Introduction to JavaSE common classes and methods (with code)

The content of this article is an introduction to common JavaSE classes and methods (with code). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1. Use for comparison of basic data types: ==

2. Use for comparison of reference data types: equals method

If reference data types use == comparison, the comparison will be Is the address value

toStringClass

Object calling toString() needs to override this method: In the encapsulated class, otherwise the address will be output

equalsMethod

'Object' calls equals() and needs to be repeated Writing method: Rewrite in the encapsulation class, otherwise the address

StringClass## will be compared.

StringBuilderClass

##String Builder is equivalent to a buffer container in memory and will be closed as the memory is closed. Disappear, Do not create the memory address of the added characters when splicing characters in the address memory, saving memory space

StringBuilder() Constructs a string builder without characters, with an initial capacity of 16 characters.

StringBuilder(String str) Constructs a string builder initialized to the specified string content

StringBuilder sb = new StringBuilder();

public StringBuilder append (any type): Add data and return the object itself (chained calls are supported).

public StringBuilder reverse(): Reverse the character sequence

public String toString(): Return the string representation of the data in this sequence. Convert to String

Disadvantages of the append method: it can splice any type, but after the splicing is completed, it becomes a string

public static String toString(int[] a): Convert the array into a string

public static void sort(int[] a): Sort the array in ascending order

Convert the wrapper class to the String class

The int type can be directly spliced ​​into a String type.

int->String

1 ""

String.valueOf() method can Convert basic type data to String type

String.valueOf(data);

Packaging class.ParseXXX method can convert basic type to String type. Note that basic types must be converted into corresponding packages. Class, the following is an example of converting int to String

int->String (key)

Integer.parseInt("100")

In java, there is a java.util.Date, which represents date and time, accurate to milliseconds.

Construction method of Date class:

Date() No-parameter construction Method: Create a Date object based on the current system time

Date(long date): Create a Date object based on the specified millisecond value. The specified millisecond value, the millisecond value that has elapsed since January 1, 1970 (the computer's base time)

Common methods:

public long getTime() Convert the date object into the corresponding Time value in milliseconds.

void setTime(long time) Set this Date object to the millisecond value that has elapsed since 00:00:00 on January 1, 1970

//请打印出1970年1月2号的时间的对象
　　　　Date date2 = new Date(24 * 60 * 60 * 1000);
　　　　System.out.println(date2);
　　//获取当前时间的毫秒值
　　　　Date date = new Date();
　　　　System.out.println(date.getTime());
　　//将date，改成1970年1,月1号 　　
　　　　date.setTime(0);
　　　　System.out.println(date);

SimpleDateFormatClass

You can use the DateFormat class, but it is an abstract class, so we have to use its subclass SimpleDateFormat constructor

SimpleDateFormat(String pattern) Constructs a SimpleDateFormat using the given pattern. The default date format symbol is the default FORMAT locale.

Parameter pattern is the pattern

Letter pattern: y represents surface M represents month d represents day H represents hour m represents minute s represents second S represents millisecond

Chinese time: March 11, 2019 11:09:33 seconds 333 milliseconds

Alphabet pattern of code: yyyy year MM month dd day HH point mm minute ss second SSS millisecond

Member method:

Formatting (Date-> Text): Date -- String

public final String format(Date date)

Parsing (Text-> Date): String -- Date

public Date parse(String source)

//根据系统时间创建Date对象
        Date date = new Date();
        System.out.println(date);

        //date不好看，格式化为中国式时间
　　　　 //SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日  HH点mm分ss秒 SSS毫秒");
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM-dd  HH:mm:ss");
        //将date格式化为String
        String time = sdf.format(date);
        System.out.println(time);

        //注意，我们一般人不会记忆毫秒值，能不能根据具体的时间(2019-03-11  11:16:02)解析成毫秒值
        //ParseException: Unparseable date: "2018年03-11  11:18:57",注意，模式必须与之前一致
        Date date1 = sdf.parse("2018年03-11  11:18:57");
        System.out.println(date1);
        System.out.println(date1.getTime());

CalendarClass

Calendar is abstract Class, due to language sensitivity, the Calendar class is not created directly when creating an object, but is created through a static method and returns a subclass object

According to the API document of the Calendar class, common methods are:

public int get(int field): Returns the value of the given calendar field.

public void set(int field, int value): Set the given calendar field to the given value.

public abstract void add(int field, int amount): Add or subtract the specified amount of time to a given calendar field according to the rules of the calendar.

public Date getTime(): Returns a Date object representing this Calendar time value (the millisecond offset from the epoch to the present).

The Calendar class provides many member constants, representing a given calendar field:

import java.util.Calendar;
public class CalendarUtil {
    public static void main(String[] args) {
        //get方法
        // 创建Calendar对象
        Calendar cal = Calendar.getInstance();
        // 设置年 
        int year = cal.get(Calendar.YEAR);
        // 设置月
        int month = cal.get(Calendar.MONTH) + 1;
        // 设置日
        int dayOfMonth = cal.get(Calendar.DAY_OF_MONTH);
        //set方法
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, 2020);
        //add方法
        cal.add(Calendar.DAY_OF_MONTH, 2); // 加2天
        cal.add(Calendar.YEAR, -3); // 减3年
        //getTime方法
        Date date = cal.getTime();
    }    
}

public static long currentTimeMillis(): Returns the current time in milliseconds.

import java.util.Date;

public class SystemDemo {
    public static void main(String[] args) {
           //获取当前时间毫秒值
        System.out.println(System.currentTimeMillis()); // 1516090531144
    }
}

import java.util.Arrays;
public class Demo11SystemArrayCopy {
    public static void main(String[] args) {
        int[] src = new int[]{1,2,3,4,5};
        int[] dest = new int[]{6,7,8,9,10};
        System.arraycopy( src, 0, dest, 0, 3);
        /*代码运行后：两个数组中的元素发生了变化
         src数组元素[1,2,3,4,5]
         dest数组元素[1,2,3,9,10]
        */
    }
}

Random random = new Random();
        /*for (int i = 0; i < 10; i++) {
            System.out.println(random.nextInt());
        }*/
        /*for (int i = 0; i < 10; i++) {
            int j = random.nextInt(10);
            System.out.println(j);
        }*/
        //来一个随机值，这个数据的范围必须是1~100,33~66 54~78
        //random.nextInt(100);//0~99 +1 -> 1~100
        /*33~66 - 33 -> 0~33
        for (int i = 0; i < 10; i++) {
            System.out.println(random.nextInt(34) + 33);
        }*/
        //54~78 - 54 -> 0~24
        for (int i = 0; i < 10; i++) {
            System.out.println(random.nextInt(25) + 54);
        }

public class Student implements Comparable<Student>{
    ....
    @Override
    public int compareTo(Student o) {
        return this.age-o.age;//升序
    }
}

ArrayList<String> list = new ArrayList<String>();
        list.add("cba");
        list.add("aba");
        list.add("sba");
        list.add("nba");
        //排序方法  按照第一个单词的降序
        Collections.sort(list, new Comparator<String>() {
            @Override
            public int compare(String o1, String o2) {
                int rs = o2.getCj() - o1.getCj();
                return rs==0 ? o1.getAge()-o2.getAge():rs;
//                return o2.charAt(0) - o1.charAt(0);
            }
        });
        System.out.println(list);

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