Nine Core Java Questions

My previous post with Eight Core Java Questions was popular so I created nine more.

Please join the discussion of the answers for these questions on Twitter

1.

Comment on this question
How many elements does this set have?

Set set = new HashSet(Arrays.asList(
        -0, +0, -0L, +0L, -0F, 0F, -0D, 0D, 0xF, 0xD, 0x0.DP+0));
System.out.println(set.size());

1. 12
2. 1
3. 6
4. 9

2.

Comment on this question
This compiles in Java 17 due to

static int[] a(int[]... a)[] { return a; }
static final int one = a(new int[0]).length; // 1

1. It confuses the compiler
2. It’s a static method
3. Backward Compatibility
4. JEP 747

3.

Comment on this question
The code below prints "23 & 52" the following due to

System.out.println(
        new BigDecimal(2.008f).toString().length() + " & " +
                new BigDecimal(2.008).toString().length());

1. A bug in Java 23
2. The randomness of floating-point
3. Length of the mantissa
4. Magic numbers

4.

Comment on this question
This code prints "hello world" due to

public static void main(String[] args) {
    System.out.println(r(604568751) + " " + r(401174283));
}

public static String r(int num) {
    return Integer.toUnsignedString(new Random(num).nextInt(), 33);
}

1. Secret code in Java
2. Using Magic numbers
3. A bug in Java 13
4. (char) 33 is '!'

5.

Comment on this question
This is class is

public class StackTrace extends Throwable { }

1. An unchecked exception
2. A checked exception
3. Neither
4. A compilation error

6.

Comment on this question

This code prints the following in Java 17

   public static void main(String[] args) {
        test(new IdentityHashMap<>());
        test(new WeakHashMap<>());
        test(new ConcurrentSkipListMap<>());
    }

    static void test(Map<String, Integer> map) {
        map.computeIfAbsent("key", (String k) -> {
            map.putIfAbsent(k, 2);
            return 1;
        });
        System.out.println(map.getClass().getSimpleName() + " " + map);
    }

IdentityHashMap {key=1}
WeakHashMap {key=1}
ConcurrentSkipListMap {key=2}

1. It depends on whether the map is concurrent or not
2. It’s a memory leak
3. It's a thread safety issue
4. The behaviour is undefined.

7.

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In Java 17, the stack trace StackTraceElement[] of a Throwable is created when

1. It is thrown
2. It is used for the first time
3. Never really exists
4. The Throwable is created

8.

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If -Dfalse=true is set on the command line, which of these is true

1. (Boolean) false
2. Boolean.parseBoolean("false")
3. Boolean.getBoolean("false")
4. "true" == "false"

9.

Comment on this question
This expression is true for powers of 2 and one other value.

n & ( n - 1 ) == 0

1. Integer.MAX_VALUE
2. Character.MIN_VALUE
3. Double.MIN_VALUE
4. Float.NaN

Hint

In the questions above, there are a total of three answers for each for B, C,and D