JAVA- basic concepts

• Identifier

1. Identifier 26 may comprise letters (case insensitive), numbers 0-9, and _ $ symbol underscore.
2. Identifiers can not begin with an array.
3. Identifiers can not be keywords.

• constant
  • Concept: During the program run, a fixed amount of unchanged
  • classification:
    1. String constants: Any partial double quotation marks, called string constants, such as: "abc", "123"
    2. Integer constants: write directly on the numbers, there is no decimal point. For example: 100, 200, -200
    3. Float constant: to write directly on the numbers, there is a decimal point, for example: 2.5, -3.14,0.0
    4. Character Constants: single quotes to all single character, called the character constants, such as: 'A', '1', 'medium'
    5. Boolean constants: only two values, true false
    6. Empty constants: null, no data on behalf of

   type of data

　　Integer byte short int long

　　Float float double

　　The type char

　　Boolean boolean

　　Note : The string is not a basic type, but reference types.

　　　　　Float may only be an approximation, not a precise value

　　　　　Data range is not necessarily related to the number of bytes

　　　　　Float among the default type is double, if you want to use a float, the need to add a suffix F,

　　　　　If is an integer, the default is int type, long type if you want to use, you need to add a suffix L

 

　　variable

　　　　1. Variable names can not be repeated

　　　　2. For float and long types, the character can not be omitted and L suffix F

　　　　3. Start from the line defined variables, until the end of a direct member of the braces so far

　　Cast

　　　　1.byte / short / char three types can occur mathematical waybill, such as addition "+"

　　　　2.byte / short / char in the three types of operation that they will first promoted to the int type, and then in the calculation

　　ASCII code table

　　　　48 ->'0' 65->'A' 97->'a'

　　Compiler optimizations:

　　　　short result = 5 + 8; // the right of the equal sign are constant compiled for all

　　　　short result = 13;