Numbers

comparator

do *NOT* use return i2-i1; as compare() method. since the result could lead overflow.

check even/odd

Do NOT use if (i % 2 == 1) it won't work for negative numbers. Use 

	if (i % 2 != 0)
//or even better with bitwise AND operator
	if ((i & 1) !=0)

silent number overflow

final long NUM=24 * 60 * 60 * 1000 * 1000; will overflow, since java will automatically think all constants are int, and result is int even if declared as long Solution is final long NUM=24L * 60 * 60 * 1000 * 1000; (better with big L)

double/float tips

1. it is possible to represent infinity as a double or a float. 2. adding a small floating-point value to a large one will not change the large value 3. binary floating-point arithmetic is only an approximation to real arithmetic. 4. (from puzzle 28)

represent infinity:

double i = 1.0 /0.0;
double i = Double.POSITIVE_INFINITY;

large float/double

Floating-point operations return the floating-point value that is closest to their exact mathematical result. Once the distance between adjacent floating-point values is greater than 2, adding 1 to a floating-point value will have no effect, because the half-way point between values won’t be reached. For the float type, the least magnitude beyond which adding 1 will have no effect is 2^25 , or 33,554,432; for the double type, it is 2^54 , or approximately 1.8 × 10^16 .

The distance between adjacent floating-point values is called an ulp, which is an acronym for unit in the last place. In release 5.0, the Math.ulp method was introduced to calculate the ulp of a float or double value.

Math.abs()

if the argument number is Integer.MIN_VALUE , the result of the method is same as Integer.MIN_VALUE

Strings

Matcher.quoteReplacement(string)

$ or \ in replacement part of replaceAll() has special meaning. With Mather.quoteReplacement(str), those chars will lost their special meanings.

Build repeated string

// repeat "foo" 100 times
String repeated = new String(new char[100]).replace("\0", "foo");

Exceptions

same method in different interface

The set of checked exceptions that a method can throw is the intersection of the sets of checked exceptions that it is declared to throw in all applicable types 

interface Type1 {
	void f() throws CloneNotSupportedException;
}
interface Type2 {
	void f() throws InterruptedException;
}
interface Type3 extends Type1, Type2 {
}
public class Arcane3 implements Type3 {
	public void f() {
		System.out.println("Hello world");
	}
	public static void main(String[] args) {
		Type3 t3 = new Arcane3();
		t3.f(); //here intersection is empty, so no try/catch
	}
}

Class, Method, variables

Initialization Procedure

see http://docs.oracle.com/javase/specs/jls/se7/html/jls-12.html#jls-12.4.2

method overload

To decide which overloadings are applicable: take the most spesific one. (parameter) For example: 

method1: void foo(Object o){...}
method2: void foo(int[] array){...}

//the foo(int[] array) would be called, since it is more spesific than the other
caller: object.foo(null);

hide v.s. overriding

Once a method is overridden in a subclass, you can’t invoke it on an instance of the subclass (except from within the subclass, by using the super keyword). You can, however, access a hidden field by casting the subclass instance to a superclass in which the field is not hidden.

E.g. 

Class Base{
	protected String name="base";
}

Class Sub extends Base{
	protected String name="sub";
}

now, say sub is an object of Sub. 

sub.a would be "sub" while:
(Base)sub.a would be "base"

static methods

	class Dog {
		public static void bark() {
			System.out.print("woof ");
		}
	}
	class Basenji extends Dog {
		public static void bark() { }
	}
	public class Bark {
		public static void main(String args[]) {
			Dog woofer = new Dog();
			Dog nipper = new Basenji();
			woofer.bark();
			nipper.bark();
		}
	}

the output is woof woof

*There is no dynamic dispatch on static methods*. When a program calls a static method, the method to be invoked is selected at compile time, based on the compile-time type of the qualifier, which is the name we give to the part of the method invocation expression to the left of the dot. In this case, the qualifiers of the two method invocations are the variables woofer and nipper , both of which are declared to be of type Dog . Because they have the same compile-time type, the compiler causes the same method to be invoked: Dog.bark . This explains why the program prints woof woof . It doesn’t matter that the runtime type of nipper is Basenji ; only its compile-time type is considered.

Comparable/Comparator

the implmentation of compareTo() or compare() method must be consistent with equals().

Performance

Variable Declaration

*A local variable declaration can appear only as a statement directly within a block.* (A block is a pair of curly braces and the statements and declarations contained within it.) 

 
 for (int i =1; i<100; i++)
	Object o = new Object();  //won't be compiled

 for (int i =1; i<100; i++){
	Object o = new Object();  //ok
 }

Others

& and &&, | and ||

shutdown hook

	public class HelloGoodbye {
		public static void main(String[] args) {
			System.out.println("Hello world");
			Runtime.getRuntime().addShutdownHook(
					new Thread() {
					public void run() {
					System.out.println("Goodbye world");
					}
					});
			System.exit(0);
		}
	}
	
	//print hello and goodby

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