Java Basic

• Java store number with fractional parts as floating point numbers
  • store in 4 parts
    • sign(+/-)
    • mantissa(value)
    • radix(10)
    • exponent(n for radix ^ n)
• Variable name must start with  letter of _
  • do not use other symbol (?/!….)
  • use upperCase letter or _ to show separate
  • no not use java words(new, int….)
• const in c++ = final in java
• commant
  • // single line comment
  • /* \n */ all comment until matching
  • /** \n */ javadoc
• convert between type
  • use typeA a = (typeB) b (same value and conver to b type)
    • A a = new B() create a B type value
• value type
  • integer
    • byte
    • short
    • int
    • long
  • float
    • float
    • double
  • other
    • boolean
    • char
  • all before as pre-build primitive types
  • all other type called reference type
    • created from classes
    • start with UpperCase letter
    • actually pointer to a memory
    • refers to object
    • ex. String = char[n]
    • test equality of Stringuse String.equals(“String 2”)
• public static void main(String [] args)
  • public means can be accessed by other
  • static means it is used but not an object
  • void means no return
• call a method in  class, use class.method() syntax;
• mathematical methods
  • Math.sqrt(x)
  • Math.pow(x,y)
  • Math.sin(x)
  • Math.cos(x)
  • Math.tan(x)
  • Math.toRadians(x): degree to Radians
  • Math.toDegrees(x): radians to Degree
  • Math.exp(x): e^x
  • Math.log(x): ln(x) for x>0
• method type
  • static means the method is not used on an object
    • when call it, just use the name
    • the function should not use any data declared inside class except class itself
  • instance(non-static) must use the object
    • when call it, need objectName.method();
    • the function can directly use data declared inside class except class itself
  • accessor
    • return a value without changing it
    • normally return type is dame to data it returned
  • mutator
    • change the value of data but not return it
    • usually use void type
    • may take parameter
• Internalising data
  • 0 to number(int, float, double…)
  • false to boolean
  • null to any other
  • can be overridden
• Constructor
  • if no constructor been written, java will create a constructor by itself
  • a class can have multiple constructor with different parameter, compiler will choose based on actual code
  • can not been called to a constructed object
  • can call when constructing a new object
  • can not have return type void, also means just a method that return nothing
• String = null can not use String.length() and cause null pointer exception
• overloading is okay for any method
• primitive copy is copy value of var to another which in different location
  • only for primitive var(int, char…)
    • because take less space than object variable
• object variable use reference, which two variable share same reference
• an object with null reference means no reference
  • may cause running time error
• this reference refers to current object been used
  • mostly used in method declaration inside class
  • can use in constructor when  parameter have same name to data object
• Garbage collection
  • dellocate an object when it have no reference to it
  • keep track to all object with reference
  • may slow down running program and use more memory
  • avoid mistake by wrong or do not dellocate memory
• Java doc
  • @param parameter
  • @return return data
  • @procon procondition
  • @postcon postcondition