1. Introduction

System is a system class. In the java.lang package of JDK, it can be seen that it is also a core language feature of java. The constructor of the System class is decorated with private and is not allowed to be instantiated. Therefore, the methods in the class are also static methods modified by static.

The Arrays class in JAVA is a tool class that implements array operations. It includes a variety of static methods that can implement array sorting and search, array comparison, and adding elements to the array. Functions such as copying and converting arrays to strings. These methods have overloaded methods for all basic types.

2. Detailed explanation of knowledge points

1. Concept

In the API, the introduction of the System class is relatively simple. We give the definition. System represents the system where the program is located and provides Corresponding system attribute information and system operations.

2. Commonly used methods

• (1) public static void gc(): Used to run the garbage collector in the JVM and complete the memory Cleaning up garbage

• (2) public static void exit(int status): Used to end the running Java program. Just pass in a number as a parameter. Usually 0 is passed in as normal status, and others are abnormal status

• (3) public static long currentTimeMillis(): Get the current system time and January 1970 The millisecond difference between 00:00 on 01st

• (4) public static Properties getProperties(): is used to obtain the specified key (string name) System property information recorded in

Code demonstration:

package com.Test;
import Test2.MyDate;
import java.awt.*;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Properties;

public class Main {

private final static String name = "磊哥的java歷險(xiǎn)記-@51博客";

/*
*public static void gc() //回收垃圾
*public static void exit(int status) //退出程序，0為正常狀態(tài)，其他為異常狀態(tài)
*public static long currentTimeMillis() //獲取當(dāng)前時(shí)間毫秒值
*public static Properties getProperties() //獲取某個(gè)屬性信息
*/
public static void main(String[] args) {
//構(gòu)造方法被私有 不能創(chuàng)建對(duì)象
//System sy = new System();
//public static void exit(int status) //退出程序，0為正常狀態(tài)，其他為異常狀態(tài)
// System.out.println("我要退出了！");
// System.exit(0);
// System.out.println("我已經(jīng)退出了！");
//public static long currentTimeMillis() //獲取當(dāng)前時(shí)間毫秒值
long timeMillis = System.currentTimeMillis();
long time = new Date().getTime();
long timeInMillis = Calendar.getInstance().getTimeInMillis();
System.out.println(timeMillis);
for(int i = 0; i < 5; i++) {
System.out.println("i love java");
}
long timeMillis2 = System.currentTimeMillis();
System.out.println(timeMillis2-timeMillis);
//publicstatic Properties getProperties() //獲取某個(gè)屬性信息
Properties properties = System.getProperties();
System.out.println(properties);
System.out.println("============="+name+"=============");
}
}

3. Precautions

The System class cannot create objects manually because the construction method is privately modified, preventing the outside world from creating objects. All methods in the System class are static and can be accessed by class name. In the JDK, there are many such classes.

4. Arrays class

The Arrays class is a tool class provided by jdk specifically for operating arrays, located in the java.util package.

4.1. Commonly used methods of the Arrays class

• (1) toString() method of Arrays - returns the string representation of the contents of a specified array..?

• (2) Arrays copyOf () //Copy the specified array, intercept or fill with null (if necessary), so that the copy has the specified length.?

• (3) Arrays sort() // Sort the specified type array in ascending numerical order.

• (4) Arrays binarySearch () //Use binary search method to search the specified type array to obtain the specified value //Must be ordered

• (5)Arrays fill() //Assign the specified type value to each element in the specified range of the specified type array

• If two arrays of a given type mutually If equal, return true.

Code Demo:

package com.Test;
import java.util.Arrays;
/* Arrays toString （） //返回指定數(shù)組內(nèi)容的字符串表示形式。
Arrays copyOf （） //復(fù)制指定的數(shù)組，截取或用 null 填充（如有必要），以使副本具有指定的長(zhǎng)度。
Arrays sort() //對(duì)指定的類(lèi)型數(shù)組按數(shù)字升序進(jìn)行排序。
Arrays binarySearch （） //使用二分搜索法來(lái)搜索制定類(lèi)型數(shù)組，以獲得指定的值 //必須有序
Arrays fill（） //將指定的類(lèi)型值分配給指定 類(lèi) 型數(shù)組指定范圍中的每個(gè)元素
Arrays equals() //如果兩個(gè)指定的類(lèi)型數(shù)組彼此相等，則返回 true。*/
public class Test{
private final static String name = "磊哥的java歷險(xiǎn)記-@51博客";
public static void main(String args[]){

//定義數(shù)組
int[] score={1,2,3};
int[] scores={1,2,3};
//數(shù)組之間比較，長(zhǎng)度，值相等，則認(rèn)為兩個(gè)數(shù)組相等，返回布爾值
System.out.println("比較值和長(zhǎng)度："+Arrays.equals(score,scores));
//判斷地址
if(score==scores){
System.out.println("score和scores比較，相等");
}else{
System.out.println("score和scores比較，不相等");
}
//定義二維數(shù)組
int[][] sc={{222,333,1,2,0},{1,2,3,2,0}};
//排序
Arrays.sort(sc[1]);
System.out.println("排序："+Arrays.toString(sc[1]));
System.out.println("按照下標(biāo)取值:"+sc[0][1]+" ");

//定義數(shù)據(jù)se
int[] se={1,2,3,4,5};
//填充數(shù)組
Arrays.fill(se,0);
System.out.println("填充："+Arrays.toString(se));
//復(fù)制值到sx，增加指定長(zhǎng)度
int[] sx=Arrays.copyOf(se,2);
//輸出sx的填充后的值
System.out.println("復(fù)制2："+Arrays.toString(sx));
int[] xb={14,20,67,34,33,23,10};
//排序xb
Arrays.sort(xb);
System.out.println(Arrays.toString(xb));
//在排序后，通過(guò)二分查找，找到34的元素，并返回下標(biāo)
int index1=Arrays.binarySearch(xb,34);
System.out.println("二分法取值："+index1);
System.out.println("============="+name+"=============");
}
}

Title:

• (1) Create an int type array A, the value of A is {1,2,3,4,5}

• (2) Copy the value of A into B with a length of 6

• (3) Compare whether A and B are the same

Experimental steps:

• (1) Declare a class Test and create two arrays

• (2) Use Arrays related methods to complete the operation

Code demonstration:

package com.Test;

import java.util.Arrays;
/*聲明一個(gè)類(lèi)Test,并且創(chuàng)建兩個(gè)數(shù)組*/
/* Arrays toString （） //返回指定數(shù)組內(nèi)容的字符串表示形式。
Arrays copyOf （） //復(fù)制指定的數(shù)組，截取或用 null 填充（如有必要），以使副本具有指定的長(zhǎng)度。
Arrays sort() //對(duì)指定的類(lèi)型數(shù)組按數(shù)字升序進(jìn)行排序。
Arrays binarySearch （） //使用二分搜索法來(lái)搜索制定類(lèi)型數(shù)組，以獲得指定的值 //必須有序
Arrays fill（） //將指定的類(lèi)型值分配給指定 類(lèi) 型數(shù)組指定范圍中的每個(gè)元素
Arrays equals() //如果兩個(gè)指定的類(lèi)型數(shù)組彼此相等，則返回 true。*/
public class Main {
private final static String name = "磊哥的java歷險(xiǎn)記-@51博客";
public static void main(String[] args){
//創(chuàng)建int類(lèi)型數(shù)組A，A的值為{1,2,3,4,5}
int[]A = new int[]{1,2,3,4,5};
//將A的值拷貝進(jìn)長(zhǎng)度為6的B中
int[]B = Arrays.copyOf(A, 6);
//比較A和B是否相同
System.out.println("兩個(gè)數(shù)組是否相等："+Arrays.equals(A, B));
System.out.println("============="+name+"=============");
}
}

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