Type casting in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type casting in Java?

Type casting is converting a variable from one data type to another. It helps when you want to treat data as a different type.

beginner

What is the difference between implicit and explicit type casting?

Implicit casting (widening) happens automatically when converting a smaller type to a bigger type (e.g., int to long). Explicit casting (narrowing) requires you to write the cast manually (e.g., double to int).

beginner

How do you explicitly cast a double to an int in Java?

Use parentheses with the target type before the value:
int x = (int) 9.7;
This cuts off the decimal part.

intermediate

What happens if you cast a large long value to an int?

If the long value is too big for int, the value wraps around and you get a different number. This is called overflow.

intermediate

Can you cast between unrelated object types in Java?

No. You can only cast between compatible types in the same class hierarchy. Otherwise, you get a ClassCastException at runtime.

Which of these is an example of implicit type casting?

A double to int

B String to int

C int to long

D long to byte

Implicit casting happens when converting smaller types to bigger types automatically, like int to long.

What is the result of casting 9.99 (double) to int?

A Error

B 10

C 9.99

D 9

Casting double to int cuts off the decimal part, so 9.99 becomes 9.

Which cast requires explicit syntax in Java?

A double to int

B float to double

C int to long

D byte to short

Casting from double to int is narrowing and requires explicit cast.

What exception occurs if you cast incompatible object types?

A ClassCastException

B IllegalArgumentException

C ArrayIndexOutOfBoundsException

D NullPointerException

Casting incompatible object types throws ClassCastException at runtime.

What happens if a large long value is cast to int?

A Value remains the same

B Value is truncated to fit int size

C Compilation error

D Value is rounded

Casting large long to int truncates the value and may cause overflow.

Explain the difference between implicit and explicit type casting in Java.

Think about when Java does the conversion for you and when you must tell it to convert.

Describe what happens when you cast a double value to an int in Java.

Focus on how the number changes and what you must write in code.

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