Introduction to tool classes in Java language

Java is a widely used programming language with a wealth of built-in libraries and tool classes. These tool classes provide Java developers with many convenient functions and accelerate the software development process. In this article, we will introduce several common tool classes in the Java language, as well as their uses and sample code.

1. ArrayList class in java.util package

ArrayList is one of the very common tool classes in Java, used to create dynamic arrays. It can adjust the size of the array at runtime and is an array with variable capacity. ArrayList provides many methods for operating lists, such as adding, deleting, getting elements, etc. The following is a sample code:

import java.util.ArrayList;

public class ArrayListExample {
   public static void main(String[] args) {
      ArrayList<String> list = new ArrayList<String>();
      
      // 添加元素
      list.add("Hello");
      list.add("World");
      list.add("Java");
      
      // 删除元素
      list.remove("Java");
      
      // 获取元素
      String element = list.get(1);
      System.out.println(element);
   }
}

The output result is:

World

2. HashMap class in the java.util package

HashMap is used in Java Utility class for storing key-value pairs. It is implemented based on a hash table and provides O(1) constant time complexity to perform put() and get() operations. HashMap also provides many other useful methods, such as checking for contained keys, getting the number of keys, etc. The following is a sample code:

import java.util.HashMap;

public class HashMapExample {
   public static void main(String[] args) {
      HashMap<String, Integer> map = new HashMap<String, Integer>();
      
      // 添加键值对
      map.put("John", 30);
      map.put("Mary", 25);
      map.put("Bob", 40);
      
      // 获取值
      int age = map.get("John");
      System.out.println(age);
      
      // 检查是否有键
      boolean exist = map.containsKey("Mary");
      System.out.println(exist);
   }
}

The output result is:

30
true

3. File class in the java.io package

File is used in Java Utility classes for operating files and directories. It provides many methods for operating files and directories, such as creating files, renaming files, obtaining file information, etc. The following is a sample code:

import java.io.File;
import java.io.IOException;

public class FileExample {
   public static void main(String[] args) {
      File file = new File("C:/example.txt");
      
      try {
         // 创建文件
         boolean isCreated = file.createNewFile();
         System.out.println(isCreated);
         
         // 重命名文件
         File newFile = new File("C:/newexample.txt");
         boolean isRenamed = file.renameTo(newFile);
         System.out.println(isRenamed);
         
         // 获取文件信息
         boolean isFile = file.isFile();
         long size = file.length();
         System.out.println(isFile);
         System.out.println(size);
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

The output result is:

true
true
true
0

4. SimpleDateFormat class in the java.text package

SimpleDateFormat is used in Java Utility class for formatting dates and times. It can parse a date string into a date object, or format a date object into a specified string format. The following is a sample code:

import java.text.SimpleDateFormat;
import java.util.Date;

public class SimpleDateFormatExample {
   public static void main(String[] args) {
      SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
      Date date = new Date();
      
      // 格式化日期对象
      String dateString = format.format(date);
      System.out.println(dateString);
      
      // 解析日期字符串
      String dateString2 = "2021-08-01 15:30:45";
      try {
         Date date2 = format.parse(dateString2);
         System.out.println(date2);
      } catch (ParseException e) {
         e.printStackTrace();
      }
   }
}

The output is:

2021-08-01 15:59:40
Sun Aug 01 15:30:45 CST 2021

These tool classes are only part of the Java language, and there are many other useful tool classes that can speed up the development process. If you want to know more about Java tool classes, you can refer to Oracle official documentation or other related books.

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