Assertions - Finding the output Questions and Answers

Q: The program is run with java -ea Example43 - what will be the output? public class Example43 { public static void main(String[] args) { String s = null; assert s != null && s.length() > 0; System.out.println("ok"); } }

The conditional-and operator short-circuits: s != null is false, so s.length() is never evaluated and there is no NullPointerException. The whole expression is false, so the assertion throws an AssertionError. Written with & instead of && it would be a NullPointerException.

Q: The program is run with java -ea Case46 - what will be the output? public class Case46 { public static void main(String[] args) { String city = "delhi"; String typed = new String("delhi"); assert city == typed : "not the same object"; System.out.println(city.equals(typed)); } }

new String(...) is required to create a distinct object, so the variable references something other than the pooled literal and the identity test fails, throwing before the print. Had the program compared with equals, as the unreached print does, the answer would have been true.

Q: The program is run with java -ea Probe50 - what will be the output? public class Probe50 { public static void main(String[] args) { char c = 'a'; assert (char)(c + 1) == 'b'; System.out.println((char)(c + 1)); } }

Arithmetic on a char promotes it to int, so c + 1 is the int 98, and the cast turns it back into the char b. The comparison against the char literal b is true, so the assertion passes and the cast value prints as b rather than 98. Dropping the cast would print 98.

Q: The program is run with java -ea Snippet52 - what will be the output? import java.util.*; public class Snippet52 { public static void main(String[] args) { int[] a = {1, 2}; int[] b = {1, 2}; assert Arrays.equals(a, b); System.out.println("equal"); } }

Arrays.equals compares length and then each element, so two distinct arrays holding the same values are reported equal. The assertion passes and equal is printed. This is the correct counterpart to the reference comparison that fails.

Q: The program is run with java -ea Listing53 - what will be the output? public class Listing53 { public static void main(String[] args) { Object o = "text"; assert o instanceof Integer; System.out.println("int"); } }

instanceof asks a question rather than performing a conversion, so it never throws; it simply answers false for a String tested against Integer. The assertion condition is false and an AssertionError is thrown. A cast would have produced ClassCastException instead.

Q: The program is run with java -ea Fragment54 - what will be the output? public class Fragment54 { public static void main(String[] args) { try { assert false : "first"; } catch (AssertionError e) { System.out.println("caught " + e.getMessage()); throw new AssertionError("second"); } } }

AssertionError can be caught explicitly by name. The handler prints caught first, then throws a new AssertionError which nothing catches, so it propagates out of main. Note the second error is created with new rather than by an assert statement, so it is thrown even without -ea.

31.

The program is run with java -ea Example43 - what will be the output?

public class Example43
{
    public static void main(String[] args)
    {
        String s = null;
        assert s != null && s.length() > 0;
        System.out.println("ok");
    }
}

A.The program prints ok
B.A NullPointerException is thrown.
C.An AssertionError is thrown.
D.Compilation fails.

Answer: C

The conditional-and operator short-circuits: s != null is false, so s.length() is never evaluated and there is no NullPointerException. The whole expression is false, so the assertion throws an AssertionError. Written with & instead of && it would be a NullPointerException.

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32.

The program is run with java -ea Program44 - what will be the output?

public class Program44
{
    public static void main(String[] args)
    {
        int i = 0;
        while (i < 3)
        {
            i++;
        }
        assert i == 3;
        System.out.println(i);
    }
}

A.The program prints 2
B.Compilation fails.
C.An AssertionError is thrown.
D.The program prints 3

Answer: D

The loop increments i until the condition fails, leaving i equal to 3. The assertion states that postcondition, finds it true and does nothing, so 3 is printed. An assertion that passes leaves no trace, which is the normal case for correct code.

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33.

The program is run with java -ea Trial45 - what will be the output?

public class Trial45
{
    public static void main(String[] args)
    {
        String a = "ab";
        String b = "ab";
        assert a == b;
        System.out.println("same");
    }
}

A.The program prints same
B.Compilation fails.
C.No output is produced.
D.An AssertionError is thrown.

Answer: A

Both are string literals with the same characters, so the compiler places a single interned String in the pool and both variables reference it. a == b compares references and is true, the assertion passes and same is printed. The result would differ with new String("ab").

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34.

The program is run with java -ea Case46 - what will be the output?

public class Case46
{
    public static void main(String[] args)
    {
        String city = "delhi";
        String typed = new String("delhi");
        assert city == typed : "not the same object";
        System.out.println(city.equals(typed));
    }
}

A.The program prints true
B.An AssertionError with message not the same object
C.The program prints false
D.Compilation fails.

Answer: B

new String(...) is required to create a distinct object, so the variable references something other than the pooled literal and the identity test fails, throwing before the print. Had the program compared with equals, as the unreached print does, the answer would have been true.

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35.

The program is run with java -ea Item48 - what will be the output?

public class Item48
{
    public static void main(String[] args)
    {
        Long x = 200L;
        Long y = 200L;
        assert x == y : "not cached";
        System.out.println(x.equals(y));
    }
}

A.The program prints false
B.Compilation fails.
C.The program prints true
D.An AssertionError with message not cached

Answer: D

200 is outside the guaranteed cache range of -128 to 127, which applies to Long just as it does to Integer, so each boxing conversion may produce a new object. The identity test is false and the assertion throws. The equals comparison in the unreached print would have been true.

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36.

The program is run with java -ea Sample49 - what will be the output?

public class Sample49
{
    public static void main(String[] args)
    {
        assert 0.1 + 0.2 == 0.3;
        System.out.println("exact");
    }
}

A.An AssertionError is thrown.
B.Compilation fails: == cannot be applied to double.
C.No output is produced.
D.The program prints exact

Answer: A

Neither 0.1 nor 0.2 is exactly representable in binary floating point, and their sum comes out slightly above 0.3. The equality is therefore false and the assertion throws. Floating-point assertions have to compare against a tolerance, for example Math.abs(sum - 0.3) < 1e-9.

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37.

The program is run with java -ea Probe50 - what will be the output?

public class Probe50
{
    public static void main(String[] args)
    {
        char c = 'a';
        assert (char)(c + 1) == 'b';
        System.out.println((char)(c + 1));
    }
}

A.An AssertionError is thrown.
B.The program prints b
C.The program prints 98
D.Compilation fails.

Answer: B

Arithmetic on a char promotes it to int, so c + 1 is the int 98, and the cast turns it back into the char b. The comparison against the char literal b is true, so the assertion passes and the cast value prints as b rather than 98. Dropping the cast would print 98.

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38.

The program is run with java -ea Snippet52 - what will be the output?

import java.util.*;

public class Snippet52
{
    public static void main(String[] args)
    {
        int[] a = {1, 2};
        int[] b = {1, 2};
        assert Arrays.equals(a, b);
        System.out.println("equal");
    }
}

A.Compilation fails.
B.No output is produced.
C.An AssertionError is thrown.
D.The program prints equal

Answer: D

Arrays.equals compares length and then each element, so two distinct arrays holding the same values are reported equal. The assertion passes and equal is printed. This is the correct counterpart to the reference comparison that fails.

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39.

The program is run with java -ea Listing53 - what will be the output?

public class Listing53
{
    public static void main(String[] args)
    {
        Object o = "text";
        assert o instanceof Integer;
        System.out.println("int");
    }
}

A.An AssertionError is thrown.
B.Compilation fails.
C.A ClassCastException is thrown.
D.The program prints int

Answer: A

instanceof asks a question rather than performing a conversion, so it never throws; it simply answers false for a String tested against Integer. The assertion condition is false and an AssertionError is thrown. A cast would have produced ClassCastException instead.

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40.

The program is run with java -ea Fragment54 - what will be the output?

public class Fragment54
{
    public static void main(String[] args)
    {
        try
        {
            assert false : "first";
        }
        catch (AssertionError e)
        {
            System.out.println("caught " + e.getMessage());
            throw new AssertionError("second");
        }
    }
}

A.caught first only
B.caught first, then an AssertionError with message second
C.An AssertionError with message second only
D.Compilation fails.

Answer: B

AssertionError can be caught explicitly by name. The handler prints caught first, then throws a new AssertionError which nothing catches, so it propagates out of main. Note the second error is created with new rather than by an assert statement, so it is thrown even without -ea.

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