Free Java 17 OCP Practice Test 2

10 more challenging exam questions for 1Z0-829 — with detailed explanations

Questions

20 min

Suggested Time

~65%

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📋 How This Practice Test Works

Test 2 covers different topics than Test 1: generics, polymorphism, collections, method references, and tricky exam patterns. Click an answer for each question.

When done, click Submit & Show Answers to see your score and detailed explanations for every question.

These questions reflect actual OCP exam difficulty. Aim for 65%+ to be exam-ready.

Question 1

What is the result of the following code?

import java.util.*;

List<? extends Number> nums = new ArrayList<Integer>();
nums.add(42);              // line 1
Number n = nums.get(0);    // line 2
System.out.println(n);

APrints 42
BThrows ClassCastException at line 1
CCompile error at line 1
DCompile error at line 2

✅ Correct Answer: C

An upper-bounded wildcard List<? extends Number> is read-only for writes. You cannot add any element (except null) because the compiler can't guarantee type safety — the list could be List<Integer>, List<Long>, or any other Number subtype. Line 1 fails to compile. Line 2 is fine because we can safely read elements as the upper bound (Number).

Question 2

What is the output?

class Animal {
    String name = "Animal";
    String getName() { return name; }
}

class Dog extends Animal {
    String name = "Dog";
    String getName() { return name; }
}

Animal a = new Dog();
System.out.println(a.name + " " + a.getName());

ADog Dog
BAnimal Dog
CAnimal Animal
DDog Animal

✅ Correct Answer: B

Fields are not polymorphic — they're resolved by the reference type (Animal), so a.name returns "Animal". Methods are polymorphic — they're resolved by the actual object type (Dog), so a.getName() returns "Dog". This is a classic OCP trap.

Question 3

What is the output?

import java.util.*;
import java.util.stream.*;

List<String> names = List.of("Alice", "Bob", "Anna", "Charlie");

Map<Character, Long> grouped = names.stream()
    .collect(Collectors.groupingBy(
        s -> s.charAt(0),
        Collectors.counting()
    ));

System.out.println(grouped.get('A'));

A1
B3
C2
Dnull

✅ Correct Answer: C

Collectors.groupingBy with a downstream collector counting() groups by first character and counts elements in each group. The result is: {A=2, B=1, C=1}. Both "Alice" and "Anna" start with 'A', so grouped.get('A') returns 2L (boxed to Long for display).

Question 4

Which method reference is INVALID for the functional interface `Function<String, Integer>`?

A String::length
B Integer::parseInt
C String::hashCode
D System.out::println

✅ Correct Answer: D

Function<String, Integer> requires a method that takes a String and returns an Integer.

(A) String::length is VALID. It effectively translates to: (String s) -> s.length(). The return type is int, which is autoboxed to Integer.

(B) Integer::parseInt is VALID. It references the static method Integer.parseInt(String s), which takes a String and returns an int.

(C) String::hashCode is VALID. It effectively translates to: (String s) -> s.hashCode(), returning an int.

(D) System.out::println is INVALID because println returns void, not Integer. It is compatible with Consumer<String>, not Function<String, Integer>.

Question 5

What is the output?

String s = "Hello";
StringBuilder sb = new StringBuilder("Hello");

System.out.println(s == "Hello");
System.out.println(s.equals(sb.toString()));

Atrue
true
Bfalse
true
Ctrue
false
Dfalse
false

✅ Correct Answer: A

First line: "Hello" literal is interned in the String pool. The variable s also references that interned literal. So s == "Hello" is true (same reference).

Second line: sb.toString() returns "Hello" (content equal to s). String.equals() compares content, so the result is true.

Question 6

What is the output?

int x = 5;
String result = switch (x) {
    case 1, 2, 3 -> "low";
    case 4, 5, 6 -> {
        String s = "medium";
        yield s.toUpperCase();
    }
    default -> "high";
};
System.out.println(result);

Amedium
BMEDIUM
Clow
DCompile error: yield not allowed

✅ Correct Answer: B

The switch expression with arrow syntax -> uses yield when the case branch is a block. The case 4, 5, 6 matches x=5, executes the block, and yields "medium".toUpperCase() which is "MEDIUM".

Question 7

What is the result?

class Counter {
    private static int count = 0;
    private int instanceCount = 0;

    public Counter() {
        count++;
        instanceCount++;
    }
}

Counter c1 = new Counter();
Counter c2 = new Counter();
Counter c3 = new Counter();

// What are the values of count and instanceCount (from c3) at this point?

Acount = 3, instanceCount (of c3) = 1
Bcount = 1, instanceCount (of c3) = 3
Ccount = 3, instanceCount (of c3) = 3
Dcount = 1, instanceCount (of c3) = 1

✅ Correct Answer: A

count is static — shared across all instances. Three constructor calls increment it three times, so count = 3.

instanceCount is an instance variable — each Counter has its own. For c3 (just created), instanceCount was initialized to 0, then the constructor incremented it to 1.

Question 8

What is the output?

try {
    try {
        throw new RuntimeException("first");
    } catch (RuntimeException e) {
        throw new IllegalStateException("second", e);
    }
} catch (Exception e) {
    System.out.println(e.getMessage());
    System.out.println(e.getCause().getMessage());
}

Afirst
second
Bfirst
null
Csecond
first
DCompile error

✅ Correct Answer: C

The inner try throws RuntimeException("first"). The inner catch wraps it: new IllegalStateException("second", e) — "second" becomes the message, e becomes the cause. The outer catch receives this IllegalStateException. e.getMessage() returns "second", and e.getCause().getMessage() returns "first" (the original wrapped exception's message).

Question 9

Which statement about sealed classes is FALSE?

AA sealed class must explicitly list all permitted subclasses with the permits clause
BSubclasses of a sealed class must be declared final, sealed, or non-sealed
CSubclasses of a sealed class can be in any module
DSealed classes enable exhaustive switch expressions

✅ Correct Answer: C

This is FALSE. Subclasses of a sealed class must be in the same module as the sealed class (or in the same package if the sealed class is in the unnamed module). They cannot be in any arbitrary module.

The other statements are TRUE: (A) Permits clause is required (except for nested classes in the same file). (B) Subclasses must specify their inheritance behavior. (D) Sealed type hierarchies allow the compiler to verify exhaustive pattern matching.

Question 10

What is the output?

import java.util.*;

Map<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);

map.merge("a", 10, Integer::sum);
map.merge("d", 100, Integer::sum);

System.out.println(map.get("a") + " " + map.get("d"));

A11 100
B10 100
C11 null
D1 100

✅ Correct Answer: A

map.merge(key, value, remappingFunction): if the key is absent or maps to null, associates it with value. Otherwise replaces with remappingFunction.apply(oldValue, value).

For "a" (existing, value=1): merge applies Integer.sum(1, 10) = 11.
For "d" (absent): merge stores 100 directly without calling the function.

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📋 How This Practice Test Works

What is the result of the following code?

✅ Correct Answer: C

What is the output?

✅ Correct Answer: B

What is the output?

✅ Correct Answer: C

Which method reference is INVALID for the functional interface Function<String, Integer>?

✅ Correct Answer: D

What is the output?

✅ Correct Answer: A

What is the output?

✅ Correct Answer: B

What is the result?

✅ Correct Answer: A

What is the output?

✅ Correct Answer: C

Which statement about sealed classes is FALSE?

✅ Correct Answer: C

What is the output?

✅ Correct Answer: A

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Which method reference is INVALID for the functional interface `Function<String, Integer>`?