Guangdong Embedded Java Training First Day notes-java basic concept

One or two binary number

1, the highest bit is 0, indicates a positive number, the highest bit is 1, indicates a negative number.

2, the corresponding negative and positive conversion between the way: through the complement of the conversion, that is: reverse and add 1.

For example: 0000 0001 means +1; after 0000 0001 is reversed: 1111 1110, then 1111 1110+1=1111 1111, binary 1111 1111 equals-1.

3, 8-bit binary number, the maximum and minimum value that can be represented.

Maximum value: 0111 1111=127, i.e. 2^ (8-1) -1=127. (because it is calculated from 0, it can represent 0~127).

Minimum value: 1000 0000 = 128, i.e. -2^ (8-1) =-128.

What are the concepts of 4, 32-bit operating system and 64-bit operating system respectively?

2^10=1024k;

2^20=1m;

2^30=1g;

2^32=4g; (4G what concept: 4G is about 4.29 billion).

As can be seen from the above calculation: the32-bit operating system supports a maximum of 4G of memory.

2^40=1t;

2^40=1e

2^50=1t;

2^60=1z;

2^64=16z;

Obviously, the 64-bit operating system supports a large amount of memory.

Ii. Types of data

1. There are two kinds of data types in Java: Basic data type, reference data type.

2, 8 kinds of basic data types:

Boolean: Boolean (1-bit, in Java, whose value can only be true or false, cannot be represented by 1 or %).

Integer: Byte (8-bit, byte-unit, one byte for one bytes), short (16-bit, shorter-integer), int (32-bit, integer), long (64-bit, long-integer)

Float type: Float (32-bit, single-precision float), double (64-bit, double-precision floating-point type)

Character type: char (16 bits, one char for one character), one char equals 2 bytes

3. Conversion between 8 basic data types

　　　　The Boolean type cannot be converted to another data type .

Conversion between the other 7 data types (conversion without losing precision) rules are as follows:

　　　　

The data type you want to convert in reverse, requires a cast.

4. Reference data type: Array, class, interface;

Data of reference data types need to be created with the New keyword.

Third, operator (operators)

1, classification: unary operator, two-dollar operator, ternary operator.

Unary operators: + + (self-increment) 、--(decrement),-(minus sign), + (plus),! (non), ~ (bitwise negation).

Binary operators: +-*/% + = = *=/=%= && (with) & (bitwise VS) | | (OR) | (bitwise OR) &&= | | = = (Assignment) = = (judging if equal)! = > >= < <=

Ternary operator (trinocular operator):? :, A?b:c judge A, if true, then B, otherwise c. For example: (4>2)? 1:0, first to determine whether 4>2 this equation is set up is 1, or 0. Therefore, it should be 1.

2. Precedence of various operators:

　　　　　　

Iv. the difference between break and continue

　　Break: Jump out of the loop, no longer execute the loop statement;

Continue: Jump out of this loop, no longer loops the rest of the content, but executes the next loop.

Guangdong Embedded Java Training First Day notes-java basic concept