Data Types

Q 1

What is type inference in Java numeric expressions?

Hard

Answer

Java determines expression types using compile-time rules such as numeric promotion, literal types, and contextual typing.

Explanation

Understanding these rules is important for overloaded methods, arithmetic expressions, assignments, and generic APIs.

Reference: Java Language Specification

Q 2

Can BigInteger and BigDecimal overflow like int and long?

Medium

Answer

They are arbitrary-precision numeric classes and do not overflow at fixed primitive widths, although operations can consume substantial memory and CPU as values grow.

Explanation

Applications should still consider resource usage when working with extremely large values.

Reference: Java BigInteger and BigDecimal APIs

Q 3

What is the difference between BigInteger and BigDecimal?

Medium

Answer

BigInteger provides arbitrary-precision integer arithmetic, while BigDecimal provides arbitrary-precision decimal arithmetic with scale and rounding support.

Explanation

BigInteger is appropriate for integers beyond long's range, while BigDecimal is useful when exact decimal calculations are required.

Reference: Java BigInteger and BigDecimal APIs

Q 4

Why can floating-point calculations produce unexpected decimal results?

Medium

Answer

Many decimal fractions cannot be represented exactly in binary floating-point format, so calculations can contain small representation errors.

Explanation

For exact decimal arithmetic, use BigDecimal with appropriate construction and rounding rules.

Reference: Java BigDecimal API

Q 5

What is the difference between float/double precision and range?

Medium

Answer

Precision describes how many significant digits can be represented accurately, while range describes the magnitude of values that can be represented.

Explanation

A floating-point type can represent a very large range while still having limited precision for many decimal values.

Reference: IEEE 754 / Java Language Specification

Q 6

What is binary numeric promotion?

Hard

Answer

Binary numeric promotion converts operands of certain arithmetic and comparison operations to a common numeric type before the operation is performed.

Explanation

byte, short, and char are generally promoted to int, while wider operands can cause promotion to long, float, or double.

Reference: Java Language Specification

Q 7

Can two different primitive types be compared using ==?

Medium

Answer

Compatible numeric primitive types can be compared after binary numeric promotion.

Explanation

For example, an int and long can be compared after the int value is promoted to long.

Reference: Java Language Specification

Q 8

What is the difference between primitive equality and reference equality?

Medium

Answer

For primitive operands, == compares their values. For reference operands, == compares whether the references identify the same object or array.

Explanation

Logical equality of objects should normally be implemented and checked with equals().

Reference: Java Language Specification

Q 9

What happens when parsing an invalid integer String?

Easy

Answer

Methods such as Integer.parseInt() throw NumberFormatException when the supplied String is not a valid integer representation.

Explanation

Applications should validate or handle invalid input when parsing external data.

Reference: Java Integer API

Q 10

Can String be converted directly to a primitive type?

Easy

Answer

String values can be parsed into primitive types using methods such as Integer.parseInt(), Long.parseLong(), and Boolean.parseBoolean().

Explanation

Parsing converts textual representations into typed values and may throw NumberFormatException for invalid numeric input.

Reference: Java Wrapper Classes API

Q 11

Can boolean be converted to int using casting?

Easy

Answer

No. Java does not define a primitive cast between boolean and numeric types.

Explanation

If an application needs a numeric representation, it must explicitly map true and false to chosen numeric values.

Reference: Java Language Specification

Q 12

When should you use long instead of int?

Easy

Answer

Use long when the required numeric range may exceed int's limits or when the domain naturally represents 64-bit values.

Explanation

Examples include large counters, database identifiers in some systems, file sizes, and epoch-based numeric values.

Reference: Java Language Specification

Q 13

What is the difference between int and Integer?

Easy

Answer

int is a primitive 32-bit signed integer, while Integer is an immutable wrapper object representing an int value.

Explanation

Integer can be null and can be used in generic collections, while int cannot be null and cannot be used directly as a generic type argument.

Reference: Java Integer API

Q 14

What is the difference between char and Character?

Easy

Answer

char is a primitive type, while Character is its wrapper class.

Explanation

Character provides object behavior and utility methods for Unicode and character classification while char provides the underlying UTF-16 code unit value.

Reference: Java Character API

Q 15

How can an int be treated as an unsigned value in Java?

Medium

Answer

Java provides methods such as Integer.toUnsignedLong(), compareUnsigned(), divideUnsigned(), and remainderUnsigned() to interpret int bit patterns as unsigned values.

Explanation

The underlying int remains a 32-bit value, but these APIs provide unsigned arithmetic and comparison semantics.

Reference: Java Integer API

Q 16

What is signed and unsigned behavior in Java primitive types?

Medium

Answer

Java's byte, short, int, and long types are signed, while char is an unsigned 16-bit type.

Explanation

Java also provides unsigned arithmetic utility methods for some integral types without changing the underlying signed primitive representation.

Reference: Java Language Specification

Q 17

Can underscores be placed anywhere in a numeric literal?

Medium

Answer

No. Java restricts underscore placement; for example, underscores cannot appear directly adjacent to the decimal point, type suffix, or at the beginning or end of a literal.

Explanation

These restrictions prevent ambiguous or confusing literal syntax.

Reference: Java Language Specification

Q 18

What are underscores in numeric literals used for?

Easy

Answer

Underscores can improve readability of numeric literals by separating groups of digits, such as 1_000_000.

Explanation

They do not change the numeric value and are removed as part of literal processing.

Reference: Java Language Specification

Q 19

What is a hexadecimal, binary, and octal integer literal?

Easy

Answer

Java supports decimal literals and non-decimal integer literals using prefixes 0x or 0X for hexadecimal, 0b or 0B for binary, and a leading 0 for octal.

Explanation

These forms are useful when working with bit patterns, masks, and low-level representations.

Reference: Java Language Specification

Q 20

What is the difference between 1, 1L, 1.0, and 1.0F?

Easy

Answer

1 is an int literal, 1L is a long literal, 1.0 is a double literal, and 1.0F is a float literal.

Explanation

Literal types influence overload resolution, assignment compatibility, and arithmetic promotion.

Reference: Java Language Specification

Q 21

Why does 10.5 have type double by default?

Easy

Answer

A floating-point literal without an F or f suffix is of type double.

Explanation

Use the F suffix when a float literal is specifically required, such as 10.5F.

Reference: Java Language Specification

Q 22

What is a numeric literal suffix in Java?

Easy

Answer

Numeric literal suffixes such as L, F, and D can explicitly specify long, float, and double literal types.

Explanation

For example, 10L is a long literal and 10.5F is a float literal.

Reference: Java Language Specification

Q 23

What is the difference between primitive literals and wrapper objects?

Medium

Answer

A primitive literal such as 10 represents a primitive value, while a wrapper object such as Integer.valueOf(10) represents that value as an object.

Explanation

The compiler can perform boxing when a primitive is used where an object is required.

Reference: Java Language Specification

Q 24

What is the difference between Integer.parseInt() and Integer.valueOf()?

Medium

Answer

parseInt() returns a primitive int, while valueOf() returns an Integer object.

Explanation

valueOf() may reuse cached Integer instances, while parseInt() is intended to produce the primitive numeric value.

Reference: Java Integer API

Q 25

What happens when a null wrapper is unboxed?

Medium

Answer

Unboxing a null wrapper object throws NullPointerException.

Explanation

This can occur implicitly in expressions such as assigning an Integer reference to an int variable or performing arithmetic with a null Integer.

Reference: Java Language Specification

Q 26

What is the difference between null and zero?

Easy

Answer

null represents the absence of an object reference, while zero is a numeric primitive value.

Explanation

They have different meanings and cannot be used interchangeably in Java.

Reference: Java Language Specification

Q 27

Can primitive data types be null?

Easy

Answer

No. Primitive types cannot contain null.

Explanation

If nullable numeric or boolean values are required, the corresponding wrapper classes such as Integer or Boolean can be used.

Reference: Java Language Specification

Q 28

What is null in Java?

Easy

Answer

null is a special reference value that indicates a reference does not currently refer to an object or array.

Explanation

Primitive variables cannot contain null, but reference variables can unless restricted by application design or type annotations.

Reference: Java Language Specification

Q 29

Why should == not be used to compare Integer values?

Medium

Answer

== compares references when both operands are Integer objects, so it may compare object identity rather than numeric value.

Explanation

Integer caching can make some comparisons appear to work while others fail, so equals() should normally be used for value comparison.

Reference: Java Integer API

Q 30

What is Integer caching in Java?

Medium

Answer

Java implementations commonly cache Integer instances for a range of small integer values, and the language specification guarantees identity for certain constant boxing cases.

Explanation

Because wrapper identity is not value equality, application code should generally use equals() rather than == for comparing Integer values.

Reference: Java Language Specification