How well do you know miscellaneous Java language semantics?

2024-06-24

Most languages have a lot of funny semantics. Java is no exception.

I also threw in a few trick questions just to make it a bit harder.

Your current results (e.g. 5 correct out of 15) are tracked at the bottom of the page.

It's only 25 questions. How hard can it be?

1.

Consider the following code segment.

public static String calculate() {
    try {
        throw new RuntimeException();
    } catch (RuntimeException e) {
        return "Cats are the best!";
    } finally {
        return "Dogs are the best!";
    }
}

What is the result of calling the calculate function?

The method generates a RuntimeException.
Compilation fails due to "unreachable statement".
"Cats are the best!"
"Dogs are the best!"
None of the above.

2.

Consider the following code segment.

List<Integer> numbers = new ArrayList<>();
numbers.add(1);

List<String> strings = (List<String>) (Object) numbers;
strings.add("Hello, world!");

strings.forEach(System.out::println);

This code does not function as intended. Which part of this code will cause an error or exception?

Creating a new ArrayList<>(), because you must provide the type between the angle brackets.
Casting a List<Integer> to a List<String>, because Integer is not equal to String.
Adding "Hello, world!" to the list, because the list is truly a list of integers.
Calling strings.forEach(System.out::println), because the same method reference cannot take arguments of multiple different types.
The code does not cause an error. 1 and "Hello, world!" are printed.

3.

Which of the following is not a keyword in the Java language?

and
const
strictfp
to
goto

4.

Consider the following code segment.

public class Main {

    public class Inner {

    }
}

What is the correct way to create an instance of the Inner class, that exists inside the top-level class Main, from an external, static function?

new Main.Inner()
new Main().new Inner()
new Main().Inner()
new Main.Inner() {}
new Inner()

5.

How does Java determine whether or not a given number n of type double is NaN?

n > Double.MAX_VALUE || n < Double.MIN_VALUE
n == 0 && n != 0
n == Double.NaN
n != n
The language relies on a native function to determine this.

6.

Consider the following code segment.

public class Output {
    public Output() {
        output(null);
    }

    public void output(Long t) {  
        System.out.println("Output channel one: " + t);  
    }  
        
    public void output(Integer t) {  
        System.out.println("Output channel two: " + t);  
    }
}

This code does not compile. What is a correct way to adjust the program so that it compiles?

Replace public void output(Integer t) with public void output(Long t)
Replace public void output(Long t) with public void output(Object t)
Replace public void output(Long t) with public void output(String t)
Replace either function with public void output(Void t)
Add a parameterized type <T> to Output and replace either function with public void output(T t)

7.

Which encoding standard is used by default for serializing the String type (e.g. to a DataOutputStream)?

ASCII
UTF-8
UTF-16
CESU8
None of the above

8.

Which of the following code segments are examples of syntactically valid expressions?

I. var x = List.of();
II. List x = List.<String>of();
III. let x = List<String>.of();

I only
III only
I and II
II and III
I and III

9.

Consider the following code segment.

public static int determineSum() {  
    int sum = 0;  
    int counter = 0;  
    int reversed = 10;  
    while (true) {  
        if (counter > 5) {  
            return sum;  
        }
        sum += counter;  
        counter++;  
        if (reversed > counter) {  
            reversed--;  
            sum += reversed;  
        }  
    }
    return -1;
}

What is the result of calling the determineSum() function?

26
50
75
-1
The provided code segment does not compile.

10.

What is the result of evaluating the following code segment?

Integer.valueOf(0) == Integer.valueOf(0)

false, because comparing two different objects by identity is always false
true, because the == operator is overridden for numerical values, like integers
false, because Integer.valueOf is an intrinsic candidate
true, because Integer.valueOf is specified to cache certain integers
The result is JVM-dependent, because a given JVM may or may not implement the function with a cache.

11.

Which of the following are valid calls of a function with the signature void count(int... numbers)?

I. count({5});
II. count(5);
III. count(new int{5});

I only
II only
III only
I and II
I and III

12.

Consider the following code segment.

public static String handle(Number a) {  
    if (a instanceof Integer) {  
        return a == null ? "Null integer!" : "Integer!";  
    } else if (a == Long.valueOf(5L)) {  
        return "Long equal to five!";  
    } else if (a == null) {  
        return "Null object!";  
    } else if (a instanceof Number) {  
        return a == null ? "Null number!" : "Number!";  
    } else {  
        return "Unknown object!";  
    }  
}

Which of the following results is not a possible return value from a call of the handle function?

"Null integer!"
"Integer!"
"Long equal to five!"
"Null object!"
"Number!"

13.

Consider the following code segment.

public static void main(String[] args) {  

    // placeholder line
    do {
        System.out.println("Hello, world!");
    } while (Math.random() > 0.5);  
}

The provided segment, in its current state, compiles successfully. Which of the following lines of code could replace // placeholder line without causing a compile error?

var Math = 2;
https://example.com
"Example";
if (true) {
/*\ *\/ **/ /* *\

14.

Consider the following method.

public int mystery(int number) {  
    if (number == 0) {  
        return 0;  
    } else if (number % 10 == 0) {  
        return 10 + mystery(number / 10);  
    } else if (number % 5 == 0) {  
        return 5 + mystery(number / 5);  
    } else {  
        return number % 10 + mystery(number / 10);  
    }  
}

What is the result of calling mystery(120500)?

0
8
32
34
The provided code segment does not compile.

15.

Consider the following code segment.

Which of the following lines of code could replace /* placeholder code */ without causing a compile error?

public void end() {
    /* placeholder code */
}

I. throw (RuntimeException) null;
II. throw (Throwable) null;
III. throw null;

II only
I and II
II and III
I and III
I, II, and III

16.

Consider the following code segment.

public class Wrapper<T> {  
    final T[] objects;  
    
    public Wrapper() {  
        /* placeholder code */  
    }  
    
}

Which of the following code segments correctly initializes objects without an exception or error?

objects = new T[0];
objects = {};
objects = (T[]) new Object[0];
objects = T[0];
objects = new Object<T>[0];

17.

An variable int x is declared, but left uninitialized. How could the variable be placed such that x could be read from without resulting in an error?

I. Statically (e.g. static int x;)
II. As an instance variable (e.g. int x;)
III. A variable in a function (e.g. int x; inside a function body)

I only
III only
I and II
II and III
I, II, and III

18.

What is printed when executing the following code segment?

StringBuilder builder = new StringBuilder("😠");
builder.append(8 ^ 7);
builder.append(0.0);

System.out.println(builder.toString().length());

19.

Which of the following expressions evaluates to true?

new String("a") == new String("a")
"a" == new String("a")
new String("a".intern()) == "a"
"a".intern() == new String("a")
"a" == new String("a".intern()).intern()

20.

What is the difference between errors and exceptions in Java?

Error types are defined by Java, while exceptions are created in user code
Error types are not used in Java, and exceptions are used in their place
Error types are used when compiling the program, while exceptions are used when running the program
Error types are intended for abnormal occurrences, while exceptions are intended for normal occurrences
Error types are for operating system errors, while exceptions are for program errors

21.

Consider the following code segment.

enum Animal {  
    CAT("Cat"),  
    DOG("Dog"),  
    HAMSTER("Hamster");  
    
    private String name;  
    public Animal(String name) {  
        this.name = name;  
    }  
    
    private String getName() {  
        return name;  
    }  
}

Why does the above code segment fail to compile?

Only constants can be fields in enums, but String is not a constant type.
The name field is not final.
The Animal constructor cannot be public.
Animal#getName is private, making it essentially meaningless.
The Animal type does not have an access modifier.

22.

Which of the following code segments successfully initializes a 2-dimensional array of int?

int[][] numbers = new int[][];
[][]int numbers = {};
[[int]] numbers = new [[int, 5], 5];
List<List<Integer>> numbers = new ArrayList<>();
int[] numbers[] = new int[5][5];

23.

Consider the following code segment.

List<Integer> numbers = new ArrayList<>() {{  
    add(5);  
}};

Which of the following is a correct statement made about the segment?

The segment does not compile, because anonymous class extension requires only one pair of curly brackets.
The segment does not compile, because the anonymous class that is created does not directly extend List.
The segment does not compile, because the add function cannot be resolved; it should be replaced with numbers.add(5).
The segment compiles, because this is an example of an instance initializer being placed directly inside of anonymous class extension.
The segment compiles, but numbers is initialized as an empty list.

24.

Consider the following code segments. Which segment(s) print Hello, world! when run, without an exception or compile error?

new Runnable() {  
    @Override
    public void run(Consumer<String> print) {
        print.accept("Hello, world!");
    }
}.run(string -> System.out.println(string));

II.

new Object() {
    void foo(String s) {
        System.out.println(s);
    }
}.foo("Hello, world!");

III.

((Consumer<String>) System.out::println).accept("Hello, world!");

I only
II only
I and II
II and III
I and III

25.

Consider the following code segment.

public class Main {  

    public static void main(String[] args) {  
        new Main();  
        new Main();  
    }  
    
    public Main() {  
        System.out.print("A");  
    }  
    
    static {  
        System.out.print("B");  
    }  
    
    {
        System.out.print("C");  
    }  
    
    static {  
        System.out.print("D");  
    }  
    
    {
        System.out.print("E");  
    }  
    
}

What is the printed when calling the main function?

"ABCDEABCDE"
"BDCEACEA"
"BCAACCEE"
"BCDEAA"
"BDACEACE"

Results

You got 0 correct out of 0.

(if there weren't 25 total, go back and answer them all!)

The average score was 11 from a sample size of 13. Do note that this includes a fair number of super nerds. Did you beat the average Java user?

Found my content on a site or in a video? Any questions or corrections? Please let me know on Discord at @GoldenStack

Note: I'm aware #1 is common to most languages with try & catch, #5 is specified by IEEE, and #13 is common to all languages with the two specific features required for the correct answer. I added them because they make good questions here :)

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