java成神之——ImmutableClass,null檢查,字符編碼,defaultLogger,可變參數,JavaScriptEngine,2D圖,類單例,克隆,修飾符基本操做

• ImmutableClass
• null檢查
• 字符編碼
• default logger
• 函數可變參數
• Nashorn JavaScript engine
  • 執行腳本文件
  • 改變js文件輸出流
  • 全局變量
• 2D圖
• 類單例
• 克隆
• 修飾符
• 結語

ImmutableClass

一旦初始化，值沒法修改
主要是用於多線程之間的數據共享，避免數據污染

class不設置setter便可實現Immutable Class

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

null檢查

對象null檢查
Object obj = null;
Objects.isNull(obj); // true
Objects.nonNull(obj); // false

集合null檢查

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);
}); // 過濾null值

字符編碼

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", "輸出", "日誌"));
logger.warning(String.format("%s: %s", "輸出", "日誌"));

日誌等級
SEVERE
WARNING
INFO
CONFIG
FINE
FINER
FINEST

函數可變參數

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

Nashorn JavaScript engine

執行腳本文件

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

改變js文件輸出流

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();

全局變量

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

engine.get("global");

2D圖

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(); 
    }
}

類單例

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();

修飾符

經常使用修飾符

    public 公開
    protected 父類和子類可用
    default 不寫權限關鍵字，默認是default，只能在當前package下是使用
    private 本類可用
    final 表示固定不變的
        final修飾的類不能被繼承
        final修飾的方法不能被子類重寫
        final修飾的變量就是常量，不能修改 final int j = 11;
    static 表示靜態，只能經過類自己訪問

其餘修飾符

    strictfp 浮點計算 
        strictfp class A{}
    volatile 多線程中，讀取最新值
        volatile boolean active;
    synchronized 只容許一個線程操做
        synchronized void SharedMethod(){}
        void SharedMethod(){
            synchronized (this){
                ...
            }
        }
    transient 不會被序列化
        public transient int limit = 55;

結語

本文章是java成神的系列文章之一

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