J2SE basics: 8. Common System Class 2: j2se Basics

J2SE basics: 8. Common System Class 2: j2se Basics
1: transformation between basic data and encapsulation types

A: Basic Data Type ---> encapsulation type (object type)

Boolean boolean_1 = new Boolean (true );

Byte ----> Byte

Short ----> Short

Char ----> Character

Int ---> Integer

Long --> Long

Float --> Float

Double --> Double

B: encapsulation type ---> basic type

Integer. intValue ---> int

Folat. floatValue ---> float

Double. doubleValue ---> Double

2: conversion between basic types and strings

Call the String. valueOf () method.
String str = String. valueOf (any data type );

3: Convert string to basic data type

Call the parse method of the encapsulation class.
Int I = Integer. parseInt ("100 ");
Double d = Double. parseDouble ("12.334 ");

4: some common Characeter methods:

Character mainly works with the String object to perform some simple verification on the String object.
String. chatAt (I );

Simple data verification

Java. util. regex package

Pattern: compile a regular expression.

Matcher: checks whether the string matches the regular expression.

5: The String object has the length method. The array has the length attribute.


Math object

1: generate random number

A: use Math object: Math. random (); to obtain A random number between 0 and 1. This random number is always less than 0 and less than 1 decimal number.

B: Use a Random object.

2: decimal operation

Math. round (): the decimal point is not retained, and the numeric value is rounded down.

12.65 ---> 13

Math. ceil (): retain the decimal place to obtain the ceiling closest to the number.

12.65 --> 13.0

Math. floor (): the floor with the nearest digit is retained.

12.65 --> 12.0

Date object

1: how to get the system time:

A: System. currentTimeMillis:

It indicates the number of milliseconds that have elapsed since January 1, lunar January 1970 yen.

B: java. util. Date ();

Subclass: java. SQL. Date: The year, month, and day when the Java program processes the Date fields in the database.

Java. SQL. Time: the Time, minute, and second used by Java programs to process the date fields in the database.

Java. SQL. TimeStamp: The year, month, hour, minute, and second used by Java programs to process date fields in the database.

C: Calendar class:

Calendar

Mutual conversion between the three objects.

Between Long and Date.

Long ---> Date

Long l = 20171110888578l;

Date date = new Date (l );

Date ---> Long

Date date = new Date ();

Long time_long = date. getTime ();

Conversion between Long and Calendar

Long ---> Calendar

Long long_time = 121110864656l;

Calendar calendar = Calendar. getInstance ();

Calendar. setTimeInMillis (long_time );

Calendar ---> Long

Calendar calendar = Calendar. getInstance ();

Long time_long = calendar. getTimeInMillis ();

Conversion between Date and Calendar

Date ---> Calendar

Calendar calendar = Calendar. getInstance ();

Calendar. setTime (new Date ());

Calendar ---> Date

Calendar calendar = Calendar. getInstance ();

Date date = calendar. getTime ();

Formatting time

Format ---> DateFormat ---> SimpleDateFormat ();

Convert date to string

Call SimpleDateFormat. format

String to date

Call SimpleDateFormat. parse.

Precise computing object

BigInteger: exact integer calculation

BigDecimal: exact decimal calculation.

Truncation and rounding.


Normal format

NumberFormat numberFormat = NumberFormat. getInstance ();

Percent format

NumberFormat numberFormat = NumberFormat. getPercentInstance ();

// Set the Maximum/minimum reserved integer bits.
NumberFormat. setMaximumIntegerDigits (4 );
NumberFormat. setMinimumIntegerDigits ();

// Set the Maximum/minimum number of digits after the decimal point
NumberFormat. setMaximumFractionDigits (2 );
NumberFormat. setMaximumFractionDigits ();
Loan currency format
NumberFormat numberFormat = NumberFormat. getCurrencyInstance ();

DecimalFormat

Number ---> string (rounding function)
DecimalFormat decimalFormat = new DecimalFormat (".##");
String result = decimalFormat. format (d );

The basis of J2EE is J2SE. Question: What is the basis of J2SE? For example, the form is not important.

Collection operations. (Very important !!!)
File IO operations. (Very important !!)
In addition, there are some common classes and methods (very important !! But it is mainly accumulated in work practices)
(For example, type conversion, date type, reflection, polymorphism, parsing configuration files, etc)
Thread. (Depending on the company's specific business to determine the importance, generally grasp the line)
Abnormal operations and custom exceptions. (The general requirements of companies are not too high)
In addition, database operations should also be mastered.
Java internal class (just understand the concept. It is an advanced usage and I feel that it is rarely used)

These are my work experiences.

What are J2SE basics?

1 Java Introduction 2 basic syntax 3 object-oriented 4 exception handling 5 arrays 6 common Class 7 container 8io stream 9 thread 10 network 11GUI