7.20實習培訓日誌-Java基礎程序設計結構

時間 2020-06-22

原文原文鏈接

Java基礎程序設計結構

在 Math 類中，爲了達到最快的性能，全部的方法都使用計算機浮點單元中的例程，若是獲得一個徹底可預測的結果比運行速度更重要的話，那麼就應該使用StrictMath類，它使用「自由發佈的 Math 庫」（ fdlibm ) 實現算法，以確保在全部平臺上獲得相同的結果。html

不要在boolean與任何數值類型之間進行強制類型轉換,防止錯誤java

不要用== 運算符測試字符串的相等性：只有字符串常量是共享的，而 + 或 substring等操做產生的結果並非共享的。算法

不要使用 char 類型，太過底層，UTF-16須要兩個代碼單元，用charAt會有問題。編程

在循環中，檢測兩個浮點數是否相等須要格外當心。json

從 Java SE 7 開始， case 標籤還能夠是字符串字面量數組

帶標籤的break服務器

int j = 10;
breakpoint:
for (int i=0;i<10;i++){
    while (j-->0){
        if(j==5){
            break breakpoint;
        }
    }
}
System.out.println(j);

能夠將標籤應用到任何語句中，甚至能夠應用到 if 語句或者塊語句中網絡

只能跳出語句塊,而不能跳入語句塊dom

Biglnteger 類實現了任意精度的整數運算， BigDecimal 實現了任意精度的浮點數運算socket

數組長度爲 0 與 null 不一樣

Arrays.copyOf ：一個數組的全部值拷貝到一個新的數組中

運行時常量池從方法區中分配

Java Stream

java8--List轉爲Map、分組、過濾、求和等操做

Map<Integer, List<Integer>> map = random.ints(size, 0, bound).boxed().sorted().collect(Collectors.groupingBy(value->value/10));

Java的網絡通訊

XML操做

定義

XML 指可擴展標記語言

XML 被設計用來傳輸和存儲數據。

語法規則

全部 XML 元素都須有關閉標籤
XML 標籤對大小寫敏感
XML 必須正確地嵌套
XML 文檔必須有根元素
XML 的屬性值須加引號
在 XML 中，空格會被保留

Java 處理 XML 的三種主流技術及介紹

Json操做

JavaScript對象表示法(JavaScript Object Notation)，是輕量級的文本數據交換格式

JSON 是存儲和交換文本信息的語法。相似 XML。

JSON 比 XML 更小、更快，更易解析。

public static void readJson(){
    try {
        JsonParser jsonParser = new JsonParser();
        JsonObject jsonObject = (JsonObject) jsonParser.parse(new FileReader("test.json"));
        System.out.println(jsonObject.get("cat").getAsString());
        System.out.println(jsonObject.get("pop").getAsBoolean());

        JsonArray array = jsonObject.get("book").getAsJsonArray();
        for (int i=0;i<array.size();i++) {
            System.out.println("====");
            JsonObject subJson = array.get(i).getAsJsonObject();
            System.out.println(subJson.get("id").getAsInt());
            System.out.println(subJson.get("name").getAsString());
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    }
}

HTTP操做

使用http的Get讀取網絡數據

public static void doGet(){
    URL url = null;
    try {
        url = new URL("http://www.dengyouquan.cn");
        URLConnection connection = url.openConnection();
    } catch (MalformedURLException e) {
        e.printStackTrace();
    }catch (IOException e) {
        e.printStackTrace();
    }
    try(BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream()));) {
        String line = null;
        while ((line=reader.readLine())!=null){
            System.out.println(line);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

使用http的Post與網絡通訊

public static void doPost() {
    URL url = null;
    HttpURLConnection connection = null;
    try {
        url = new URL("http://www.dengyouquan.cn");
        connection = (HttpURLConnection) url.openConnection();
        connection.addRequestProperty("encoding", "UTF-8");//添加請求屬性
        connection.setDoInput(true);//容許輸入
        connection.setDoOutput(true);//容許輸出
        connection.setRequestMethod("POST");//POST請求
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    if (connection == null)
        return;
    //writer與reader順序一致
    try (BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(connection.getOutputStream()));
            BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
    ) {
        writer.write("?p=6");
        writer.flush();
        String line = null;
        while ((line = reader.readLine()) != null) {
            System.out.println(line);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

使用HttpClient進行Get通訊

public static void doHttpClientGet(){
    HttpClient httpClient = HttpClientBuilder.create().build();
    HttpGet httpGet = new HttpGet("http://www.dengyouquan.cn");
    try (CloseableHttpResponse httpResponse = (CloseableHttpResponse) httpClient.execute(httpGet);){
        HttpEntity entity = httpResponse.getEntity();
        if(entity!=null){
            String  entityStr= EntityUtils.toString(entity,"utf-8");
            System.out.println(entityStr);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

使用HttpClient進行Post通訊

public static void doHttpClientPost(){
    try {
        HttpClient httpClient =  HttpClientBuilder.create().build();
        HttpPost httpPost = new HttpPost("http://www.dengyouquan.cn");
        List<BasicNameValuePair> params = new ArrayList<>();
        params.add(new BasicNameValuePair("p","6"));
        httpPost.setEntity(new UrlEncodedFormEntity(params,"UTF-8"));
        HttpResponse httpResponse = httpClient.execute(httpPost);
        HttpEntity httpEntity = httpResponse.getEntity();
        System.out.println(EntityUtils.toString(httpEntity,"UTF-8"));
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Socket操做

Socket是應用層與TCP/IP協議族通訊的中間軟件抽象層，它是一組接口。

HTTP1.0默認是短鏈接，無狀態的.

Http1.1默認是長鏈接

半包:接受方沒有接受到一個完整的包，只接受了部分；

粘包：發送方發送的多個包數據到接收方接收時粘成一個包。

TCP：流，會有半包和粘包

UDP:數據包，無半包和粘包

ServerSocket和Socket

使用Socket時用BufferedReader會阻塞

readLine讀取socket流的時候產生了阻塞

socket通訊服務器端

Socket編程中,儘可能用PrintWriter取代BufferedWriter

PrintWriter的println方法自動添加換行，BufferedWriter須要顯示調用newLine方法

PrintWriter的print、println方法能夠接受任意類型的參數

PrintWriter和BufferedWriter的使用區別

Scanner的平均耗時是BufferedReader的10倍左右

java Scanner與BufferedReader讀取鍵盤輸入性能比較

public class ServerSocket1 {
    private static final int PORT = 8888;

    public static void main(String[] args) {
        try (ServerSocket serverSocket = new ServerSocket(PORT)) {
            System.out.println("服務器已啓動...");
            int j = 0;
            while (j++<Integer.MAX_VALUE){
                Socket socket = serverSocket.accept();
                new Thread(new ServerSocketHandler(socket)).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

class ServerSocketHandler implements Runnable {
    private Socket socket;

    ServerSocketHandler(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        try(BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()))){
            String line = null;
            System.out.println("服務器已鏈接...");
            boolean exit = false;
            while (!exit){
                System.out.println("等待消息");
                String msg = reader.readLine();
                switch (msg){
                    case "":
                        continue;
                    case "date":
                        LocalDateTime localDateTime = LocalDateTime.now();
                        System.out.println("localDateTime:"+localDateTime);
                        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy年MM月dd日");
                        writer.write(dateTimeFormatter.format(localDateTime)+System.lineSeparator());
                        break;
                    case "exit":
                        System.out.println("server close...");
                        writer.write("exit"+System.lineSeparator());
                        exit = true;
                        break;
                    default:
                        writer.write("commend not found!"+System.lineSeparator());
                }
                writer.flush();
            }
        } catch (IOException e) {
            System.out.println("客戶端異常退出。。。");
        }
    }
}

socket通訊客戶端

public class ClientSocket implements Runnable {
    private static final String ADDRESS = "127.0.0.1";
    private static final int PORT = 8888;
    public static void main(String[] args) {
        new Thread(new ClientSocket()).start();
    }

    @Override
    public void run() {
        try(Socket socket= new Socket(ADDRESS,PORT);
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
            Scanner scanner = new Scanner(System.in);){
            boolean exit = false;
            System.out.println("客戶端已鏈接...");
            while (!exit) {
                writer.write(scanner.nextLine() + System.lineSeparator());
                writer.flush();
                String line = reader.readLine();
                System.out.println("客戶端收到消息："+line);
                if(line.equals("exit")){
                    exit = true;
                }
            }
        } catch (IOException e) {
            System.out.println("無服務器");
        }
    }
}