Compile-time polymorphism in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is compile-time polymorphism in Java?

Compile-time polymorphism is when the method to be called is decided during the compilation of the program. It is also called method overloading.

beginner

How does method overloading demonstrate compile-time polymorphism?

Method overloading allows multiple methods with the same name but different parameters. The compiler decides which method to call based on the arguments used.

intermediate

Can compile-time polymorphism be achieved using method overriding?

No, method overriding is runtime polymorphism because the method to call is decided during program execution, not at compile time.

intermediate

What are the requirements for method overloading in Java?

Methods must have the same name but differ in the number, type, or order of parameters. Return type alone is not enough to overload a method.

beginner

Example of compile-time polymorphism using method overloading:

class Calculator {
  int add(int a, int b) { return a + b; }
  double add(double a, double b) { return a + b; }
}

The compiler chooses which add method to call based on argument types.

What decides which overloaded method to call in Java?

A The programmer manually

B The JVM at runtime

C The operating system

D The compiler based on method parameters

Compile-time polymorphism means the compiler decides which method to call based on the method's parameters.

Which of the following is NOT a valid way to overload a method?

A Changing only the return type

B Changing the number of parameters

C Changing the order of parameters

D Changing the type of parameters

Changing only the return type does not overload a method because the compiler cannot distinguish methods by return type alone.

Compile-time polymorphism is also known as:

A Method overloading

B Method overriding

C Dynamic binding

D Inheritance

Method overloading is compile-time polymorphism because the method call is resolved during compilation.

Which polymorphism type is determined at runtime?

A Method overloading

B Runtime polymorphism

C Compile-time polymorphism

D Static polymorphism

Runtime polymorphism is when the method to call is decided during program execution, such as with method overriding.

In Java, which keyword is NOT related to compile-time polymorphism?

A static

B final

C @Override

D public

The '@Override' annotation relates to method overriding (runtime polymorphism), not compile-time polymorphism.

Explain compile-time polymorphism and how Java achieves it.

Think about how Java chooses which method to run before the program starts.

Describe the difference between compile-time and runtime polymorphism with examples.

One is decided by the compiler, the other by the program while running.

Practice

1.

What is compile-time polymorphism in Java?

easy

A. Using different method names for different tasks

B. Changing the method behavior at runtime based on object type

C. Using the same method name with different parameters in the same class

D. Creating multiple classes with the same name

Solution

1. Step 1: Understand method overloading

   Compile-time polymorphism is also called method overloading, where methods share the same name but differ in parameters.

2. Step 2: Differentiate from runtime polymorphism

   Runtime polymorphism uses method overriding, changing behavior based on object type at runtime, not compile-time.

3. Final Answer:

   Using the same method name with different parameters in the same class -> Option C

4. Quick Check:

   Compile-time polymorphism = method overloading [OK]

Hint: Same method name, different parameters means compile-time polymorphism [OK]

Common Mistakes:

• Confusing compile-time with runtime polymorphism
• Thinking method overriding is compile-time polymorphism
• Believing different method names are polymorphism

2.

Which of the following is the correct syntax for method overloading in Java?

public class Calculator {
    public int add(int a, int b) { return a + b; }
    public int add(int a, int b, int c) { ? }
}

easy

A. return a + b + c;

B. return a + b;

C. return a * b * c;

D. return a - b - c;

Solution

1. Step 1: Check method parameters

   The second add method has three parameters, so it should add all three values.

2. Step 2: Write correct return statement

   Return the sum of a, b, and c to correctly overload the add method.

3. Final Answer:

   return a + b + c; -> Option A

4. Quick Check:

   Overloaded method sums all parameters [OK]

Hint: Overloaded methods must handle all their parameters correctly [OK]

Common Mistakes:

• Returning sum of only two parameters in three-parameter method
• Using wrong operators like multiplication or subtraction
• Syntax errors like missing semicolon

3.

What will be the output of the following Java code?

class Demo {
    void show(int a) { System.out.println("Int: " + a); }
    void show(String a) { System.out.println("String: " + a); }
}
public class Test {
    public static void main(String[] args) {
        Demo d = new Demo();
        d.show(5);
        d.show("Hello");
    }
}

medium

A. Int: 5\nString: Hello

B. String: 5\nInt: Hello

C. Int: 5\nInt: Hello

D. Compilation error

Solution

1. Step 1: Identify overloaded methods

   There are two show methods: one takes int, the other takes String.

2. Step 2: Match method calls to parameters

   d.show(5) calls show(int), printing "Int: 5"; d.show("Hello") calls show(String), printing "String: Hello".

3. Final Answer:

   Int: 5\nString: Hello -> Option A

4. Quick Check:

   Method overloading calls correct method by parameter type [OK]

Hint: Method chosen by parameter type at compile time [OK]

Common Mistakes:

• Confusing parameter types and outputs
• Expecting runtime polymorphism behavior
• Thinking it causes compilation error

4.

Find the error in this code snippet related to compile-time polymorphism:

class Test {
    void display(int a) { System.out.println(a); }
    void display(int a, int b) { System.out.println(a + b); }
    void display(int a) { System.out.println(a * 2); }
}

medium

A. Incorrect method parameter types

B. Duplicate method display(int a) causes compilation error

C. Missing return type in one method

D. No error, code compiles fine

Solution

1. Step 1: Check method signatures

   Two methods have the exact same name and parameter list: display(int a).

2. Step 2: Understand method overloading rules

   Method overloading requires different parameter lists; duplicate signatures cause compilation error.

3. Final Answer:

   Duplicate method display(int a) causes compilation error -> Option B

4. Quick Check:

   Duplicate method signatures cause compile error [OK]

Hint: Overloaded methods must differ in parameter list [OK]

Common Mistakes:

• Thinking method bodies affect overloading
• Ignoring duplicate parameter lists
• Assuming code compiles without error

5.

Consider this class:

class Printer {
    void print(int a) { System.out.println("Number: " + a); }
    void print(String a) { System.out.println("Text: " + a); }
    void print(int a, String b) { System.out.println(a + " and " + b); }
}

Which call will cause a compile-time error?

hard

A. print("Test")

B. print(10, "Hello")

C. print(5)

D. print("Hello", 10)

Solution

1. Step 1: Check method signatures

   Methods accept (int), (String), and (int, String) parameters.

2. Step 2: Match call parameters

   print("Hello", 10) tries to call (String, int), which does not exist, causing compile error.

3. Final Answer:

   print("Hello", 10) -> Option D

4. Quick Check:

   No matching method for (String, int) call [OK]

Hint: Check parameter order and types carefully for overloaded methods [OK]

Common Mistakes:

• Assuming parameter order doesn't matter
• Thinking all combinations are allowed
• Ignoring method signature mismatch