Type casting in Java - Mini Project: Build & Apply

Type Casting in Java

📖 Scenario: You are working on a simple calculator program that needs to convert numbers between different types to perform calculations correctly.

🎯 Goal: Learn how to convert (cast) numbers from one type to another in Java and see the results.

📋 What You'll Learn

Create variables with specific types and values

Create a variable to hold a casted value

Use explicit type casting to convert a double to an int

Print the original and casted values

💡 Why This Matters

🌍 Real World

Type casting is used when you need to convert data from one type to another, such as when reading input, performing calculations, or working with different APIs.

💼 Career

Understanding type casting is essential for software developers to handle data correctly and avoid errors in programs.

1

Create a double variable

Create a variable called originalValue of type double and set it to 9.78.

Java

// Create a variable called originalValue of type double and set it to 9.78
// Your code here

Hint

Use the syntax: double variableName = value;

2

Create an int variable for the casted value

Create a variable called castedValue of type int to hold the casted value.

Java

double originalValue = 9.78;
// Create an int variable called castedValue
// Your code here

Hint

Declare an int variable without assigning a value yet.

3

Cast the double to int

Assign the value of originalValue casted to int to the variable castedValue using explicit type casting.

Java

double originalValue = 9.78;
int castedValue;
// Cast originalValue to int and assign to castedValue
// Your code here

Hint

Use (int) originalValue to cast the double to int.

4

Print the original and casted values

Print the values of originalValue and castedValue using System.out.println with the exact text:
Original value: 9.78
Casted value: 9

Java

double originalValue = 9.78;
int castedValue;
castedValue = (int) originalValue;
// Print the originalValue and castedValue with the exact text
// Your code here

Hint

Use System.out.println("Original value: " + originalValue); and similarly for castedValue.

Practice

1.

What is type casting in Java?

easy

A. Writing a loop

B. Creating a new variable

C. Declaring a method

D. Changing a value from one data type to another

Solution

1. Step 1: Understand the meaning of type casting

   Type casting means converting a value from one data type to another, like from int to double or vice versa.

2. Step 2: Identify the correct description

   Among the options, only changing a value's data type matches the definition of type casting.

3. Final Answer:

   Changing a value from one data type to another -> Option D

4. Quick Check:

   Type casting = Changing data type [OK]

Hint: Type casting means changing data types [OK]

Common Mistakes:

• Confusing type casting with variable declaration
• Thinking type casting creates new variables
• Mixing type casting with control structures

2.

Which of the following is the correct syntax for explicit type casting from double to int in Java?

easy

A. int x = (int) 3.14;

B. int x = int(3.14);

C. int x = 3.14 as int;

D. int x = cast int 3.14;

Solution

1. Step 1: Recall Java explicit casting syntax

   In Java, explicit casting uses parentheses with the target type before the value, like (int) value.

2. Step 2: Match syntax with options

   Only int x = (int) 3.14; uses the correct syntax: int x = (int) 3.14;. Others are invalid in Java.

3. Final Answer:

   int x = (int) 3.14; -> Option A

4. Quick Check:

   Explicit cast uses (type) value [OK]

Hint: Use (type) before value for explicit cast [OK]

Common Mistakes:

• Using function-like cast syntax (e.g., int(3.14))
• Using 'as' keyword which Java doesn't support
• Writing 'cast' keyword which is invalid

3.

What is the output of the following Java code?

double d = 9.99;
int i = (int) d;
System.out.println(i);

medium

A. 9.99

B. 10

C. 9

D. Compilation error

Solution

1. Step 1: Understand explicit cast from double to int

   Casting a double to int truncates the decimal part, it does not round.

2. Step 2: Apply casting to value 9.99

   When casting 9.99 to int, the decimal .99 is dropped, resulting in 9.

3. Final Answer:

   9 -> Option C

4. Quick Check:

   Double to int cast truncates decimals = 9 [OK]

Hint: Casting double to int drops decimals, no rounding [OK]

Common Mistakes:

• Assuming casting rounds the number
• Expecting compilation error due to cast
• Confusing output with original double value

4.

Identify the error in the following Java code snippet:

byte b = 130;

medium

A. Type mismatch: possible lossy conversion from int to byte

B. No error, code runs fine

C. Syntax error: missing cast operator

D. Runtime exception: value too large

Solution

1. Step 1: Understand byte range in Java

   Byte type can hold values from -128 to 127 only.

2. Step 2: Check assigned value 130

   130 is outside byte range, so Java reports a compile-time error about possible lossy conversion.

3. Final Answer:

   Type mismatch: possible lossy conversion from int to byte -> Option A

4. Quick Check:

   Byte range exceeded causes compile error [OK]

Hint: Byte values must be between -128 and 127 [OK]

Common Mistakes:

• Thinking code runs without error
• Expecting runtime exception instead of compile error
• Missing need for explicit cast

5.

Given the code below, what will be the output?

int a = 300;
byte b = (byte) a;
System.out.println(b);

Note: byte range is -128 to 127.

hard

A. 44

B. -56

C. 300

D. Compilation error

Solution

1. Step 1: Understand casting int 300 to byte

   When casting int to byte, Java keeps only the lowest 8 bits of the int value.

2. Step 2: Calculate byte value from 300

   300 in binary is 00000001 00101100 (16 bits). The lowest 8 bits are 00101100 (decimal 44).

3. Step 3: Confirm signed byte value

   300 mod 256 = 44. However, since byte is signed, the value wraps around to -56.

4. Final Answer:

   -56 -> Option B

5. Quick Check:

   Int to byte cast wraps around signed byte range = -56 [OK]

Hint: Casting int to byte keeps lowest 8 bits and wraps signed range [OK]

Common Mistakes:

• Assuming output is original int value
• Confusing signed byte range with unsigned
• Expecting compilation error without cast