1Z0-819: Java SE 11 DeveloperDemo

A class declared final cannot be subclassed. Declaring class Y extends X is therefore a compile-time error.

Synchronizing on the shared test object makes each count++ operation mutually exclusive, protecting its read-modify-write sequence. Alternatively, AtomicInteger.incrementAndGet() performs the increment atomically. A volatile field provides visibility but does not make the compound ++ operation atomic.

A method reference can replace a lambda or anonymous functional-interface implementation when it only delegates to an existing named method with compatible parameters. testName::compareToIgnoreCase supplies the mapped member as the argument to the testName instance method, and Member::print invokes print on each streamed Member.

peek performs its action as stream elements are consumed, so neither peek executes until collect begins. The two println("-----") statements execute beforehand. The peek before the stateful sorted() operation prints the original list order while the sort consumes the input; the peek after it prints the natural alphabetical order. strFruits is assigned but never printed. Java Stream API

An interface that inherits methods with override-equivalent signatures must have at least one method whose return type is return-type-substitutable for every other inherited method. D inherits a() from B returning Collection and from C returning Path; those return types are incompatible, so the inherited methods cause a compile-time error.

JDBC 4.0 drivers available on the class path are automatically discovered and loaded through the Java service-provider mechanism when DriverManager initializes. Explicitly calling Class.forName to load a JDBC driver is therefore no longer required for standard modern JDBC drivers.

The entries are sorted alphabetically by their currency-code keys and collected into a LinkedHashMap, which preserves insertion order. The reciprocal exchange-rate values, formatted to two decimal places, are CNY 6.42, EUR 0.84, GBP 0.75, and USD 1.00.

Java rejects assigning a superclass-typed reference (SomeClass) to a subclass-typed reference (AnotherClass) without an explicit cast, because the reference could otherwise refer to a different SomeClass subclass. Therefore, compilation fails at ac = sc;.

A null Person argument causes checkPerson to create a Person named Mary. A non-null argument is returned unchanged. The first call therefore assigns p to Mary, and the second call assigns p1 to that same Mary object. toString() returns the person's name, so both print operations display Mary.

A named Java module is declared by a module declaration in module-info.java. The java.base module defines the Java platform’s foundational APIs and exports its core platform API packages. A module declaration can have no directives, while access between named modules requires the relevant readability and export relationships rather than mere co-location. Java Language Specification, §7.7 Module java.base

Test.class directly depends on java.base for core Java types and on java.sql because it uses java.sql.Timestamp. The jdeps --summary option prints a dependency summary for the analyzed class, producing Test.class -> java.base and Test.class -> java.sql; it does not recursively report java.sql's own dependencies in this invocation.

Java method overloading requires methods with the same name to differ in their parameter lists; a return type alone cannot distinguish them. public float getVal() and public long getVal() therefore have the same method signature and cause a compile-time error.

A local variable referenced from a lambda expression must be final or effectively final. Because i is modified by i++, it is not effectively final, so the code does not compile.

value is a required String[] annotation element. It can be supplied as an array initializer containing strings, or—because the element is named value—by using the single-element shorthand form. A nested array is not valid for String[], and omitting the required element is invalid.

Adding a byte and a short promotes both operands to int, producing the int value 25. That value cannot be implicitly narrowed to byte or short for overload selection. It can be boxed as an Integer and passed to the process(Object) overload, which prints Object value 25.

The accumulated value has type double, while N is an arbitrary subtype of Number; a double cannot be converted to every possible N. Declaring sum to return Double makes return sum; valid because Java boxes the double result into a Double.

The java.desktop module defines the AWT and Swing user-interface toolkits. An application that uses either toolkit therefore requires java.desktop.

The java --show-module-resolution option reports the modules resolved at startup, revealing runtime module dependencies. The jdeps --list-deps option analyzes class files and lists their module dependencies; jdeps is the JDK dependency analyzer.

An overriding instance method must have the same name, parameter types, and return type as the inherited method. Automobile declares wheels() with no parameters; changing Car.wheels(int i) to Car.wheels() implements that abstract method. Dynamic dispatch through the Automobile reference then invokes the implementation that prints 4.

L is the required checked exception because p() declares throws L. A multi-catch cannot include related exception types such as L and its subclass M. N is unrelated to L and is unchecked, so either catch (L e) or catch (L | N e) is legal and handles the checked exception requirement.

A local variable declared in a try block is scoped only to that block and is not visible in its associated catch block. Therefore, the reference to x in line 2 cannot be resolved and compilation fails. Integer.parseInt(String, int) parses an integer using the supplied radix, but that behavior is not reached because of the scope error.

A try-with-resources statement implicitly closes its resource, and closing a FileInputStream can throw IOException. That checked exception must be caught or declared. Replacing the catch with catch (Exception e) catches both FileNotFoundException and IOException; alternatively, declaring throws IOException on main() handles the exception that may arise during implicit resource closing.

B.print() validly overrides the package-access instance method in A because an overriding method may provide broader access. Although the reference is cast to A, it refers to a B instance, so Java’s runtime method lookup invokes the overriding B.print() method and prints Derived class.

A covariant return type permits an implementing method to return a subtype of the interface method’s declared return type, so B is a subtype of A. Because new B(str) is used, B must be concrete rather than abstract. Since B is a subtype of A, A cannot be final.