试试这个：

String after = before.trim().replaceAll(" +", " ");

参见

• String.trim（）
  
  
  
  
  • 返回字符串的副本，省略前导和尾随空格。
  
  

  只需一个 replaceAll 也可以这样做>，但这比 trim（）解决方案的可读性低得多。尽管如此，这里提供的只是为了展示正则表达式可以做什么：

  It's also possible to do this with just one replaceAll, but this is much less readable than the trim() solution. Nonetheless, it's provided here just to show what regex can do:

      String[] tests = {
          "  x  ",          // [x]
          "  1   2   3  ",  // [1 2 3]
          "",               // []
          "   ",            // []
      };
      for (String test : tests) {
          System.out.format("[%s]%n",
              test.replaceAll("^ +| +$|( )+", "$1")
          );
      }
  

  有3个替代品：

  
  
  • ^ _ + ：字符串开头的任何空格序列
    
    
    
    
    • 匹配并替换为 $ 1 ，它捕获空字符串
    
    
    • ^_+ : any sequence of spaces at the beginning of the string
      • Match and replace with $1, which captures the empty string
      
      
      • 匹配和替换为 $ 1 ，它捕获空字符串
      
      
      • Match and replace with $1, which captures the empty string
      
      
      • 匹配和替换使用 $ 1 ，捕获单个空格
      
      
      • Match and replace with $1, which captures a single space
      
      
      • regular-expressions.info/Anchors
      
      
      • regular-expressions.info/Anchors