每日要点

字符串Sring

Sting 是不变字符串 对字符串的修改操作会产生新的字符串对象
而不是修改原来的对象

例子1:

        String str1 = "abc";
        String str2 = new String("abc");
        System.out.println(str1 == str2);
        String str3 = new String("abc");
        String str4 = "ab" + "c";
        System.out.println(str2 == str3);
        System.out.println(str1 == str4);
        // 两个面试题：
        // 1.输出str5的结果
        // 2.下面的语句总共创建了几个字符串对象
        String str5 = 1 + 2 + "a" + "b" + 3 + 4;
        System.out.println(str5);
        String str6 = "hello, world";
        // length() - 获得字符串的长度(字符数量)
        System.out.println(str6.length());
        // charAt() - 获取指定位置的字符
        char ch = str6.charAt(1);
        System.out.println(ch);
        // compareTo() / equals() - 字符串比较
        // compareToIgnoreCase()
        System.out.println(str6.compareTo("Hello, world"));
        System.out.println(str6.compareToIgnoreCase("Hello, world"));
        System.out.println(str6.equals("Hello, world"));
        System.out.println(str6.equalsIgnoreCase("Hello, world"));
        // 字符串链接
        String str7 = str6.concat("! Goodbye!");
        System.out.println(str6);
        System.out.println(str7);
        // - 用新内容替换字符串中指定内容
        str7 = str7.replaceAll("Good", "Fuck");
        str7 = str7.replace("o", "@");
        System.out.println(str7);
        String str8 = "床前明月光,疑是地上霜,举头望明月,低头思故乡";
        // 将字符串用指定的字符拆分成多个字符串
        String[] strsArray = str8.split(",");
        for (String str : strsArray) {
            System.out.println(str);
        }
        // 从字符串中取出指定范围的子串
        String str9 = str8.substring(7);
        System.out.println(str9);
        String str10 = str8.substring(6, 11);
        System.out.println(str10);
        String str11 = "   hello   world   ";
        // 去掉字符串左右两端的空格
        System.out.println(str11.trim());
        int a = 123;
        String str12 = "" + a;
        // 静态方法 - 将基本数据类型转换成对应的字符串
        String str13 = String.valueOf(a);
        System.out.println(str12 == str13);
        System.out.println(str12.equals(str13));
        System.out.println(str12);
        System.out.println(String.valueOf(str13));
        // 检查字符串是否以指定的内容结尾
        String str14 = "program.exe";
        System.out.println(str14.endsWith("exe"));
        // 检查字符串是否以指定的内容开头
        String str15 = "http://www.baidu.com";
        System.out.println(str15.startsWith("http://"));

StingBuffer和StringBuilder

StingBuffer和StringBuilder代表可变字符串
对字符串做修改时不会创建新的字符串对象
StringBuffer是线程安全的 - 多个线程可以操作同一个StringBuffer对象
StringBuilder是线程不安全的 - 多个线程同时操作StringBuilder时会发生错误
Stringbuilder是Java 5引入的可变字符串类型 它拥有更好的性能
例子2

//      StringBuffer sb = new StringBuffer("hello");
        StringBuilder sb = new StringBuilder("hello");
        // 在字符串末尾追加新内容
        sb.append(" world!");
        sb.append(" goodbye!");
        // 在字符串指定位置插入新内容
        sb.insert(5, " good");
        sb.insert(0, "hi! ");
        System.out.println(sb);
        // 删除指定位置的字符
        sb.deleteCharAt(10);
        // 删除指定范围的字符
        // 第一个参数是开始索引(从0数)
        // 第二个参数是结束索引(从1数)
        sb.delete(0, 9);
        System.out.println(sb);
        // 字符串内容反转
        sb.reverse();
        System.out.println(sb);
        // 修改字符串指定位置的字符
        sb.setCharAt(5, '#');
        System.out.println(sb);

结论

如果需要频繁的修改一个字符串请不要使用Sring因为每次修改都有可能创建
新的字符串对象 所以在这种场景下应该使用StingBuilder或者StringBuffer

/*      String str = "";
        long start = System.nanoTime();
        for (int i = 0; i < 100000; i++) {
            str += 'a';
        }
        long end = System.nanoTime();
        System.out.println((end - start) / 1e9 + "s");*/
//      StringBuffer str = new StringBuffer();
        StringBuilder str = new StringBuilder();
        long start = System.nanoTime();
        for (int i = 0; i < 10000000; i++) {
            str.append('a');
        }
        long end = System.nanoTime();
        System.out.println((end - start) / 1e9 + "s");

练习

1.窗口滚动字段

public class MyFrame extends JFrame {
    
    public MyFrame() {  
        
        this.setTitle("练习");
        this.setSize(350, 100);
        this.setResizable(false);
        this.setLocationRelativeTo(null);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        
        JLabel jLabel = new JLabel("欢迎来到成都千锋JavaEE1601班              ");
        jLabel.setFont(new Font("微软雅黑", Font.PLAIN, 18));
        this.add(jLabel);
        
        Timer timer = new Timer(200, e -> {
            // jLabel = new JLabel("hello"); // compiler error
//          String startStr = str.substring(0, 1);
//          jLabel.setText(str.substring(1) + startStr);
            
             String str = jLabel.getText();
             str = str.substring(1) + str.charAt(0);
             jLabel.setText(str);
            
/*          StringBuilder sb = new StringBuilder(jLabel.getText());
            char ch = sb.charAt(0);
            sb.deleteCharAt(0).append(ch);
            jLabel.setText(sb.toString());*/
        });
        timer.start();
    }
    
    @Override
    public void paint(Graphics g) {
        super.paint(g);
    }
    
    public static void main(String[] args) {
        new MyFrame().setVisible(true);
    }
}

正则表达式

正则表达式 - regular expression
定义字符串的匹配模式
例子1:

    // [] 任取其一 {}一组 
    public static void main(String[] args) {
        String tel = "13821424679";
        // boolean isValid = str.matches("1[3478][0-9]{9}");
        boolean isValid = tel.matches("1[3478]\\d{9}");
        System.out.println(isValid);
        
        String qq = "123421";
        boolean isValidQQ = qq.matches("[1-9][0-9]{4,11}");
        System.out.println(isValidQQ);
        
        String username = "sadwqf";
        // boolean isValidUid = username.matches("[a-zA-Z0-9_]{6,20}");
        // boolean isValidUid = username.matches("[^a-zA-Z0-9_]{6,20}");
        boolean isValidUid = username.matches("\\w{6,20}");
        // boolean isValidUid = username.matches("\\W{6,20}");
        System.out.println(isValidUid);
        
        String email = "jackfrued@126.com";
        System.out.println(email.matches("^\\w+([-+.]\\w+)*@\\w+([-.]\\w+)*\\.\\w+([-.]\\w+)*$"));
        
        String msg = "操你大爷, 日你二爷, 干你三爷, f U C k你四爷";
        msg = msg.replaceAll("[操日干]|[fF]\\s*[Uu]\\s*[Cc]\\s*[Kk]\\s*", "*");
        System.out.println(msg);
        
        String sentence = "You go your way, I will go mine!";
        String words[] = sentence.split("[\\s,!]");
        for (String word : words) {
            System.out.println(word);
        }
    }

例子2:

        Pattern.matches("\\w{6,20}", "adswq");
        Pattern pattern = Pattern.compile("a*b", Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher("BaabaaaBaBaaaaaba");
        while(matcher.find()) {
            System.out.println(matcher.group());
        }

练习:写一个方法,生成指定长度的验证码
foo(6) ==> "89dsv8"
foo(4) ==> "T9ax"
工具类:

public final class MyUtil {
    private static final String CHARS = "abcdefghijklmnopqrstuvwxyzABVDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    private MyUtil() {
        throw new AssertionError();
    }
    
    public static String generateVC(int len) {
        
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < len; i++) {
            int randomIndex = (int) (Math.random() * CHARS.length());
            char ch = CHARS.charAt(randomIndex);
            sb.append(ch);
        }
        return sb.toString();
    }
}

测试:

        for (int i = 0; i < 10; i++) {
            System.out.println(MyUtil.generateVC(10));
        }

作业

写一个方法,生成指定长度的验证码
foo(6) ==> "89dsv8"
foo(4) ==> "T9ax"
发一个消息 ,把消息加密、解密 凯撒密码

    public static String getCode(int num) {
        String code ="";
        int i = 0;
        while (i < num) {
            int randomNum = (int) (Math.random() * 75 + 48);
            String ch = (char) randomNum + "";
            boolean isValid = ch.matches("[a-zA-Z0-9]");
            if (isValid) {
                code += ch; 
                i ++;
            }
        } 
        return code;
    }

    public static String CaesarEncrypt(String msg) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < msg.length(); i++) {
            sb.append((char) (msg.charAt(i) + 3));
        }
        return sb.toString();
    }
    public static String CaesarDecrypt(String msg) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < msg.length(); i++) {
            sb.append((char) (msg.charAt(i) - 3));
        }
        return sb.toString();
    }
    public static void main(String[] args) {
        String code = getCode(6);
        System.out.println(code);
        System.out.println(CaesarEncrypt("五大服务器"));
        System.out.println(CaesarDecrypt(CaesarEncrypt("asafwq")));
    }