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.