java成神之——ImmutableClass,null检查,字符编码,defaultLogger,可变参数,JavaScriptEngine,2D图,类单例,克隆,修饰符基本操作

• ImmutableClass
• null检查
• 字符编码
• default logger
• 函数可变参数
• Nashorn JavaScript engine
  • 执行脚本文件
  • 改变js文件输出流
  • 全局变量
• 2D图
• 类单例
• 克隆
• 修饰符
• 结语

ImmutableClass

一旦初始化，值无法修改
主要是用于多线程之间的数据共享，避免数据污染

class不设置setter即可实现Immutable Class

public class ImmutableIntArray {
    private final int[] array;
    public ImmutableIntArray(int[] array) {
        this.array = array.clone();
    }
    public int[] getValue() {
        return this.clone();
    }
}

null检查

对象null检查
Object obj = null;
Objects.isNull(obj); // true
Objects.nonNull(obj); // false

集合null检查

List<Object> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(null);
list.add(3);
list.stream().filter(Objects::nonNull).forEach(item -> {
    System.out.println(item);
}); // 过滤null值

字符编码

BufferedWriter wr = Files.newBufferedWriter(Paths.get("demo.txt"),StandardCharsets.UTF_8)
BufferedReader reader = Files.newBufferedReader(Paths.get("demo.txt"),StandardCharsets.UTF_8)
String s = new String(Files.readAllBytes(Paths.get("demo.txt")), StandardCharsets.UTF_8);
byte[] bytes = "aaaa".getBytes(StandardCharsets.UTF_8);

default logger

private static final Logger logger = Logger.getLogger(TestController.class.getName());
logger.log(Level.INFO, String.format("%s: %s", "输出", "日志"));
logger.warning(String.format("%s: %s", "输出", "日志"));

日志等级
SEVERE
WARNING
INFO
CONFIG
FINE
FINER
FINEST

函数可变参数

void fn(int... x) {
    for (int item : x) {
        System.out.println(item);
    }
}
fn(1,2,3,4,5);

Nashorn JavaScript engine

执行脚本文件

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");
engine.eval(new FileReader("C:\Users\26401\Desktop\" + "demo.js"));

改变js文件输出流

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");
StringWriter stringWriter = new StringWriter();
engine.getContext().setWriter(stringWriter);
engine.eval(new FileReader("C:\Users\26401\Desktop\" + "demo.js"));
return stringWriter.toString();

全局变量

engine.put("global", "globaldata");
engine.eval("print(global);");

engine.get("global");

2D图

package com.springlearn.learn.model;
import javax.swing.*;
import java.awt.*;
// MyPanel extends JPanel, which will eventually be placed in a JFrame
public class MyPanel extends JPanel {
    // custom painting is performed by the paintComponent method
    @Override
    public void paintComponent(Graphics g){
        // clear the previous painting
        super.paintComponent(g);
        // cast Graphics to Graphics2D
        Graphics2D g2 = (Graphics2D) g;
        g2.setColor(Color.red); // sets Graphics2D color
        // draw the rectangle
        g2.drawRect(0,0,100,100); // drawRect(x-position, y-position, width, height)
        g2.setColor(Color.blue);
        g2.fillRect(200,0,100,100); // fill new rectangle with color blue
    }
}

public class Test{
    JFrame f;
    MyPanel p;

    public Test(){
        f = new JFrame();
        // get the content area of Panel.
        Container c = f.getContentPane();
        // set the LayoutManager
        c.setLayout(new BorderLayout());
        p = new MyPanel();
        // add MyPanel object into container
        c.add(p);
        // set the size of the JFrame
        f.setSize(400,400);
        // make the JFrame visible
        f.setVisible(true);
        // sets close behavior; EXIT_ON_CLOSE invokes System.exit(0) on closing the JFrame
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static void main(String[] args) throws Exception {
		DemoApplication t = new DemoApplication(); 
	}
}

类单例

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return INSTANCE;
    }
}

public class Singleton {
    private static class InstanceHolder {
        static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getInstance() {
        return InstanceHolder.INSTANCE;
    }
    private Singleton() {}
}

克隆

public class Test implements Cloneable {
    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

Test test = new Test();
Test testcopy = (Test) test.clone();

修饰符

常用修饰符

    public 公开
    protected 父类和子类可用
    default 不写权限关键字，默认是default，只能在当前package下是使用
    private 本类可用
    final 表示固定不变的
        final修饰的类不能被继承
        final修饰的方法不能被子类重写
        final修饰的变量就是常量，不能修改 final int j = 11;
    static 表示静态，只能通过类本身访问

其他修饰符

    strictfp 浮点计算 
        strictfp class A{}
    volatile 多线程中，读取最新值
        volatile boolean active;
    synchronized 只允许一个线程操作
        synchronized void SharedMethod(){}
        void SharedMethod(){
            synchronized (this){
                ...
            }
        }
    transient 不会被序列化
        public transient int limit = 55; 

结语

本文章是java成神的系列文章之一

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