Method overriding in Java - Time & Space Complexity

Time Complexity: Method overriding

O(n)

Understanding Time Complexity

Let's see how the time it takes to run a program changes when we use method overriding in Java.

We want to know if calling an overridden method takes more time as the program grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Bark");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}
    

This code shows a class Animal with a method sound, and a Dog class that changes (overrides) this method. We call the sound method on a Dog object through an Animal reference.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling the overridden method sound()
• How many times: Called once in this example, but could be called many times in a larger program

How Execution Grows With Input

Calling an overridden method takes about the same time no matter how many classes or objects you have.

Input Size (n)	Approx. Operations
10	10 method calls, each takes similar time
100	100 method calls, each takes similar time
1000	1000 method calls, each takes similar time

Pattern observation: Each method call takes a fixed amount of time, so total time grows linearly with the number of calls.

Final Time Complexity

Time Complexity: O(n)

This means if you call the overridden method n times, the total time grows in a straight line with n.

Common Mistake

[X] Wrong: "Overriding a method makes the program slower because it adds extra steps."

[OK] Correct: The program uses a simple way to find the right method to run, so each call still takes about the same time as a normal method call.

Interview Connect

Understanding how method overriding affects time helps you explain how programs stay efficient even when using flexible designs.

Self-Check

"What if we called the overridden method inside a loop that runs n times? How would the time complexity change?"

Practice

1.

What is method overriding in Java?

easy

A. When a child class provides a new version of a method from its parent class

B. When a method calls itself repeatedly

C. When two methods have the same name but different parameters

D. When a method is hidden from other classes

Solution

1. Step 1: Understand method overriding concept

   Method overriding means a child class changes the behavior of a method inherited from its parent class.

2. Step 2: Compare options with definition

   When a child class provides a new version of a method from its parent class correctly describes this. When a method calls itself repeatedly is recursion, C is method overloading, D is incorrect.

3. Final Answer:

   When a child class provides a new version of a method from its parent class -> Option A

4. Quick Check:

   Method overriding = child changes parent method [OK]

Hint: Overriding means child changes parent's method behavior [OK]

Common Mistakes:

• Confusing overriding with overloading
• Thinking overriding changes method signature
• Mixing overriding with recursion

2.

Which of the following is the correct syntax to override a method in Java?

class Parent {
    void show() { System.out.println("Parent"); }
}
class Child extends Parent {
    ?
}

easy

A. private void show() { System.out.println("Child"); }

B. @Override void show() { System.out.println("Child"); }

C. void Show() { System.out.println("Child"); }

D. void show(int x) { System.out.println("Child"); }

Solution

1. Step 1: Check method signature and annotation

   To override, method name and parameters must match exactly. Using @Override annotation helps catch mistakes.

2. Step 2: Analyze options

   private void show() { System.out.println("Child"); } uses private access modifier, narrowing from package-private (not allowed for overriding; causes hiding). void show(int x) { System.out.println("Child"); } changes parameters (overloading). void Show() { System.out.println("Child"); } has method name mismatch ('Show' vs 'show' due to case sensitivity). @Override void show() { System.out.println("Child"); } matches method signature exactly and uses @Override correctly.

3. Final Answer:

   @Override void show() { System.out.println("Child"); } -> Option B

4. Quick Check:

   @Override + same signature = correct override [OK]

Hint: Use @Override and same method signature to override [OK]

Common Mistakes:

• Narrowing access modifier (e.g., to private)
• Changing method parameters (overloading instead)
• Method name case mismatch

3.

What will be the output of the following code?

class Parent {
    void greet() { System.out.println("Hello from Parent"); }
}
class Child extends Parent {
    @Override
    void greet() { System.out.println("Hello from Child"); }
}
public class Test {
    public static void main(String[] args) {
        Parent obj = new Child();
        obj.greet();
    }
}

medium

A. Hello from Parent

B. Compilation error

C. Runtime error

D. Hello from Child

Solution

1. Step 1: Understand dynamic method dispatch

   When a parent reference points to a child object, overridden methods in child are called at runtime.

2. Step 2: Trace the method call

   obj is Parent type but refers to Child instance. Calling greet() runs Child's version.

3. Final Answer:

   Hello from Child -> Option D

4. Quick Check:

   Parent ref + Child object calls Child method [OK]

Hint: Parent reference calls child's overridden method at runtime [OK]

Common Mistakes:

• Expecting parent method output
• Confusing compile-time and runtime method binding
• Thinking method overloading applies here

4.

Identify the error in the following code snippet:

class Parent {
    void display() { System.out.println("Parent"); }
}
class Child extends Parent {
    @Override
    void Display() { System.out.println("Child"); }
}

medium

A. Method name case mismatch causes no overriding

B. Missing return type in Child class method

C. Cannot override a method without parameters

D. No error, code is correct

Solution

1. Step 1: Check method names carefully

   Parent method is 'display' (lowercase d), Child method is 'Display' (uppercase D). Java is case-sensitive, so these are different methods.

2. Step 2: Understand @Override annotation effect

   @Override expects exact match. Here, it causes a compile-time error because method names differ in case.

3. Final Answer:

   Method name case mismatch causes no overriding -> Option A

4. Quick Check:

   Method names must match exactly, including case [OK]

Hint: Method names must match case exactly to override [OK]

Common Mistakes:

• Ignoring case sensitivity in method names
• Assuming @Override is optional and ignoring errors
• Confusing overriding with overloading

5.

Consider the following classes:

class Animal {
    String sound() { return "Some sound"; }
}
class Dog extends Animal {
    @Override
    String sound() { return "Bark"; }
}
class Cat extends Animal {
    @Override
    String sound() { return "Meow"; }
}
public class Test {
    public static void main(String[] args) {
        Animal[] animals = {new Dog(), new Cat(), new Animal()};
        for (Animal a : animals) {
            System.out.println(a.sound());
        }
    }
}

What is the output of this program?

hard

A. Bark
Some sound
Meow

B. Some sound
Some sound
Some sound

C. Bark
Meow
Some sound

D. Compilation error due to array of different types

Solution

1. Step 1: Understand polymorphism with overridden methods

   Each object calls its own overridden sound() method at runtime, even if referenced as Animal.

2. Step 2: Trace the loop output

   Dog's sound() returns "Bark", Cat's returns "Meow", Animal's returns "Some sound".

3. Final Answer:

   Bark Meow Some sound -> Option C

4. Quick Check:

   Array of Animals calls each overridden sound() correctly [OK]

Hint: Array of parent type calls each child's overridden method [OK]

Common Mistakes:

• Expecting all calls to use parent method
• Thinking array cannot hold different subclass objects
• Confusing overriding with overloading