Java basic syntax

Posted by dnszero on Fri, 29 Oct 2021 14:20:02 +0200

Java foundation is the cornerstone of learning Java EE, big data and Android development

1. Keywords and reserved words

Definition and characteristics of keyword s

Definition: a string with special meaning given by the Java language for special purposes

Features: all letters in the keyword are lowercase

reserved word

The existing Java version has not been used, but later versions may be used as keywords

Such as const, goto

2. Identifier


The character sequence used by Java when naming various variables, methods, classes and other elements is called an identifier

Tip: any place where you can name yourself is called an identifier

For example: class name, variable name, method name, interface name

Naming rules for identifiers:

The following naming rules are not met, and the compilation cannot pass

  • It consists of 26 English letters in case, 0-9_ Or $composition
  • Cannot start with a number
  • Keywords and reserved words cannot be used
  • Java is strictly case sensitive and has no limit on length
  • Cannot contain spaces

Naming conventions in Java

Package name: all letters are lowercase when composed of multiple words

Class name and interface name: when composed of multiple words, all words are capitalized

Variable name and method name: when composed of multiple words, the first word is lowercase, and the second word is capitalized

Constant name: all letters are capitalized, and multiple words are connected with underscores

Note: try to "see the name and know the meaning" when naming

3. Variables

Concept of variable

  • A storage area in memory
  • The data in this area can vary within the same data type range
  • Variable is the most basic storage unit in the program, including variable type, variable name and stored value.

Role of variables

  • Used to save data in memory

Use variable attention

  • Each variable in Java must be declared before use
  • Use the variable name to access the data of this area
  • Scope of variable: the pair of {} in which it is defined
  • The variable value is valid only if it is within its scope
  • Variables with duplicate names cannot be defined within the same scope

Use of variables

Use format: data type variable name = variable value;

package day02;

public class Variable {

	public static void main(String[] args) {
		// Definition of variables
		int age = 12;
		// Use of variables
		int number;
		number = 10;


Type of variable

be careful:

  • The Java integer constant defaults to int. when declaring a long variable, if the assigned value exceeds int, you need to add an 'l' or 'l' at the end
  • Java floating point constant is double by default. When declaring float constant, you need to add 'f' and 'f'

Use of various data types

package day02;

public class Variable {

	public static void main(String[] args) {
		// Integer type: int, long
		int a = 10;
		// When declaring a long type, add 'L' or 'L' after it
		long b = 10000000l;
		// Floating point type: float, double
		double c = 15.5;
		// When declaring a float type, add 'F' or 'F' after it
		float d = 16.5f;
		// Character type: char
		char e = 'a';
		// char type can only be assigned one character	
//		char f = 'aa';
		// boolean type: boolean
		// Boolean types have only two values: true and false
		boolean g = true;


Automatic conversion of basic data types

When the data type variable with small capacity and the data type variable with large capacity are operated, the result will be automatically converted to the data type with large capacity
In particular, when byte, char and short are calculated, the result is int

Hadron conversion of basic data types

Reverse operation of automatic conversion

1. Strong conversion character () is required
2. Note: forced type conversion may result in precision loss (not rounding or truncation)

String type variable

1.String is a reference data type
2. When declaring a String type variable, use double quotation marks ""
3.String can operate with 8 basic data type variables, and the operation can only be connection operation:+
4. The operation result is String

package day02;

public class Variable {

	public static void main(String[] args) {
		String a = "Student number";
		int b = 111;
		String c = a + b;


Exercise 1

package day02;

public class Variable {

	public static void main(String[] args) {
		char c = 'a';
		int num = 10;
		String str = "hello";
		System.out.println(c + num + str);	// 107hello
		System.out.println(c + str + num);	// ahello10
		System.out.println(c + (num + str));	// a10hello
		System.out.println((c + num) + str);	// 107hello
		System.out.println(str + num + c);	// hello10a


Exercise 2

Output '* *'

package day02;

public class Variable {

	public static void main(String[] args) {
		System.out.println("*	*");	// *	*
		System.out.println('*' + '\t' + '*');	// 93
		System.out.println('*' + "\t" + '*');	// *	*
		System.out.println('*' + '\t' + "*");	// 51*
		System.out.println('*' + ('\t' + "*"));	// *	*	


Conversion between base and base

There are 10 kinds of people in the world who know binary and don't know binary

All numbers exist in binary form at the bottom of the computer.

Binary integers have the following three forms

Positive number: three in one

Negative number:

  • Original code: the highest bit is the symbol number
  • Inverse code: except for the symbol code, the original code changes from 1 to 0 and 0 to 1
  • Complement: complement + 1

At the bottom of the computer, they are stored in the form of complement (positive and negative numbers are the same)

There are specific binary conversion functions in the Java API

Daily test

  1. What are the naming rules for identifiers?
    26 English letters, underscore (), dollar sign ($), 0-9 numbers, cannot start with a number
  2. What are the naming conventions for identifiers?
    Package name: word lowercase
    Class name and interface name: big hump
    Variable name and method name: small hump
    Constant name: word underscore connection
  3. How are Java variables divided by data type? It also points out the 8 basic data types of Java and the size of their memory space.
    Basic data type
    Byte (1 byte), short (2 bytes), int (4 bytes), long (8 bytes)
    char (1 character)
    float (4 bytes), double (8 bytes)
    Reference data type
    Class, interface, array
  4. Describes the operation rules for automatic type promotion between basic data type variables.
    byte,short,char -> int -> long -> float -> double
  5. Explain the use rules of forced conversion between basic data type variables and possible problems of strong conversion.
    Large capacity - > small capacity
    Strong conversion character: ()
    Precision loss

Topics: Java