Oracle 1z0-830 : Java SE 21 Developer Professional

  • Exam Code: 1z0-830
  • Exam Name: Java SE 21 Developer Professional
  • Updated: Aug 12, 2026     Q & A: 85 Questions and Answers

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Oracle 1z0-830 Exam Syllabus Topics:

SectionWeightObjectives
Topic 1: Functional Programming and Streams15%- Optional class, primitive streams
- Lambda expressions, functional interfaces, method references
- Stream API: create, intermediate/terminal operations, parallel streams, grouping, partitioning
Topic 2: Handling Date, Time, Text, Numeric and Boolean Values12%- Use primitives and wrapper classes, evaluate expressions and apply type conversions
- Manipulate text, text blocks, String, StringBuilder and StringBuffer
- Use Date-Time API: LocalDate, LocalTime, LocalDateTime, Period, Duration, Instant, ZonedDateTime
Topic 3: Modules and Packaging5%- Module system: module-info.java, exports, requires, provides, uses
- Create and use JAR files, modular and non-modular builds
Topic 4: Controlling Program Flow10%- Decision constructs: if-else, switch expressions and statements, pattern matching
- Loops: for, enhanced for, while, do-while, break, continue, return
Topic 5: Using Object-Oriented Concepts20%- Enums, nested classes, local variable type inference
- Inheritance, abstract classes, sealed classes, interfaces, polymorphism
- Classes, records, objects, constructors, initializers, methods, fields, encapsulation
- Overloading, overriding, Object class methods, immutable objects
Topic 6: Concurrency and Multithreading10%- Synchronization, locks, concurrent collections, thread safety
- Thread lifecycle, Runnable, Callable, ExecutorService, virtual threads
Topic 7: Handling Exceptions8%- Exception hierarchy, try-catch-finally, multi-catch, try-with-resources
- Create and use custom exceptions, throw, throws
Topic 8: Advanced Features and Annotations3%- Generics, type parameters, wildcards, type erasure
- Annotations, built-in annotations, custom annotations
Topic 9: Working with Arrays and Collections12%- Collections Framework: List, Set, Map, Deque, Queue, sorting, searching
- Declare, instantiate, initialize, use arrays and multidimensional arrays
Topic 10: Java I/O and Localization5%- File I/O, NIO.2, streams, readers/writers, serialization
- Resource bundles, locale, formatting messages, numbers, dates

Oracle Java SE 21 Developer Professional Sample Questions:

1. Given:
java
Stream<String> strings = Stream.of("United", "States");
BinaryOperator<String> operator = (s1, s2) -> s1.concat(s2.toUpperCase()); String result = strings.reduce("-", operator); System.out.println(result); What is the output of this code fragment?

A) -UNITEDSTATES
B) UNITED-STATES
C) United-STATES
D) -UnitedStates
E) United-States
F) -UnitedSTATES
G) UnitedStates


2. Given:
java
try (FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos)) {
fos.write("Today");
fos.writeObject("Today");
oos.write("Today");
oos.writeObject("Today");
} catch (Exception ex) {
// handle exception
}
Which statement compiles?

A) fos.writeObject("Today");
B) fos.write("Today");
C) oos.writeObject("Today");
D) oos.write("Today");


3. Which three of the following are correct about the Java module system?

A) If a package is defined in both a named module and the unnamed module, then the package in the unnamed module is ignored.
B) Code in an explicitly named module can access types in the unnamed module.
C) We must add a module descriptor to make an application developed using a Java version prior to SE9 run on Java 11.
D) The unnamed module exports all of its packages.
E) If a request is made to load a type whose package is not defined in any known module, then the module system will attempt to load it from the classpath.
F) The unnamed module can only access packages defined in the unnamed module.


4. Given:
java
Object myVar = 0;
String print = switch (myVar) {
case int i -> "integer";
case long l -> "long";
case String s -> "string";
default -> "";
};
System.out.println(print);
What is printed?

A) string
B) It throws an exception at runtime.
C) nothing
D) integer
E) long
F) Compilation fails.


5. Given:
java
public class Test {
static int count;
synchronized Test() {
count++;
}
public static void main(String[] args) throws InterruptedException {
Runnable task = Test::new;
Thread t1 = new Thread(task);
Thread t2 = new Thread(task);
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(count);
}
}
What is the given program's output?

A) It's always 1
B) It's either 0 or 1
C) It's always 2
D) It's either 1 or 2
E) Compilation fails


Solutions:

Question # 1
Answer: F
Question # 2
Answer: C
Question # 3
Answer: A,D,E
Question # 4
Answer: F
Question # 5
Answer: E

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