quick Check:

   Object lifecycle = created with new and referenced [OK]

Hint: Objects live only while referenced, created with new [OK]

Common Mistakes:

• Thinking objects live forever
• Believing manual deletion is needed
• Assuming objects are created without new

2. Which of the following is the correct way to create a new object of class Car in Java?

easy

A. Car myCar = Car();

B. Car myCar = new Car();

C. new Car myCar();

D. Car myCar = create Car();

Solution

1. Step 1: Recall Java object creation syntax

   In Java, to create an object, use the syntax: ClassName variable = new ClassName();

2. Step 2: Match options with correct syntax

   Car myCar = new Car(); matches the correct syntax. Other options have syntax errors or invalid keywords.

3. Final Answer:

   Car myCar = new Car(); -> Option B

4. Quick Check:

   Use new keyword to create objects [OK]

Hint: Use 'new' keyword with class name and parentheses [OK]

Common Mistakes:

• Omitting 'new' keyword
• Incorrect order of keywords
• Using invalid method-like syntax

3. What will be the output of the following Java code?

class Demo {
    public static void main(String[] args) {
        Demo obj1 = new Demo();
        Demo obj2 = obj1;
        obj1 = null;
        if (obj2 != null) {
            System.out.println("Object is alive");
        } else {
            System.out.println("Object is gone");
        }
    }
}

medium

A. Object is alive

B. Object is gone

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Analyze object references

   Initially, obj1 points to a new Demo object. Then obj2 is assigned the same reference as obj1.

2. Step 2: Check null assignment and condition

   obj1 is set to null, but obj2 still references the object. The if condition checks obj2 != null, which is true.

3. Final Answer:

   Object is alive -> Option A

4. Quick Check:

   Object lives while referenced = true [OK]

Hint: Object lives if any reference points to it [OK]

Common Mistakes:

• Assuming object is gone when one reference is null
• Confusing reference variables with objects
• Expecting compilation or runtime errors

4. Identify the error in the following code related to object lifecycle:

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

medium

A. Compilation error due to null assignment

B. No error, prints length of string

C. NullPointerException at runtime

D. String object is not created

Solution

1. Step 1: Understand object reference and null assignment

   The variable s initially references a String object. Then it is set to null, so it no longer points to any object.

2. Step 2: Analyze method call on null reference

   Calling s.length() when s is null causes a NullPointerException at runtime.

3. Final Answer:

   NullPointerException at runtime -> Option C

4. Quick Check:

   Calling method on null reference causes exception [OK]

Hint: Don't call methods on null references [OK]

Common Mistakes:

• Thinking null assignment causes compile error
• Expecting output instead of exception
• Ignoring null pointer risks

5. Consider the following code snippet:

class Node {
    Node next;
    int value;
    Node(int val) { value = val; }
}

public class Test {
    public static void main(String[] args) {
        Node a = new Node(1);
        Node b = new Node(2);
        a.next = b;
        b = null;
        // Which nodes are eligible for garbage collection here?
    }
}

Which nodes are eligible for garbage collection after b = null;?

hard

A. Neither a nor b nodes are eligible

B. Only the node originally referenced by b is eligible

C. Both nodes are eligible

D. Only the node referenced by a is eligible

Solution

1. Step 1: Analyze references after assignment

   Variable a references a Node with value 1. This node's next points to the Node with value 2.

2. Step 2: Check if nodes are still reachable

   Even though b is set to null, the Node with value 2 is still referenced by a.next. So both nodes are still reachable and not eligible for garbage collection.

3. Final Answer:

   Neither a nor b nodes are eligible -> Option A

4. Quick Check:

   Objects reachable via references are not collected [OK]

Hint: Objects reachable from any reference stay alive [OK]

Common Mistakes:

• Assuming null variable means object is collected
• Ignoring references inside objects
• Confusing variable null with object eligibility