Java.lang Class - Finding the output Questions and Answers

71.

What will be the output of the program?

public class Demo71
{
    private static void report()
    {
        String s = "Mississippi";
        int count = 0;
        for (int i = 0; i < s.length(); i++) if (s.charAt(i) == 's') count++;
        System.out.println(count);
    }

    public static void main(String[] args)
    {
        report();
    }
}

A.The program prints 5
B.The program prints 2
C.The program prints 4
D.The program prints 3

Answer: C

Mississippi contains four lower case s characters, in two adjacent pairs. Counting with charAt compares chars directly, so no string allocation happens. The two i characters and the two p characters are not counted, which is where a quick guess of 2 comes from.

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72.

What will be the output of the program?

public class Snippet72
{
    void execute()
    {
        String s = "abc";
        String t = s.replace('z', 'y');
        System.out.println((s == t) + " " + s.equals(t));
    }

    public static void main(String[] args)
    {
        Snippet72 demo = new Snippet72();
        demo.execute();
    }
}

A.The program prints false false
B.The program prints true false
C.The program prints false true
D.The program prints true true

Answer: D

replace returns the original object when there is nothing to change, so no new String is allocated and both variables reference the same one. Identity and equality are therefore both true. Replacing a character that does occur would have produced a new object.

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73.

What will be the output of the program?

public class Listing73
{
    public static void main(String[] args)
    {
        Long a = 127L;
        Long b = 127L;
        System.out.println((a == b) + " " + a.equals(127L) + " " + a.equals(127));
    }
}

A.The program prints true true false
B.The program prints false true true
C.The program prints true false false
D.The program prints true true true

Answer: A

Long caches the same -128 to 127 range as Integer, so the identity test is true. equals against a long literal boxes it to Long and matches. equals against the int literal 127 boxes it to Integer, and a Long is never equal to an Integer whatever the value, so that one is false.

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74.

What will be the output of the program?

public class Fragment74
{
    static void show()
    {
        System.out.println("10" + 20 + 30);
        System.out.println(10 + 20 + "30");
    }

    public static void main(String[] args)
    {
        show();
    }
}

A.102030 then 102030
B.The program prints 102030 then 3030
C.102030 then 1020 30
D.The program prints 60 then 60

Answer: B

Addition is left associative. The first line starts with a String, so every following value is joined as text, giving 102030. The second adds 10 and 20 as ints first and only then meets the String, giving 3030. Same operands, different order, different answer.

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75.

What will be the output of the program?

public class Example75
{
    void run()
    {
        char[] letters = {'j', 'a', 'v', 'a'};
        System.out.println(letters.length + " " + String.valueOf(letters));
    }

    public static void main(String[] args)
    {
        new Example75().run();
    }
}

A.The program prints 1 java
B.The program prints 4 [C@1b6d
C.The program prints 4 java
D.The program prints 4 java4

Answer: C

String.valueOf has an overload taking a char array, which builds a String from its contents rather than calling toString on the array. So it prints java. Passing the array to println directly would have shown the type and hash instead.

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76.

What will be the output of the program?

public class Program76
{
    public static void main(String... args)
    {
        Object[] items = new String[2];
        items[0] = "ok";
        System.out.println(items[0]);
        items[1] = Integer.valueOf(1);
    }
}

A.ok, then a ClassCastException
B.Compilation fails.
C.ok, and the program ends normally
D.ok, then an ArrayStoreException

Answer: D

Array covariance lets a String array be referenced as an Object array, so the assignment compiles. The first store is a String and succeeds, and the print happens. The second stores an Integer into what is really a String array, which is checked at runtime and throws ArrayStoreException.

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77.

What will be the output of the program?

public class Trial77
{
    Trial77()
    {
        String a = null;
        System.out.println(String.valueOf(a));
        System.out.println(a + "");
    }

    public static void main(String[] args)
    {
        new Trial77();
    }
}

A.Both lines print null
B.A NullPointerException is thrown on the first line
C.The first prints an empty line, the second prints null
D.Both lines print an empty line

Answer: A

String.valueOf checks for null and returns the text null rather than throwing, and concatenation applies the same conversion. So both lines print the four characters null. It is calling a method ON the null reference that fails, which neither line does.

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78.

What will be the output of the program?

public class Case78
{
    public static void main(String[] args) throws Exception
    {
        int octal = 070;
        int hex = 0x1A;
        int plain = 70;
        System.out.println(octal + ", " + hex + ", " + plain);
    }
}

A.The program prints 70, 26, 70
B.The program prints 56, 26, 70
C.The program prints 70, 1, 70
D.The program prints 56, 1, 70

Answer: B

A leading zero marks an octal literal, so 070 is 7 times 8, which is 56, not 70. The 0x prefix marks hexadecimal, so 1A is 26. Without a prefix, 70 is just seventy, which is what makes the padded form so easy to misread. Accidental octal is a classic source of quiet arithmetic bugs.

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79.

What will be the output of the program?

public class Sketch79
{
    private static void report()
    {
        System.out.println(Math.max(Integer.MIN_VALUE, Integer.MIN_VALUE + 1));
    }

    public static void main(String[] args)
    {
        report();
    }
}

A.An ArithmeticException is thrown.
B.The program prints -2147483648
C.The program prints -2147483647
D.The program prints 2147483647

Answer: C

Adding one to the smallest int stays inside the range and gives -2147483647, which is the larger of the two, so max returns it. No overflow occurs here because the addition moves toward zero. Only crossing an endpoint wraps.

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80.

What will be the output of the program?

public class Item80
{
    void execute()
    {
        String s = "a";
        for (int i = 0; i < 3; i++) s += s;
        System.out.println(s + " " + s.length());
    }

    public static void main(String[] args)
    {
        Item80 demo = new Item80();
        demo.execute();
    }
}

A.The program prints aaaa 4
B.The program prints aa 2
C.The program prints aaa 3
D.The program prints aaaaaaaa 8

Answer: D

Each pass doubles the string rather than adding one character, because s is appended to itself. Starting from one character, three doublings give eight. Each iteration also discards the previous String, which is why a builder is preferred in a loop.

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Java.lang Class - Finding the output Questions and Answers

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