Here are some odd quiz questions in Java which might have interesting answers.

Questions

void arrays

Which of the following doesn’t compile

void[] voids = new void[1]; (1)
Void[] voids = new Void[1]; (2)
Void[] voids = { }; (3)
Object voids = Array.newInstance(void.class, 1); (4)

See the answer at the end.

null reference

What does this do?

Thread t = null;
t.yield();
  1. A compiler error

  2. A runtime error

  3. The JVM crashes

  4. The JVM pauses briefly

What does this do?

Creating an array of generic

Which of the following creates an array of a generic type T which extends Object.

T[] ts = new T[1]; (1)
T[] ts = { new T(); }; (2)
T[] ts = (T[]) new Object[1]; (3)
T[] ts = Array.newArray(T.class, 1); (4)

Multiplying characters

The following code compiles but what does it print?

char ch = '1';
ch /= 0.9;
System.out.println(ch);

prints

  1. 1

  2. 1.111111111111111

  3. 1.9

  4. 6

Decimals

Which of the following prints 0.3

int x = 3;
System.out.println(x / 10); (1)
System.out.println(x * 0.1); (2)
System.out.println(x / 10.0); (3)
System.out.println(0 + '.' + x); (4)
System.out.println(0 + "." + x); (5)

try with null

What is NOT true about this code?

    public static void main(String... args) {
        try (PrintWriter pw = null) { }
    }
  1. It compiles but it doesn’t if you replace PrintWriter with Writer.

  2. It produces no error at compile time or runtime.

  3. The code won’t compile if { } is replaced with ;.

  4. It doesn’t compile.

Answers

void arrays

void[] voids = new void[1]; (1)
Void[] voids = new Void[1]; (2)
Void[] voids = { }; (3)
Object voids = Array.newInstance(void.class, 1); (4)
1 a void[] isn’t allowed.
2 An array of any class is allowed even the Void class
3 Also creates an empty Void[]
4 This triggers a runtime error, but not a compile error. The compiler has very little knowledge of how libraries work and doesn’t pick this up as an issue.

null reference

This code

Thread t = null;
t.yield();

is the same as

Thread t = null;
Thread.yield(); // the method is static

So the answer is 4 as it just pauses briefly.

Creating an array of generic

Which of the following creates an array of a generic type T which extends Object.

T[] ts = new T[1]; (1)
T[] ts = { new T(); }; (2)
T[] ts = (T[]) new Object[1]; (3)
T[] ts = Array.newArray(T.class, 1); (4)

Only option (3) compiles. This makes it clear that the raw array is an Object array not the runtime type of T[]

Multiplying characters

The following code compiles but what does it print?

char ch = '1';
ch /= 0.9;
System.out.println(ch);

prints 6.

Each char is an unsigned 16-bit value which holds the unicode of the character. '1' has a unicode of 49. 49 / 0.9 is 54.4444444 however this is cast back to a char in the /= operation so ch = (char) 54 which is the unicode for '6'

Each assignment operator implicitly converts the result back to the type of the variable so ch /= 0.9 is like ch = (char) ((double) ch / 0.9);

Decimals

Which of the following prints 0.3

int x = 3;
System.out.println(x / 10); (1)
System.out.println(x * 0.1); (2)
System.out.println(x / 10.0); (3)
System.out.println(0 + '.' + x); (4)
System.out.println(0 + "." + x); (5)
1 This uses integer division; 3 / 10 is 0 with 3 remainder.
2 0.1 has a representation error which means it is slight larger than 0.1 and when multipied with 3 gets 0.30000000000000004
3 This prints 0.3 as 10.0 can be represented without error.
4 This uses integer addition i.e. 0 + (int) '.' + x which is 49
5 This uses String addition so is ok.

In short, lines 3 and 5 print 0.3

try with null

What is NOT true about this code?

    public static void main(String... args) {
        try (PrintWriter pw = null) { }
    }
  1. It compiles but it doesn’t if you replace PrintWriter with Writer. True as Writer.close() throws an IOException but PrintWriter.close() doesn’t

  2. It produces no error at compile time or runtime. True as a null Closeable is silently ignored at runtime.

  3. The code won’t compile if { } is replaced with ;. True as ; is not allowed as a nexted statement of a try-with-resource block unlike while, if or for

  4. It doesn’t compile. False, it compiles.

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