Assertions - Pointing out the errors Questions and Answers

91.

Which line is an inappropriate use of an assertion?

public class Sample91
{
    public static void main(String[] args)
    {
        boolean open = true;
        boolean locked = false;
        assert open = locked;
        System.out.println(open);
    }
}

A.System.out.println(open);
B.Every line above is correct as written.
C.assert open = locked;
D.boolean locked = false;

Answer: C

Both sides are boolean, so the assignment is a valid boolean expression and this compiles. It also overwrites open with false and then asserts that false, so it always fails under -ea and silently corrupts the variable without it. A single = where == was meant is at its most dangerous between two booleans.

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92.

Which line contains the error?

public class Sample92
{
    static void run(String command)
    {
        assert command.equals("go");
        System.out.println("running " + command);
    }

    public static void main(String[] args)
    {
        run(args.length > 0 ? args[0] : null);
    }
}

A.run(args.length > 0 ? args[0] : null);
B.System.out.println("running " + command);
C.The code is correct as it stands.
D.assert command.equals("go");

Answer: D

Calling equals on a null reference throws NullPointerException rather than returning false, so the assertion cannot report anything useful. Putting the literal first, "go".equals(input), is null-safe and returns false as intended.

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93.

Which line will cause a compilation error?

public class Sample93
{
    private char[] grades = {'A', 'B', 'A'};

    int topCount()
    {
        int found = 0;
        for (char g : grades)
        {
            assert g == "A";
            if (g == 'A') found++;
        }
        return found;
    }

    public static void main(String[] args)
    {
        System.out.println(new Sample93().topCount());
    }
}

A.assert g == "A";
B.if (g == 'A') found++;
C.private char[] grades = {'A', 'B', 'A'};
D.Nothing here is wrong.

Answer: A

A char cannot be compared with a String; the types are incomparable and the line does not compile. The double quotes make "A" a String object, while the char literal needs single quotes. Changing the comparison to grade == 'A' fixes it.

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94.

Which line contains the error?

public class Sample94
{
    private static final int RETRIES = 3;

    static int attempt(java.util.Random rng)
    {
        assert true;
        assert RETRIES == 3;
        return rng.nextInt(RETRIES);
    }

    public static void main(String[] args)
    {
        System.out.println(attempt(new java.util.Random(7)));
    }
}

A.return rng.nextInt(RETRIES); depends on the assertions having passed
B.Both assertions are redundant: neither condition can ever be false.
C.assert true; does not compile
D.assert RETRIES == 3; does not compile

Answer: B

Both conditions are constants the compiler can evaluate, so neither can ever report anything. They compile and pass, costing a little time under -ea and documenting nothing. An assertion earns its place only when it states something that could conceivably be false.

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95.

Which line will cause a compilation error?

public class Sample95
{
    static boolean isReady() { return true; }

    public static void main(String[] args)
    {
        assert isReady;
        System.out.println("ready");
    }
}

A.System.out.println("ready");
B.Every line above is correct as written.
C.assert isReady;
D.static boolean isReady() { return true; }

Answer: C

The parentheses are missing, so this names a variable rather than calling the method, and no variable called isReady exists. The compiler reports that it cannot find the symbol. Adding the parentheses makes it a boolean-valued call and the assertion compiles.

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Assertions - Pointing out the errors Questions and Answers

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