Hidden Features of Java

Answer 1

Every class file starts with the hex value 0xCAFEBABE to identify it as valid JVM bytecode.

(Explanation)

Answer 2

Transfer of control in a finally block throws away any exception. The following code does not throw RuntimeException -- it is lost.

public static void doSomething() {
    try {
      //Normally you would have code that doesn't explicitly appear 
      //to throw exceptions so it would be harder to see the problem.
      throw new RuntimeException();
    } finally {
      return;
    }
  }

From http://jamesjava.blogspot.com/2006/03/dont-return-in-finally-clause.html

Answer 3

final initialization can be postponed.

It makes sure that even with a complex flow of logic return values are always set. It's too easy to miss a case and return null by accident. It doesn't make returning null impossible, just obvious that it's on purpose:

public Object getElementAt(int index) {
    final Object element;
    if (index == 0) {
         element = "Result 1";
    } else if (index == 1) {
         element = "Result 2";
    } else {
         element = "Result 3";
    }
    return element;
}

Answer 4

As of Java 1.5, Java now has a much cleaner syntax for writing functions of variable arity. So, instead of just passing an array, now you can do the following

public void foo(String... bars) {
   for (String bar: bars)
      System.out.println(bar);
}

bars is automatically converted to array of the specified type. Not a huge win, but a win nonetheless.

Answer 5

The addition of the for-each loop construct in 1.5. I <3 it.

// For each Object, instantiated as foo, in myCollection
for(Object foo: myCollection) {
  System.out.println(foo.toString());
}

And can be used in nested instances:

for (Suit suit : suits)
  for (Rank rank : ranks)
    sortedDeck.add(new Card(suit, rank));

The for-each construct is also applicable to arrays, where it hides the index variable rather than the iterator. The following method returns the sum of the values in an int array:

// Returns the sum of the elements of a
int sum(int[] a) {
  int result = 0;
  for (int i : a)
    result += i;
  return result;
}

Link to the Sun documentation

Answer 6

My favorite: dump all thread stack traces to standard out.

windows: CTRL-Break in your java cmd/console window

unix: kill -3 PID