Java tips and tricks

[npa]

"i accidently get NullPointerExceptions when checking
what strings equal, how can i avoid it easily?":

String a = null;
if(a.equals("Hello")){ // oops exception!
}

// this may be solved with:
if(a != null && a.equals("Hello"))

// but you can just do:
if("Hello".equals(a)){ // and avoid checking null at all.

It is good practice to put the constant value on the
left-hand-side so accidents dont occur in other languages
(javascript, c, c++) where they evaluate assignment to
be either true or false and hence a valid expression in
an "if" statement.

[Belisarius]

Wow, some of that is real hacker stuff. But who wouldn't want to use main()?

[npa]

well that was just my of showing that main() isn't
needed as the jvm loads and runs any "static" blocks
before anything else (including main) so if you
feel like it you do not need to use him (main).

generally you use static blocks for loading
things (i use it to connect to the database),
other people use it to load tables of constants:

private static final int POWERS_OF_2[];
	static {
		//
		//
		//
		//
		for(int k = 0; k < 32; k++){
			POWERS_OF_2[k] = Math.pow(k, 2);
		}
	}
	
	//
	// now we can referrence powers of two faster
	// than trying to calculate them
	//
}

[npa]

"i want to count the number of set-bits in an interger!":

(can't take credit for this one):

public final static int getBits(int i) {
        i = ((i & 0xaaaaaaaa) >> 1) + (i & 0x55555555);
        i = ((i & 0xcccccccc) >> 2) + (i & 0x33333333);
        i = ((i >> 4) + i) & 0x0f0f0f0f;

        i += i >> 8;
        i += i >> 16;

        return i & 0xff;
    }