Java basics 2: java Basics

Java basics 2: java Basics
1 keyword

• Definition: a word with special meanings given by the java language.
• Feature: All letters in the keyword are in lower case.

 

• Keywords used to define data types
  • Class, interface, byte, short, int, long, float, double, char, boolean, void

 

• Keywords used to define data type values
  • True, false, null

 

• Keywords used to define Process Control
  • If, else, switch, case, default, while, do, for, break, continue, return

 

• Keyword used to modify the access modifier
  • Private, protected, public

 

• Keywords used to define classes, functions, and variable Modifiers
  • Abstract, final, static, synchronized

 

• Keywords used to define the relationship between classes
  • Extends, implements

 

• Defines instance creation and instance reference, and determines the instance's keywords
  • New, this, super, instanceof

 

• Keyword used for Exception Handling
  • Try, catch, finally, throw, throws

 

• Keyword used for the package
  • Package, import

 

• Other modifier keywords
  • Native, strictfp, transient, volatile, assert

 

2 Identifier

• Some custom names in the program.
• It consists of 26 uppercase/lowercase letters, numbers 0-9, _, and $.
• Define valid identifier rules:
  • ① Numbers cannot start
  • ② Keywords cannot be used
• Java is case sensitive.
• Note: When you start a name, try to be considerate.

 

3. Notes

• The text used for annotation to indicate the interpreter is a annotation.
• This improves the readability of the Code.
• Annotation format in java:
  • Single line comment

// Single line comment

• • Multi-line comment

/* Multi-line comment */

• • Document comment

/** Document comment */

 

• Annotation Version of HelloWorld

Package java002; /*** this is the java program used to demonstrate HelloWorld */public class HelloWorld {/*** main function * @ param args */public static void main (String [] args) {/* output the Hello World statement */System. out. print ("Hello World ");}}

 

 

4. constants and variables

• A constant represents a value that cannot be changed.
• Classification of constants in java:
  • ① Integer constant, All integers.
  • ② Decimal constant, all decimal places.
  • ③ Boolean constant. Only two values are true or false.
  • ④ Character constant, which identifies a number, letter, or character with single quotation marks.
  • ⑤ A String constant that identifies one or more characters with double quotation marks.
  • ⑥ Null constant. Only one value is null.
• There are four manifestations of integers:
  • Binary
  • Octal
  • Decimal
  • Hexadecimal

• Variable concept:
  • A storage area in the memory
  • This region has its own name (variable name) and type (data type)
  • Data in this region can change continuously within the same type.
• Why define variables?
  • It is used to store constants of the same type continuously and can be reused.
• Notes on using variables
  • Scope of the Variable
  • Initialization value
• Define the variable format:

Data type variable name = initialization value;

 

• Java is a strongly typed language. It defines specific data types for each type of data and allocates memory space of different sizes in the memory.

 

• Example: Use of Variables

Package java002. * Description: Variable */public class VariableDemo {public static void main (String [] args) {// data type variable name = initialization value; int num = 100; System. out. print ("num value:" + num );}}

Package java002. * Description: Variable */public class VariableDemo {public static void main (String [] args) {// data type variable name = initialization value; byte B = 127; short s = 128; int num = 100; long l = 12345678910L; float f = 2.3f; double d = 4.5; System. out. print ("the value of B is:" + B); System. out. print ("s value:" + s); System. out. print ("num value:" + num); System. out. print ("l value:" + l); System. out. print ("f value:" + f); System. out. print ("the value of d is:" + d );}}

 

• Automatic type conversion and forced type conversion
  • Small-range type --> large-range type, automatic type conversion.
  • Otherwise, type conversion is forced.

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {int x = 3; x = x + 5; System. out. print ("x =" + x );}}

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {byte x = 3; int y = x + 5; System. out. print ("y =" + y );}}

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {byte x = 3; byte y = (byte) (x + 5 ); system. out. print ("y =" + y );}}

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {byte x = 3; byte y = (byte) (x + 200 ); system. out. print ("y =" + y );}}

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {System. out. print ('A'); System. out. print ('A' + 1 );}}

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {byte B = 4; B = 4 + 3; System. out. print (B );}}

 

5. Operator 5.1: Arithmetic Operator

• +,-, *,/, And %

Package java002. * Description: type conversion */public class TypeConverse {public static void main (String [] args) {int result = 0; int a = 10; int B = 3; System. out. print ("addition:" + (a + B); System. out. print ("subtraction:" + (a-B); System. out. print ("multiplication:" + (a * B); System. out. print ("Division:" + (a/B); System. out. print ("remainder:" + (a % B ));}}

Addition: 13 subtraction: 7 multiplication: 30 Division: 3 remainder: 1

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int I = 1; System. out. println (I ++); System. out. println (I );}}

1 2

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int I = 1; System. out. println (++ I); System. out. println (I );}}

2 2

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3, B; B = a ++; System. out. print ("a =" + a + ", B =" + B );}}

A = 4, B = 3

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3, B; B = ++ a; System. out. print ("a =" + a + ", B =" + B );}}

A = 4, B = 4

 

5.2 value assignment operator

• =

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3; int B = 4; int c = 5; System. out. print ("a =" + a + ", B =" + B + ", c =" + c );}}

• + =

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3; a + = 2; int B = 4; int c = 5; system. out. print ("a =" + a + ", B =" + B + ", c =" + c );}}

• -=

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3; a-= 2; int B = 4; int c = 5; system. out. print ("a =" + a + ", B =" + B + ", c =" + c );}}

• * =

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3; a * = 2; int B = 4; int c = 5; system. out. print ("a =" + a + ", B =" + B + ", c =" + c );}}

• /=

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3; a/= 2; int B = 4; int c = 5; system. out. print ("a =" + a + ", B =" + B + ", c =" + c );}}

• % =

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {int a = 3; a % = 2; int B = 4; int c = 5; system. out. print ("a =" + a + ", B =" + B + ", c =" + c );}}

 

Package java002;/*** packages /8/30. * Note: */public class OperateDemo {public static void main (String [] args) {byte B = 1; B + = 3; System. out. print ("B =" + B );}}