java抢救计划！

在键盘上读取：

Scanner sc = new Scanner(System.in);
int a  = sc.nextInt();

定义数组

int qwq[] = new int[105];

生成随机数：

//包含java.util.Random;
//生成0-9的随机数
int a = new Random().nextInt(10);

注意 static方法可以直接调用 不必创建对象

GUI部分

窗口居中（JFrame）

import java.util.*;
import java.awt.*;
import javax.swing.*;

public class Main
{
	public static void main(String[] args)
	{
		JFrame test = new JFrame("测试窗口(居中)");
		test.setSize(300,300);
		Dimension a  = Toolkit.getDefaultToolkit().getScreenSize();
		test.setLocation((a.width - test.getWidth()) / 2,(a.height - test.getHeight())/2);
		test.setVisible(true);
		test.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	}
}

加法计算器（带有异常处理）

import javax.management.OperationsException;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.IOException;

public class Main extends JFrame
{
	public Main()
	{
	super("计算器");
	JPanel p = new JPanel();
	p.setLayout(new FlowLayout());
	
	
	JTextField yi = new JTextField(5);
	JTextField jia = new JTextField("+");
	jia.setEditable(false);
	JTextField er= new JTextField(5);
	JButton dengyu = new JButton("=");
	JTextField daan = new JTextField(5);
	daan.setEditable(false);
	
	p.add(yi); p.add(jia); p.add(er); p.add(dengyu); p.add(daan);
	
	this.add(p);
	
	this.setSize(300,300);
	Dimension  a = Toolkit.getDefaultToolkit().getScreenSize();
	Dimension b = this.getSize();
	this.setLocation((a.width-b.width)/2,(a.height - b.height)/2);
	this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	
	
	class Dengyuhao implements ActionListener
	{
		public void actionPerformed(ActionEvent e)
		{
			String a,b,c;double a1,b1,c1;
			
			a=yi.getText();
			b=er.getText();
			
			try
			{
				a1= Double.parseDouble(a);
				b1=Double.parseDouble(b);
				
			}
			catch(Exception e1)
			{
				daan.setText("错误");
				return;
			}
			c1=a1+b1;
			c=String.valueOf(c1);
			daan.setText(c);
		}
	}
	Dengyuhao dengyu1=new Dengyuhao();
	dengyu.addActionListener(dengyu1);
}
	public static void main(String[] args)
	{
		Main frame=new Main();
		frame.setVisible(true);
	}
}

文件读写

字符流：

import java.io.*;

public class Main {

public static void main(String[] args) {
		try
		{
			FileWriter fw=new FileWriter("D:/out.txt");
			for(int i=1;i<=100;i++)
			fw.write(i+"");
			
			fw.close();
			
			
			FileReader fr = new FileReader("D:/out.txt");
	        int cnt;  
	        while((cnt = fr.read()) != -1)
	        {  
	            System.out.print((char)cnt);  
	        }  
	        fr.close();
		}
		catch(IOException  e)
	{
		System.out.println("错误");
	}
}
}

字节流

import java.io.*;

public class Main {
	   public static void main(String[] args){
	    	File file1 = new File("d:\\out.txt");
	        try {
	            FileOutputStream fout = new FileOutputStream("d:\\out.txt");
	            for(int i=1;i<=100;i++) {
	            byte[] str=String.valueOf(i).getBytes();
	            fout.write(str);
	            fout.write(13);
	            fout.write(10);     
	            }
	            fout.close();
	        } catch (Exception e) 
	        {
	            e.printStackTrace();
	        }
	        try {
	        	FileInputStream fin = new FileInputStream("d:\\out.txt");
	            int data;
	            data = fin.read();
	        	while(data!=-1) {
	        		System.out.print((char)data);
	        		data = fin.read(); 
	        	}
	        } catch (Exception e) {
	            e.printStackTrace();
	        }
	    }
}