Java基础入门：语言基础-典型引用类型介绍：字符串

字符串相关

• String 是引用类型
• 基本操作
  • 创建字符串

    // 双引号
    String s1 = "Hello World";
    
    // 利用构造器
    String s2 = new String("Hello World");
    
    // StringBuilder
      StringBuilder sb = new StringBuilder();
      sb.append('h');
      sb.append('e');
      sb.append('l');
      sb.append('l');
      sb.append('o');
  
      String s4 = sb.toString();
  
      // StringBuffer - Thread Safe 线程安全
      StringBuffer stringBuffer = new StringBuffer();
      stringBuffer.append('h')
                  .append('e')
                  .append('l')
                  .append('l')
                  .append('o')
                  .append(' ')
                  .append("world");
  
      String s5 = stringBuffer.toString();
  

  • 获取字符串长度

    int length1 = s1.length();
    System.out.println("The length of s1：" + length1);
    System.out.println("The length of s2：" + s2.length());
  

  • 获取特定位置字符：index取值范围：[0, length - 1]，不要越界

    char ch1 = s2.charAt(0);
    System.out.println("index 0 of s2：" + ch1);
    char ch2 = s2.charAt(10);
    System.out.println("index 10 of s2：" + ch2);
    
    // 遍历字符串
    for (int i = 0; i < s2.length(); i++) {
        System.out.println("ch in s2：" + s2.charAt(i));
    }
  

  • 字符串连接

    // 用+连接
    String str1 = s1 + s2;
    System.out.println("Str1：" + str1);
  
    // concat函数（方法）
    String str2 = s1.concat(s2);
    String str3 = s2.concat(s1);
    String str4 = s1.concat(s2).concat(s3).concat(s4);
    System.out.println("Str2：" + str2);
    System.out.println("Str3：" + str3);
    System.out.println("Str4：" + str4);
  

  • 格式化字符串

    // 方法一
    String formatS = String.format("int value: %d, float value: %f, " +
          "2 bit double value: %.2f, " +
          "string value: %s", 100, 3.1415f, 1.23456, "java");
    System.out.println("s after formatting: " + formatS);
    
    // 方法二
    System.out.printf("SOUF: %s %s", s1, s2);
  

  • 字符串与数字相互转化

    // String -> Numeric: 用包装类的parseXXX
    int intNum = Integer.parseInt("100");
    long longNum = Long.parseLong("1000000");
    double doubleNum = Double.parseDouble("3.1415");
  
    System.out.printf("int num: %d\n", intNum);
    System.out.printf("long num: %d\n", longNum);
    System.out.printf("double num: %.2f\n", doubleNum);
  
    // Numeric -> String
    String numberStr1 = 100 + "";
    System.out.println(numberStr1);
  
    String numberStr2 = String.valueOf(1.618);
    System.out.println(numberStr2);
  
    String numberStr3 = String.valueOf(1000);
    System.out.println(numberStr3);
  

  • 其他常见字符串操作函数

    // isEmpty
    System.out.println(s5.isEmpty());
    System.out.println(new String().isEmpty());
   
    // equals
    System.out.println(s1.equals(s2));
    
    // indexOf & lastIndexOf
    System.out.println(s3.indexOf('L'));
    System.out.println(s3.lastIndexOf('L'));
    
    // toLowerCase & toUpperCase
    System.out.println(s3.toLowerCase());
    System.out.println(s4.toUpperCase());
    
    // substring 范围：[Start, End)
    System.out.println(s1.substring(0, 3));
    
    // trim
    String st = "  Hello  ";
    System.out.println(st.trim());