Data Types

Q 31

Why are wrapper classes immutable?

Medium

Answer

Wrapper objects such as Integer, Long, and Boolean are immutable, meaning their represented value cannot change after construction.

Explanation

Immutability makes wrapper objects safe to share and simplifies their behavior in collections and concurrent code.

Reference: Java Wrapper Classes API

Q 32

What are wrapper classes in Java?

Easy

Answer

Wrapper classes represent primitive values as objects, such as Integer for int, Long for long, and Boolean for boolean.

Explanation

They are required in APIs that work with objects, including generic collections, and also provide useful conversion and utility methods.

Reference: Java Wrapper Classes API

Q 33

What is unboxing?

Easy

Answer

Unboxing is the automatic conversion of a wrapper object to its corresponding primitive type.

Explanation

If the wrapper reference is null, unboxing can throw NullPointerException.

Reference: Java Language Specification

Q 34

What is autoboxing?

Easy

Answer

Autoboxing is the automatic conversion of a primitive value into its corresponding wrapper object, such as int to Integer.

Explanation

It allows primitive values to be used in contexts requiring reference types, such as generic collections.

Reference: Java Language Specification

Q 35

What is the difference between implicit and explicit casting?

Easy

Answer

Implicit casting occurs automatically for permitted conversions such as int to long, while explicit casting uses a cast expression such as (int) longValue.

Explanation

Explicit casts are required when the conversion may lose information or when reference types need a checked narrowing conversion.

Reference: Java Language Specification

Q 36

Can a byte variable store the result of byte + byte without casting?

Medium

Answer

No, because the arithmetic expression is evaluated as int. An explicit cast to byte is normally required.

Explanation

The cast should be used carefully because the resulting int may be outside the byte range.

Reference: Java Language Specification

Q 37

Why does byte + byte produce an int in Java?

Medium

Answer

Binary numeric promotion converts byte operands to int before performing the arithmetic operation.

Explanation

Therefore, assigning the result directly to a byte requires an explicit cast if the result is known to fit.

Reference: Java Language Specification

Q 38

What is type promotion in arithmetic expressions?

Medium

Answer

Java promotes smaller integral types such as byte, short, and char to int during most arithmetic operations.

Explanation

This means an expression involving two byte values generally has type int rather than byte.

Reference: Java Language Specification

Q 39

Can char store an integer value in Java?

Medium

Answer

char can store an integer value within its 16-bit unsigned range, and Java permits conversions between char and integral types with the appropriate conversion rules.

Explanation

The numeric value represents a UTF-16 code unit when interpreted as a character.

Reference: Java Language Specification

Q 40

What is the difference between char and String?

Easy

Answer

char represents one UTF-16 code unit, while String represents an immutable sequence of characters.

Explanation

A String can contain many characters and can also represent Unicode code points that require multiple UTF-16 code units.

Reference: Java Character and String APIs

Q 41

Why is BigDecimal preferred over double for financial calculations?

Medium

Answer

BigDecimal provides decimal arithmetic with explicit precision and rounding control, while binary floating-point types such as double can represent many decimal fractions only approximately.

Explanation

Financial calculations often require predictable decimal results and controlled rounding, making BigDecimal more appropriate.

Reference: Java BigDecimal API

Q 42

What is the difference between float and double?

Easy

Answer

float uses 32-bit floating-point representation, while double uses 64-bit representation and provides greater precision.

Explanation

double is normally preferred for general floating-point calculations unless memory or domain constraints favor float.

Reference: Java Language Specification

Q 43

What is NaN in Java?

Medium

Answer

NaN means Not-a-Number and represents an undefined or unrepresentable floating-point result.

Explanation

NaN has special comparison behavior: it is not equal to itself using ==, so Double.isNaN() should be used to test for NaN.

Reference: Java Double API

Q 44

What happens when a floating-point value is divided by zero?

Medium

Answer

Floating-point division by positive or negative zero can produce positive or negative infinity, while zero divided by zero produces NaN.

Explanation

These results follow IEEE 754 floating-point semantics rather than throwing ArithmeticException.

Reference: Java Language Specification

Q 45

What happens when an integer is divided by zero?

Easy

Answer

Integer division by zero throws ArithmeticException.

Explanation

This differs from floating-point division, where division by zero can produce infinity or NaN according to IEEE 754 rules.

Reference: Java Arithmetic API

Q 46

What is integer division in Java?

Easy

Answer

When both operands are integral types, division performs integer division and discards the fractional part toward zero.

Explanation

For example, 5 / 2 produces 2, while 5.0 / 2 produces 2.5.

Reference: Java Language Specification

Q 47

What happens when floating-point arithmetic overflows?

Medium

Answer

Floating-point overflow can produce positive or negative infinity rather than wrapping like signed integer arithmetic.

Explanation

IEEE 754 semantics also define special values such as NaN and signed zero.

Reference: Java Language Specification

Q 48

What happens when an integer value overflows its primitive type?

Medium

Answer

For signed integer arithmetic, overflow wraps around according to the fixed-width two's-complement representation rather than automatically throwing an exception.

Explanation

For example, incrementing Integer.MAX_VALUE produces Integer.MIN_VALUE in ordinary int arithmetic.

Reference: Java Language Specification

Q 49

Why does assigning a long to int require casting?

Easy

Answer

A long has a wider range than int, so converting it to int may lose information.

Explanation

Java requires an explicit cast to make this potentially lossy conversion visible in the code.

Reference: Java Language Specification

Q 50

Why can int be assigned to long without casting?

Easy

Answer

An int can be assigned to a long through an automatic widening conversion because every int value can be represented by long.

Explanation

The reverse conversion is narrowing and requires an explicit cast because a long may contain values outside the int range.

Reference: Java Language Specification

Q 51

What is narrowing primitive conversion?

Medium

Answer

Narrowing conversion converts a value to a type with a smaller range or precision, such as long to int.

Explanation

Narrowing conversions generally require an explicit cast and may lose information or change the resulting value.

Reference: Java Language Specification

Q 52

What is widening primitive conversion?

Medium

Answer

Widening conversion converts a value to a type that can represent its range without normally losing magnitude, such as int to long.

Explanation

Widening conversions are generally performed automatically by Java when the conversion is permitted.

Reference: Java Language Specification

Q 53

What is type casting in Java?

Easy

Answer

Type casting is the conversion of a value from one type to another compatible type, either explicitly or implicitly.

Explanation

Casting is common when converting numeric types or working with reference types in an inheritance hierarchy.

Reference: Java Language Specification

Q 54

Do local variables have default values in Java?

Easy

Answer

No. Local variables do not receive automatic default values and must be definitely assigned before they are read.

Explanation

This compile-time rule prevents many accidental uses of uninitialized local variables.

Reference: Java Language Specification

Q 55

What is the default value of each primitive instance variable in Java?

Easy

Answer

Numeric primitives default to zero, char defaults to the null character, and boolean defaults to false.

Explanation

These defaults apply to fields and array elements. Local variables must be explicitly initialized before use.

Reference: Java Language Specification

Q 56

Why is boolean size not specified as exactly 1 bit in Java?

Medium

Answer

The Java language specification defines the boolean type by its logical values rather than requiring a specific storage size for variables or fields.

Explanation

The JVM specification also does not require a particular memory representation for boolean values, allowing implementations flexibility.

Reference: Java Virtual Machine Specification

Q 57

What values can boolean hold in Java?

Easy

Answer

A boolean can have the value true or false.

Explanation

Unlike some languages, Java does not allow integers such as 0 or 1 to be used directly as boolean values.

Reference: Java Language Specification

Q 58

What is the char data type in Java?

Easy

Answer

char is a 16-bit unsigned type representing a UTF-16 code unit.

Explanation

A char represents one UTF-16 code unit, not necessarily one complete Unicode character because some Unicode code points require surrogate pairs.

Reference: Java Language Specification

Q 59

What is the size of double in Java?

Easy

Answer

double is a 64-bit IEEE 754 floating-point type.

Explanation

double is the default floating-point type in Java and provides greater precision than float.

Reference: Java Language Specification

Q 60

What is the size of float in Java?

Easy

Answer

float is a 32-bit IEEE 754 floating-point type.

Explanation

float provides less precision than double and should generally be used only when its memory characteristics or domain requirements justify it.

Reference: Java Language Specification