java.md

Java

An application language, not a script language or a system languge

Static method does not operate on any objects

A package is a set of related classes

In java, everything is declared inside no class. No global variable and function. This can lead to somewhat verbose code.

//     inline comment
/* */  multi-line comment
/** */ document comment

File name must be same with class name

// @Foo.java
// pacakge package.path;
public class Foo {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

javac /package/path/Foo.java
java package.path.Foo

integer

byte  1    int   4
short 2    long  8
Integer.MIN_VALUE / MAX_VALUE
Byte, Short, Long class also have these two values

bigger than long: java.math.BigInteger

one_million = 1_000_000
            = 0b1111_0100_0010_0100_0000
all integers are signed, call Byte.toUnsignedInt(b) to do conversion

floating-point

float  = 3.14F
double = 3.14D
0x1.0p-10 // hex floating-point number uses p instead of e

Float|Double.POSITIVE_INFINITY|NEGATIVE_INFINITY|NaN
every NaN is different, check using = will fail
use isNaN(x) | isInfite(x) | isFinite(x)

precise numerical w/o roundoff: java.math.BigDecimal

char and bool

char UTF-16 encoding
'J', '\u004A', '\'', '\\'

boolean := false | true

variable

int total = 0;
int total = 0, count; // count is uninitialized int
Random gen = new Random();

name starts with letter, can contain _ and $
but $ is conventionally used for auto-generated names
but letters and digits can be from any alphabet, like lambda and pi

conventionally, a variable is declared as late as possible,
just before you need it for the first time

constant

final int DAYS_PER_WEEEK = 7;
final: like const keyword in C
once it is initialized, it is final (cannot be changed)
but this initilization can happen later than declaration

operators

>> right shift with 0
>>> right shift with sign bit
n >> k, k is moduloed first to be with range of sizeof(n)
k = [0,31]

int n = 1;
n << 31 => Integer.MIN_VALUE
n << 32 => 1
n << 33 => 2

big number

java.math.BigInteger
java.math.BigDecimal

BigInteger n = BigInteger.valueOf(42L)
BigInteger n = new BigInteger("42")

java has no operator overloading
// 5 * (n + k)
BigInteger r = BigInteger.valueOf(5).multiply(n.add(k))

BigDecimal.valueOf(n,e) // nx10-e
BigDecimal.valueOf(2,0).substract(BigDecimal.valueOf(11,1))