Blinded by the lights

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”.

Imagine you need to implement a class that extends an abstract class and implements an interface when both define a method with the same name - test().

Example

interface Implementable { public void test(); }
abstract class Superclass { public abstract void test(); }

public class Test extends Superclass implements Implementable {
    /* definition of load method(s)..... */
}

The following will not compile, getting (among others) duplicate method test() error:

interface Implementable { public void test(); }
abstract class Superclass { public abstract void test(); }

public class Test extends Superclass implements Implementable {
    public void Superclass.test() { }
    public void Implementable.test() { }
}

Solution

The correct implementation looks as follows:

interface Implementable { public void test(); }
abstract class Superclass { public abstract void test(); }

public class Test extends Superclass implements Implementable {
    public void test() { }
}

In such case, you need to define the body of test() method ONLY ONCE.