Object interaction in Java - Step-by-Step Execution

Concept Flow - Object interaction

Create Object A

↓

Create Object B

↓

Object A calls method on Object B

↓

Object B executes method

↓

Return result to Object A

↓

Object A uses result

This flow shows how one object creates another, calls its method, and uses the returned result.

Execution Sample

Java

class Printer {
  void printMessage(String msg) {
    System.out.println(msg);
  }
}

class User {
  void sendPrint(Printer p) {
    p.printMessage("Hello from User");
  }
}

public class Main {
  public static void main(String[] args) {
    Printer printer = new Printer();
    User user = new User();
    user.sendPrint(printer);
  }
}

User object calls a method on Printer object to print a message.

Execution Table

Step	Action	Object	Method Called	Parameter	Output
1	Create Printer object	Printer	N/A	N/A	Printer instance created
2	Create User object	User	N/A	N/A	User instance created
3	User calls sendPrint	User	sendPrint	Printer object	No output
4	sendPrint calls printMessage	Printer	printMessage	"Hello from User"	Prints: Hello from User
5	printMessage completes	Printer	printMessage	N/A	Method returns void
6	sendPrint completes	User	sendPrint	N/A	Method returns void
7	main completes	Main	main	N/A	Program ends

💡 Program ends after main method completes

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
printer	null	Printer instance	Printer instance	Printer instance	Printer instance
user	null	null	User instance	User instance	User instance

Key Moments - 3 Insights

Why does User need a Printer object to call printMessage?

Because printMessage belongs to Printer, User must have a reference to Printer to call its method, as shown in step 3 of the execution_table.

Does printMessage return any value to sendPrint?

No, printMessage returns void (no value), so sendPrint just calls it and continues, as seen in steps 4 and 5.

What happens if User tries to call printMessage without a Printer object?

It causes an error because printMessage is not in User; User must have a Printer reference to call it, shown by the need for parameter in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output when printMessage is called?

A No output

B Prints: Hello from User

C Returns a string

D Throws an error

💡 Hint

Check step 4 in the execution_table where printMessage prints the message.

At which step does the User object call a method on the Printer object?

A Step 2

B Step 5

C Step 3

D Step 7

💡 Hint

Look at step 3 in the execution_table where sendPrint is called with Printer as parameter.

If the Printer object was not created, what would happen at step 3?

A NullPointerException error occurs

B sendPrint calls printMessage anyway

C User calls sendPrint successfully

D Program prints a default message

💡 Hint

Refer to variable_tracker and step 3 in execution_table; without Printer instance, calling method causes error.

Concept Snapshot

Object interaction in Java:
- One object holds reference to another
- Calls methods on that object using dot notation
- Methods can return values or be void
- Objects must be created before interaction
- Example: user.sendPrint(printer) calls printer.printMessage()

Full Transcript

This example shows how two objects interact in Java. First, a Printer object is created, then a User object. The User object calls its sendPrint method, passing the Printer object. Inside sendPrint, the Printer's printMessage method is called with a message. The message is printed to the console. The methods return void, so no values are passed back. The program ends after main completes. This demonstrates how objects communicate by calling each other's methods using references.

Practice

1.

What does it mean when two objects in Java interact?

Choose the best explanation.

easy

A. One object calls a method of another object to perform a task.

B. Two objects share the same memory location.

C. Objects are created using the same class.

D. Objects are stored in the same variable.

Solution

1. Step 1: Understand object interaction meaning

   Object interaction means objects communicate by calling each other's methods to work together.

2. Step 2: Evaluate options

   Only One object calls a method of another object to perform a task. describes calling methods between objects, which is how interaction happens.

3. Final Answer:

   One object calls a method of another object to perform a task. -> Option A

4. Quick Check:

   Object interaction = method calls between objects [OK]

Hint: Objects interact by calling methods on each other [OK]

Common Mistakes:

• Thinking objects share memory to interact
• Confusing object creation with interaction
• Assuming variables store multiple objects

2.

Which of the following is the correct syntax to call a method start() on an object car in Java?

easy

A. start(car);

B. car.start();

C. car->start();

D. start.car();

Solution

1. Step 1: Recall Java method call syntax

   In Java, to call a method on an object, use objectName.methodName();.

2. Step 2: Check each option

   car.start(); matches correct syntax. Options A, B, and C are invalid Java syntax.

3. Final Answer:

   car.start(); -> Option B

4. Quick Check:

   Method call = object.method() [OK]

Hint: Use objectName.methodName() to call methods [OK]

Common Mistakes:

• Using arrow (->) like in C++
• Reversing method and object order
• Calling method like a function with object as argument

3.

Consider the following Java code:

class Light {
    boolean isOn = false;
    void toggle() {
        isOn = !isOn;
    }
    boolean status() {
        return isOn;
    }
}

class Room {
    Light light = new Light();
    void switchLight() {
        light.toggle();
    }
    boolean lightStatus() {
        return light.status();
    }
}

public class Main {
    public static void main(String[] args) {
        Room room = new Room();
        System.out.println(room.lightStatus());
        room.switchLight();
        System.out.println(room.lightStatus());
    }
}

What is the output when this program runs?

medium

A. false
true

B. true
false

C. false
false

D. true
true

Solution

1. Step 1: Check initial light status

   Initially, isOn is false, so room.lightStatus() prints false.

2. Step 2: Toggle light and check status again

   Calling room.switchLight() toggles isOn to true. Then room.lightStatus() prints true.

3. Final Answer:

   false true -> Option A

4. Quick Check:

   Initial false, toggled true = false then true [OK]

Hint: Track boolean changes step-by-step [OK]

Common Mistakes:

• Assuming toggle sets true first without initial check
• Confusing method calls and variable values
• Ignoring initial value of isOn

4.

Find the error in this Java code snippet involving object interaction:

class Printer {
    void print(String message) {
        System.out.println(message);
    }
}

public class Main {
    public static void main(String[] args) {
        Printer printer;
        printer.print("Hello World");
    }
}

medium

A. The print statement syntax is incorrect.

B. The method 'print' is not defined in the Printer class.

C. The object 'printer' is declared but not initialized before use.

D. The class Printer should be public.

Solution

1. Step 1: Check object declaration and initialization

   The object 'printer' is declared but never assigned a new Printer instance.

2. Step 2: Understand consequences of uninitialized object

   Calling a method on an uninitialized object causes a NullPointerException at runtime.

3. Final Answer:

   The object 'printer' is declared but not initialized before use. -> Option C

4. Quick Check:

   Uninitialized object causes runtime error [OK]

Hint: Always initialize objects before calling methods [OK]

Common Mistakes:

• Assuming declaration equals initialization
• Thinking method absence causes error here
• Ignoring runtime NullPointerException

5.

You have two classes, BankAccount and Customer. A Customer has a BankAccount object. You want to add a method transferTo in BankAccount that transfers money to another BankAccount. Which of the following best shows how objects interact to perform this transfer?

class BankAccount {
    double balance;
    void deposit(double amount) { balance += amount; }
    void withdraw(double amount) { balance -= amount; }
    void transferTo(BankAccount other, double amount) {
        // Fill in this method
    }
}

class Customer {
    BankAccount account = new BankAccount();
}

hard

A. deposit(amount); withdraw(amount);

B. other.withdraw(amount); deposit(amount);

C. balance -= amount; other.balance += amount;

D. withdraw(amount); other.deposit(amount);

Solution

1. Step 1: Understand transfer logic

   To transfer money, withdraw from current account and deposit into the other account.

2. Step 2: Check method calls for interaction

   Calling withdraw(amount) on this object and deposit(amount) on the other object shows interaction.

3. Final Answer:

   withdraw(amount); other.deposit(amount); -> Option D

4. Quick Check:

   Transfer = withdraw from one, deposit to another [OK]

Hint: Transfer = withdraw from self, deposit to other [OK]

Common Mistakes:

• Withdrawing from the wrong account
• Directly changing balance without methods
• Depositing before withdrawing