Java Programming

113 flashcards by MentisLibrary

Java as working developers use it: primitives and objects, classes interfaces and records, generics and type erasure, the collections framework, streams and lambdas, threads and executors, exceptions, and the JVM and build tooling.

What is the signature of a Java entry point method?

public static void main(String[] args). It must be public and static so the JVM can call it without an instance, return void, and take an array of strings.

Name Java's eight primitive types.

byte, short, int, long, float, double, char and boolean. They hold values directly and are not objects.

What is the difference between a primitive variable and a reference variable?

A primitive variable holds the value itself. A reference variable holds a reference to an object on the heap, so assignment copies the reference and both names see the same object.

What is fixed about a Java array once created?

Its length. arr.length is a final field, not a method, and there is no way to grow the array; you allocate a new one and copy.

When can you use the enhanced for loop?

Over any array or any type implementing Iterable. It gives you each element in order with no index.

Why does a switch statement need break?

Because cases fall through by design: once a label matches, execution continues into the following cases until a break or the end of the block.

What does the arrow form of switch change?

Each label runs only its own branch with no fall through, and the switch can be used as an EXPRESSION that produces a value.

What does / do with two integers?

Integer division: the fractional part is discarded, truncating toward zero. % gives the remainder, which keeps the sign of the dividend.

What does var do in a local declaration?

It infers the variable's type from the initializer at compile time. The variable is still statically typed and its type never changes.

What does final mean on a variable, a method and a class?

On a variable it can be assigned only once. On a method it cannot be overridden. On a class it cannot be extended.

What does static mean on a member?

It belongs to the class rather than to any instance: one copy shared by all objects, callable as ClassName.member with no instance.

What does the pattern form of instanceof add?

It tests the type and binds a typed variable in the same expression, so the explicit cast disappears.

How do you declare a method taking any number of arguments?

With varargs: Type... name as the LAST parameter. Inside the method it is an ordinary array.

How do && and & differ on booleans?

&& short-circuits: the right operand is not evaluated when the left already decides the result. & always evaluates both.

What are the wrapper classes, and what is autoboxing?

Integer, Long, Double, Boolean and the rest wrap a primitive as an object. Autoboxing is the compiler inserting the conversion for you in both directions.

Why can == on two Integer objects be true for small numbers and false for large ones?

Integer.valueOf caches boxed values from -128 to 127, so small values are the same object. Larger values allocate a new object each time.

Why is String immutable in Java?

Its character data never changes after construction, so strings can be shared safely between threads, cached in the string pool, and used as reliable hash map keys.

Why does == sometimes work on strings and sometimes not?

== compares references. Compile-time literals are interned into the string pool, so identical literals are the same object; strings built at run time are not.

When should you use StringBuilder?

When building a string in a loop or from an unknown number of parts. It appends into a growable buffer and produces the string once.

What does String.format give you over concatenation?

A template with typed conversions, so numbers, padding and locale-aware formatting are expressed once instead of assembled by hand.

What happens when an int overflows in Java?

It wraps silently using two's complement arithmetic. There is no exception unless you use the Math.addExact family, which throws ArithmeticException.

Why must money never be stored in a double?

double is binary floating point, so decimal fractions such as 0.1 have no exact representation and errors accumulate over repeated arithmetic.

What does a char actually hold?

A single 16-bit UTF-16 code unit, not a full character. Characters outside the basic multilingual plane occupy two char values.

What does an enum give you over integer constants?

A fixed set of typed instances, so only declared values are possible. Enums can hold fields, define methods, and are usable in switch and EnumMap.

What does declaring a record generate?

A final class with private final fields, a canonical constructor, accessors named after the components, and value-based equals, hashCode and toString.

What is Optional for, and what is it not for?

Expressing a possibly-absent RETURN value so the caller must handle the empty case. It is not intended for fields or parameters.

When does Java convert numeric types automatically?

Widening conversions are implicit and lossless: int to long to float to double. Narrowing conversions need an explicit cast and can silently lose data.

What does a constructor do that a method does not?

It runs when the object is created, has the class's name, declares no return type, and cannot be called again on an existing object.

How does one constructor call another in the same class?

With this(...) as the very first statement. super(...) calls a superclass constructor and must also be first, so a constructor uses one or the other.

What are Java's four access levels?

public everywhere, protected in the package and in subclasses, package-private with no keyword within the package only, and private within the class.