Object class
Universal superclass, all methods should be overriden if default implement is not suitable. |
toString() |
return memory address by default |
equals(); |
same as == by default, compare memory address, not the content |
compareTo() |
same as <> by default, memory address instead of content |
hashCode() |
return unique integer mapping an object to a hash table. |
Objects.hash(Object... args) |
a simple hash algorithm |
getClass() |
polymorphic, return class name |
ovrridden equals() usually means override hashCode() too to return the same integer if 2 object are the same.
Wrapper class
Wraps ( boxing) or unwraps ( unboxing) primary types to object type |
1. for methods required object type as parameters |
2. for java container classes require object items |
intObj = Integer (6) |
boxing int into Integer |
int intObj.intValue() |
unboxing |
Wrapper object are immutable, final
8 wrapper class
Boolean |
Byte |
Character |
short |
Integer |
Long |
Float |
Double |
Wrapper class constructor
public Xxxx(xxx) |
create from primitive |
public Xxxx(String) |
create from string |
No default constructor;
All wrapper classes are final
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Wrapper class methods
Overriden toString(), equals() and compareTo() methods based on object content value instead of memory address. |
obj.xxxValue(): used to get primary values |
new Xxx(PARAM): Constructor |
X.parseXxx(String): parse values from string as primitive |
X.valueOf: value from string as Wrapper class |
wrapper class equals() check the type first, if not same type return false.
Math
import static java.lang.Math.*; |
all members are static |
Math.PI |
Math.abs() |
Math.power(base, exp) |
Math.sqrt() |
Math.random |
[0,1) |
Math.round() |
-0.5->0,-0.6->-1, 0.5->1 |
Math random numbers
a*Math.random()+b |
a for scale, b=shift |
(int)(Math,random*a)+b |
[b,a-1) |
(int)((Math,random+1)*a)+b |
[b,a] |
(int)(Math,random*20)+5 |
[5,24] |
(int)(Math,random*21)+5 |
[5,25] |
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Java random number generators
java.lang.Math.random() |
double [0,1), need scale,shift,casting |
java.Util.Random; |
flexible types & methods,reproducible |
java.secure.SecureRandom; |
<=128bit, cryptographically secure |
java.util.concurrent.ThreadLocalRandom; |
more efficient with multi-thread app |
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