Blinded by the lights

Example

Can you predict the output of the following code?

class SubTest extends Test {
    public int aNumber;

    public SubTest() {
        aNumber = 17;
    }

    public void doubleANumber() {
        System.out.println("Inside SubTest.doubleANumber()");
        aNumber *= 2;
    }
}

public class Test {
    public int aNumber;

    public Test() {
        aNumber = 6;
    }

    public void doubleANumber() {
        System.out.println("Inside Test.doubleANumber()");
        aNumber *= 2;
    }

    public static void main(String[] args) {
        Test t = new SubTest();
        t.doubleANumber();
        System.out.println(”The value of aNumber is ” + t.aNumber);
    }
}

Result

The output is:
Inside SubTest.doubleANumber()
The value of aNumber is 6

As you see, it was not the reference type but the real object type of t variable that decided which method was invoked at runtime. This way, aNumber of SubTest class was modified. This variable shadowed aNumber of Test class. Therefore, Test.aNumber variable was untouched (didn’t change at all).

A few weeks ago I described how to format output using System.out.printl() and System.out.println() methods…

Try to run the code below:

double avgAge = 245 / 34;
System.out.format("Average age is %d.", avgAge);

Without doubts you’ll get an exception thrown at runtime:

Average age is Exception in thread "main"
java.util.IllegalFormatConversionException:
d != java.lang.Double
    at java.util.Formatter$FormatSpecifier.failConversion(Unknown Source)
    at java.util.Formatter$FormatSpecifier.printInteger(Unknown Source)
    at java.util.Formatter$FormatSpecifier.print(Unknown Source)
    at java.util.Formatter.format(Unknown Source)
    at java.io.PrintStream.format(Unknown Source)

The reason is becase wrong conversion label was used. To recall, conversion label is ‘%’ sign followed by a letter b, c, d, f, or s inside System.out.format().

In the above example, %d meant that an integral type (byte, Byte, short, Short, int, Integer, long, Long, BigInteger) would be used. However, finally there was a floating point passed (avgAge). This example would work if this conversion label were used: %f.

So, remember to be careful with conversion label!

Always contruct try-catch blocks in a way that assumes catching Exceptions from more detailed to more generic.

Example

try {
    String [] tab = null;
    System.out.println(tab[3]);
} catch (Exception e) {
    System.out.println(”Exception”);
} catch (NullPointerException e) {
    System.out.println(”NullPointerException”);
}

The above presented snippet will cause compilation error:

Unreachable catch block for NullPointerException. It is already handled by the catch block for Exception

Solution

try {
    String [] tab = null;
    System.out.println(tab[3]);
} catch (NullPointerException e) {
    System.out.println(”NullPointerException”);
} catch (Exception e) {
    System.out.println(”Exception”);
}

This way, if you invoke this snippet you’ll get the following output on the screen: “NullPointerException”.

Usually to print some ouptut you’d use System.out.printl() or System.out.println() method. It’s easy to use but you can face some difficulties in providing output formatted in a specific way.

To format output you can use either of the following methods (they work the same)

• System.out.format(String format, Object... args)
• System.out.format(Locale l, String format, Object... args)
• System.out.printf(String format, Object... args)
• System.out.printf(Locale l, String format, Object... args)

This is the way how those methods should be used:

%[argIndex$][flags][width][.precision] conversion

where:

• argIndex - index of the argument passed within the args array
• flags:
  • - - left justify
  • + - print sign (e.g. +25, -45.7)
  • 0 - pad this argument with zeroes
  • , - use local specific separators (e.g. 124,567,678.877)
  • ( -enclose a negative number in parenthesis
• width - minimum number of characters to print (nice to keep clean columns)
• precision - number of digits that will be printed after decimal delimiter (for floating points)
• conversion:
  • b - boolean
  • c - basic types that represent Unicode characters (char, Character, byte, Byte, short, Short)
  • d - integral types (byte, Byte, short, Short, int, Integer, long, Long, BigInteger)
  • f - floating point (float, Float, double, Double, BigDecimal)
  • s - string (for any argument type)

It’s a rule that == is used to compare references, not values. To compare values for equality equals() method should be used.
However, as of Java 5, you can also use == to check if two objects’ values are the same… But it works only for the following wrapper objects, if their primitive counterparts’ values are the same:

• Boolean
• Byte
• Character for values: \u0000 (0) ÷ \u007f (127)
• Byte and Integer for values: -128 ÷ 127