20 April 2018

Java Puzzles

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();

A compiler error
A runtime error
The JVM crashes
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
1.111111111111111
1.9
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) { }
    }

It compiles but it doesn’t if you replace PrintWriter with Writer.
It produces no error at compile time or runtime.
The code won’t compile if { } is replaced with ;.
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) { }
    }

It compiles but it doesn’t if you replace PrintWriter with Writer. True as Writer.close() throws an IOException but PrintWriter.close() doesn’t
It produces no error at compile time or runtime. True as a null Closeable is silently ignored at runtime.
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
It doesn’t compile. False, it compiles.