Assertions - Pointing out the errors Questions and Answers

Q: Which line will cause a compilation error? public class Sample89 { private final java.util.List<String> queue = new java.util.LinkedList<String>(); void drain() { while (!queue.isEmpty()) { assert; queue.remove(0); } } public static void main(String[] args) { new Sample89().drain(); } }

Correct answer: assert;The assert statement requires a boolean expression - either assert expr; or assert expr : detail; - so a bare assert; is a syntax error. The rest of the method is sound: queue.isEmpty() and queue.remove(0) are both valid on a List, and removing index 0 repeatedly is a legitimate way to drain one.

81.

Which line will cause a compilation error?

public class Sample81
{
    static final int MAX = 10;

    public static void main(String[] args)
    {
        assert MAX = 20;
        System.out.println(MAX);
    }
}

A.assert MAX = 20;
B.static final int MAX = 10;
C.System.out.println(MAX);
D.Nothing here is wrong.

Answer: A

Two faults meet on one line and the compiler reports both: a final field cannot be assigned, and the assignment yields an int where a boolean is required. Either alone would be fatal. The author almost certainly meant ==.

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82.

Which line contains the error?

public class Sample82
{
    public static void main(String[] args)
    {
        String s = null;
        assert s != null & s.length() > 0;
        System.out.println("ok");
    }
}

A.No line here is in error.
B.assert s != null & s.length() > 0;
C.String s = null;
D.System.out.println("ok");

Answer: B

The single ampersand is the non-short-circuiting operator, so both sides are evaluated and s.length() throws NullPointerException despite the guard to its left. Written with && the second operand would be skipped and the assertion would fail cleanly instead.

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83.

Which line rests on a mistaken assumption?

public class Sample83
{
    public static void main(String[] args)
    {
        Object o = "text";
        assert o.getClass() == Object.class;
        System.out.println(o.getClass().getSimpleName());
    }
}

A.System.out.println(o.getClass().getSimpleName());
B.Every line above is correct as written.
C.assert o.getClass() == Object.class;
D.Object o = "text";

Answer: C

getClass reports the runtime class, which is String, not the declared type Object, so the comparison is false. The assertion confuses the type of the variable with the type of the object. An instanceof test against Object would have been true, since every object is one.

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84.

Which line will cause a compilation error?

public class Sample84
{
    public static void main(String[] args)
    {
        int x = 5;
        {
            int x = 10;
            assert x == 10;
        }
        System.out.println(x);
    }
}

A.assert x == 10;
B.int x = 5;
C.The code is correct as it stands.
D.int x = 10;

Answer: D

Java does not allow a local variable to shadow another local in an enclosing block, so the inner declaration is a compile error: x is already defined. The assertion would have been true had the declaration been legal. Fields can be shadowed by locals; locals cannot shadow locals.

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85.

Which line rests on a mistaken assumption?

public class Sample85
{
    public static void main(String[] args)
    {
        assert -7 % 3 == 2 : -7 % 3;
        System.out.println(-7 % 3);
    }
}

A.assert -7 % 3 == 2 : -7 % 3;
B.System.out.println(-7 % 3);
C.public static void main(String[] args)
D.Nothing here is wrong.

Answer: A

In Java the remainder takes the sign of the dividend, so -7 % 3 is -1, not 2. The assertion assumes the mathematical modulus, which is always non-negative. Getting a non-negative result requires adding the divisor back: ((-7 % 3) + 3) % 3.

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86.

Which line will cause a compilation error?

public class Sample86
{
    static int total(int[] values)
    {
        int sum = 0;
        for (int v : values) sum += v;
        return sum;
    }

    public static void main(String[] args)
    {
        int[] scores = {4, 8, 15};
        assert scores.length() == 3;
        System.out.println(total(scores));
    }
}

A.No line here is in error.
B.assert scores.length() == 3;
C.int[] scores = {4, 8, 15};
D.for (int v : values) sum += v;

Answer: B

An array exposes length as a field, not a method, so the parentheses make this a call to a method that does not exist and the line fails to compile. The print on the next line shows the correct form. String is the opposite way round, which is what keeps this confusing.

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87.

Which line will cause a compilation error?

public class Sample87
{
    private String title = "concurrency";
    private String author = "goetz";

    String describe()
    {
        assert title.length == 11;
        return title + " by " + author;
    }

    public static void main(String[] args)
    {
        System.out.println(new Sample87().describe());
    }
}

A.return title + " by " + author;
B.Every line above is correct as written.
C.assert title.length == 11;
D.private String author = "goetz";

Answer: C

String exposes length as a METHOD, so it needs parentheses and the field form does not compile. This is the exact mirror of arrays, which use a field with no parentheses. The print on the next line is written correctly.

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88.

Which line will cause a compilation error?

public class Sample88
{
    private final java.util.Map<String, Integer> tally =
        new java.util.HashMap<String, Integer>();

    void record(String key)
    {
        Assert key != null;
        tally.put(key, tally.size() + 1);
    }

    public static void main(String[] args)
    {
        Sample88 s = new Sample88();
        s.record("first");
        System.out.println(s.tally);
    }
}

A.tally.put(key, tally.size() + 1);
B.s.record("first");
C.The code is correct as it stands.
D.Assert key != null;

Answer: D

Java keywords are lowercase, so Assert with a capital A is read as a type name and the line does not compile. There is no such class in scope. The keyword is simply assert, and no import can make the capitalised form work.

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89.

Which line will cause a compilation error?

public class Sample89
{
    private final java.util.List<String> queue =
        new java.util.LinkedList<String>();

    void drain()
    {
        while (!queue.isEmpty())
        {
            assert;
            queue.remove(0);
        }
    }

    public static void main(String[] args)
    {
        new Sample89().drain();
    }
}

A.assert;
B.queue.remove(0);
C.while (!queue.isEmpty())
D.Nothing here is wrong.

Answer: A

Correct answer: assert;

The assert statement requires a boolean expression - either assert expr; or assert expr : detail; - so a bare assert; is a syntax error. The rest of the method is sound: queue.isEmpty() and queue.remove(0) are both valid on a List, and removing index 0 repeatedly is a legitimate way to drain one.

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90.

Which line contains the error?

public class Sample90
{
    public static void main(String[] args)
    {
        int age = 30;
        assert (age > 0);
        System.out.println(age);
    }
}

A.System.out.println(age);
B.No line here is in error.
C.assert (age > 0);
D.int age = 30;

Answer: B

Nothing here is wrong. assert is a statement rather than a method, but wrapping its condition in parentheses is still valid, because those simply group the expression. It compiles, the condition is true and the program prints 30. The parentheses are redundant, not an error.

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Assertions - Pointing out the errors Questions and Answers

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