Basic concepts of Java-java programming

2017-10-06 15:31:39

First, the basic composition of Java programs

Ii. data types and identifiers

• Data type

• Identifier

The generalized set of strings used to define various object names is called identifiers, and identifiers are generally categorized as user-defined identifiers and system identifiers .

# User-defined identifiers

Naming rules:

~ Number cannot be the beginning of the identifier, can be preceded by a letter, an underscore "_" or "$"

~ Cannot have spaces in identifiers

~ Cannot use Java operators, such as "+", "-" and so on

~ Cannot use Java keywords

~ Strictly case-sensitive

# System-specific identifiers

System-specific identifiers, also known as keywords, are the special terms used by the Java compiler itself, and they have specific grammatical meanings. such as: abstract and so on.

• Constant

A constant is the amount that the value of a program remains constant throughout the course of its operation.

~ Integer constant value has three forms: decimal number, such as 54,-74.

Octal number, in Java, the representation of the octet constant begins with 0, as 0125 represents the decimal 85.

Hexadecimal number, the representation of a hexadecimal constant in Java begins with 0x or 0X, such as 0x100, which represents a decimal 256.

integer int constants account for 32-bit memory by default, and long integer types are 64 bits, represented as numbers followed by L, or L.

~ Real constant values have two forms: the decimal number form, which consists of a number and a decimal point, and must have decimal points, such as 12.34.

Scientific notation, such as 1.75e5 or 326E3, where e or E must be preceded by a number, and the number after E or E must be an integer .

~ character and string constant values: Use single quotation marks for one character, using double quotes as strings.

~ Boolean constant Value: false and True.

~ constants are defined and used: The format is final type constant name = constant value, constant name = constant value;

The general constant names are in uppercase letters, which are defined at the time of declaration.

Maximum and minimum constants for Java integers: type int: integer.max_value,integer.min_value

Long Type: Long.max_value,long.min.value

Float type: float.max_value,float.min_value

Double Type: Double.max_value,double.min_value

Positive Infinity: float.positive_infinity,double.positive_infinity

Negative Infinity: float.negative_infinity,double.negative_infinity

0/0 non-stereotyped: Float.nan,double.nan

• Variable

~ Integer variable A total of four types of byte (8-bit), short (16-bit), int (32-bit), long (64-bit), Java does not provide any unsigned integer type , which is the difference from C + +.

~ Real variable has float (32-bit) and double (64-bit) type

~ the type bit char (16-bit ) of the character variable, using the Unicode character set .

~ Boolean variable Value concept a total of two types of false,true, accounting for 1 bits.

Member variables in Java have default values, integers and reals are 0,boolean false, and local variables must be self-assigned.

• Operators and expressions

In Java, the operator C + + is similar to the one in ++,--,+/. And so on

• Array

In the process of programming to solve real problems, it is often to deal with a large number of the same type of data, and this data to be repeatedly referenced, at this time, the use of arrays such data type is a wise choice.

~ Declaration and use of one-dimensional arrays

Type array name []=new type[number of elements];

You can use the array name. length to get the length value of the array.

There are two main ways to assign an array: Assign a value directly at declaration, type array name []={< array 1>,< array 2>,< array 3> ｝

A loop can be used if the array element is assigned a regular value.

~ Declaration and use of two-dimensional arrays

Type array name [][]=new type[number of rows elements] [number of column elements], such as int myarray[][]=new int[5][6];

The assignment of two-dimensional array elements can also be done at the time of declaration, for example:

int array[][] = {{20,25,2,4},{2,4,5,6}};

The Declaration and use of a Java multidimensional array is very flexible, and he can allocate memory for each dimension from the highest dimension, and for creating a two-dimensional array, you can use the following more flexible declaration method:

Type arrn[][] = new type[arrnum1][];

Arrn[0] = new TYPE[1];

ARRN[1] = new TYPE[3];

Arrn[arrnum1-1] = new TYPE[8];

~ Copying of arrays

Copy the arr1 array to arr2, you can use System.arraycopy (arr1,0,arr2,0,arr1.lenght) to copy all the elements in arr1 to ARR2, and the subscript starts at 0.

• String

~ Constant Strings String

The Declaration and initialization of string strings.

String s = "hello,world!";

You can also use a character array to initialize

Char ch[] = {' A ', ' r ', ' t '};

string s = new string (CH);

You can use ' + ' to concatenate strings.

Basic concepts of Java-java programming