JAVA学习札记

字符串操作：

字符串分割：String array[] = str.split(" "); 

字符串截取：str.substring(0,2);

字符串比较：str1.equals(str2)

数组操作：

array定义：type[] 变量名 = new type[数组中元素的个数]; //eg: int[] a = new int[10];

                type变量名[] = new type[数组中元素的个数]; //eg: int a[] = new int[10];

                type[] 变量名 = new type[]{逗号分隔的初始化值}; //eg: int[] a = new int[]{1,2,3,4}; 或 int[] a = {1,2,3,4};

二维数组：  type[][] i = new type[2][3];（推荐）或：type i[][] = new type[2][3];

列表操作：

List定义：List<String> list=new ArrayList();

List赋值：list.add("apple");

List移除元素：list.remove("apple");

ArrayList初始化：ArrayList<String> list = new ArrayList(Arrays.asList("apple", "banana"));

判断是否包含某元素：list.contains("apple")

List长度：list.size();

List遍历：

方法（1）：for(String str:list){System.out.println(str);}

方法（2）：for (Iterator<String> s = list.iterator(); s.hasNext();) {String str=s.next(); 
            System.out.println(str);}

方法（3）：for(int i=0;i<list.size();i++){System.out.println(list.get(i));}

hashmap操作：

HashMap定义：HashMap<String, Double> map = new HashMap<String, Double>(); 

HashMap赋值：map.put("key1", "value1");

判断是否包含key：map.containsKey("apple")

获取key-value：map.get("apple")

正则匹配：

Pattern pattern=Pattern.compile("[^a-zA-Z]");

Matcher matcher=pattern.matcher(str);

while(matcher.find()){System.out.println(matcher.group());}

JAVA读文件：

1.按行读取文件内容

package readFile;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class Main {
    public static void main(String[] args) throws FileNotFoundException,IOException{
        FileReader reader = new FileReader("resource/test.txt"); //input path
        BufferedReader br = new BufferedReader(reader);
        String str = null;
        while((str = br1.readLine()) != null) {
                System.Out.println(str);
        }
　　　　 br.close();
    }
}

JAVA 写文件

package writeFile;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException; 

public class Main {
    public static void main(String[] args) throws FileNotFoundException,IOException{
    BufferedWriter writer = new BufferedWriter(new FileWriter(new File("resource/wt.txt")));//write path
    //FileWriter writer = new FileWriter("resource/wt.txt");//该方法也可以
    writer.write("hello	world
");
    writer.close()
    }    
}            

PS:若希望输入的新内容不覆盖文件已有的内容，则改为

BufferedWriter writer = new BufferedWriter(new FileWriter(new File("resource/wt.txt",true)));或

FileWriter writer = new FileWriter("resource/wt.txt",true);