table of Contents
Basic data types
A total of 8 Java data types, integer (byte, Short, int , Long), float (float, Double ), string, Boolean.
- Hex: Decimal can not start with 0, octal begin with 0, hexadecimal with 0x or 0X beginning.
- Integer: l L or to add precision to prevent deletion (overflow) at the rear when using long. Java default integer type int .
- Float: To add F or f or an error occurs later when using the float. Java default decimal type double .
To avoid 4.35 * 100 = 4.34999999 problem, use Math.round () is rounded.
To avoid unequal 2.0-1.9 == 0.1, using the Math.abs (), if the absolute difference between the two numbers is less than 1e-6, 0 is determined, i.e., considered equal .
- Character types: Java use unicode encoding , character encoding from 0x0000-0xffff. Two bytes.
- char type
- Escape
- Boolean: true, false
Variables and Constants
- Identifier: letters, underscores, dollar symbols , numbers, and the first character can not be a number.
- Keywords:
- variable:
- It must be a valid identifier; you can not use the keyword; do not repeat; select meaningful variable names.
- Local variables static variables shield
- Constant: final variable name Data type [= value].
- final variables belonging to member variables (class member variables) must be assigned when defining time.
- Can define the time constant in the method belongs, after the assignment.
- Variable range:
- Member variables into static variables (plus static int a; across class, use the class variable name reference.) And instance variables (general int a).
- Local variables (variables defined method) shield member variable (class variables defined)
Operators
- Assignment operator: =
- Arithmetic operators:
- Increment decrement operators: ++ a, a ++, - a, a--.
- To change the value or the value of the difference between first use.
- Unary operator, can not be placed outside the parentheses.
- Comparison operators:
- Can not be used in tandem
- Logical Operators:
- &&, ||: boolean operand must be the type of short-circuit
- Bitwise logical operations may be performed: &, |, ^
- Bitwise Operators:
- Bitwise: & bitwise and, bitwise or |, ~ bitwise, bitwise exclusive OR ^;
- The shift operators: left <<, >> right, unsigned shift right >>>
- Tip: Right shift is divided by the equivalent of 2, left shift is equivalent to dividing by two.
- Ternary operator: the conditional expression? Value 1: Value 2; conditional expression is true the entire expression evaluates to 1 if the value of 2.
- There is a return value, must be used.
- Operator Precedence
Type Conversion
- Implicit type conversions: byte <short <int <long <float <double. char type can also be converted to an int and more advanced types. Converted to a byte, short possible overflow, because only one byte byte, short two bytes, but more than 32,767 the number will be negative, char is unicode encoded as two bytes.
- Explicit type conversion:
- In addition boolean, conversion between other basic types in a method to achieve explicit type conversions. int a = (int) 1.2;
- Assign a value to byte, short, int, long time variables, variables can not exceed the range, otherwise it will error, you must use a cast.
- Mandatory conversion will result in inaccurate data, the decimal point is lost, overflow.
Code Specification
- Code comments: // single-line comments, multiline comments /*...*/, document annotation /**...*/
- Note: Multi-line comments can not be nested multiline comments
Identifier naming convention
- Class Name: usually nouns, capitalized the first letter of all words
- Method name: commonly used verbs, the first word lowercase, uppercase first letter of subsequent words
- Variable: the first word lowercase, uppercase first letter of subsequent words
- Constant: in all uppercase letters
