Method overriding in Java - Step-by-Step Execution

Concept Flow - Method overriding

Parent class defines method

↓

Child class defines method with same signature

↓

Create child class object

↓

Call method on child object

↓

Child's method runs, overrides parent

↓

Output from child method

Method overriding happens when a child class provides its own version of a method defined in its parent class. When called on a child object, the child's method runs instead of the parent's.

Execution Sample

Java

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

This code shows a parent and child class both with a greet method. The child overrides the parent's greet. When calling greet on a child object, the child's greet runs.

Execution Table

Step	Action	Object Type	Method Called	Output
1	Create Parent reference to Child object	Child	N/A	N/A
2	Call greet() on obj	Child	Child.greet()	Hello from Child
3	End of main method	N/A	N/A	N/A

💡 Program ends after main method finishes

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
obj	null	Child object reference	Child object reference	Child object reference

Key Moments - 2 Insights

Why does calling greet() on a Parent type variable run the Child's greet method?

Because the actual object is of Child type (see execution_table step 1 and 2), Java uses the object's real type to decide which method to run, not the variable's declared type.

What happens if the Child class does not override greet()?

Then calling greet() on the Child object runs the Parent's greet method, because no overriding method exists in Child (not shown in table but implied).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, which method is called when obj.greet() runs?

A Parent.greet()

B Child.greet()

C Both Parent.greet() and Child.greet()

D No method is called

💡 Hint

Check the 'Method Called' column in execution_table step 2

At which step is the Child object created and assigned to obj?

A Step 3

B Step 2

C Step 1

D No object is created

💡 Hint

Look at the 'Action' and 'Object Type' columns in execution_table step 1

If obj was declared as Parent and assigned a Parent object, which greet() method would run?

A Parent.greet()

B Child.greet()

C Both methods

D No method

💡 Hint

Refer to key_moments explanation about object type deciding method call

Concept Snapshot

Method overriding lets a child class provide its own version of a method from the parent class.
When calling the method on a child object, the child's method runs, even if the variable type is the parent.
The method signature must match exactly.
This enables polymorphism and dynamic method dispatch in Java.

Full Transcript

This example shows method overriding in Java. The Parent class has a greet method. The Child class extends Parent and overrides greet with its own version. In main, a Parent type variable obj holds a Child object. When obj.greet() is called, Java runs Child's greet method because the actual object is Child. The execution table traces creating the Child object, calling greet, and the output. The variable tracker shows obj pointing to the Child object throughout. Key moments clarify why the Child method runs despite the Parent type variable. The quiz tests understanding of method calls and object types. The snapshot summarizes the key points about method overriding and dynamic dispatch.

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