java String split 按定长分割

from:http://www.rgagnon.com/javadetails/java-0438.html

Some notes from A. Gonzales about String.split()

Special cases using String.split():

public class StringSplit {
  public static void main(String args[]) throws Exception{
    System.out.println(
        java.util.Arrays.toString(
        "  s".split(" ")
    ));
    // output : [, , s]

    System.out.println(
        java.util.Arrays.toString(
        "".split("")
    ));
    // output : []

    System.out.println(
        java.util.Arrays.toString(
        "  ".split(" ")
    ));
    // output : []

    System.out.println(
        java.util.Arrays.toString(
        "      ".split(" ")
    ));
    // output : []

    System.out.println(
        java.util.Arrays.toString(
        " s ".split(" ")
    ));
    // output : [, s]
    }
}

---

It's important to note that an invocation like:

param = req.getParam(...);
String[] words = param.split(" ");
String firstWord = words[0];

will generate a NullPointerException if param.equals(" ").

---

Using split() with a space can be a problem. Consider the following :

public class StringSplit {
  public static void main(String args[]) throws Exception{
    String testString = "Real  How To";  // extra space

    System.out.println(
        java.util.Arrays.toString(
        testString.split(" ")
    ));
    // output : [Real, , How, To]
    }
}

We have an extra element. The fix is to specify a regular expression to match one or more spaces.

public class StringSplit {
  public static void main(String args[]) throws Exception{
    String testString = "Real  How To";

    System.out.println(
        java.util.Arrays.toString(
        testString.split("\\s+")
    ));
    // output : [Real, How, To]
    }
}

---

Since String.split() is based on regular expression, you can make some complex operations with a simple call!

    String testString = "{RealHowto}{java-0438.html}{usage of String.split()}";
    System.out.println(java.util.Arrays.toString(
        testString.split("[{}]")
    ));

    // output : [, RealHowto, , java-0438.html, , usage of String.split()]
    // note : extra empty elements :-(

---

To split a long string into into fixed-length parts. In this example, we split in groups of 3 characters :

    String testString = "012345678901234567890";
    System.out.println(java.util.Arrays.toString(
        testString.split("(?<=\\G.{3})")
    ));

    // output : [012, 345, 678, 901, 234, 567, 890]

---

To split but keep the separator :

   String testString = "RealHowto!java-0438.html!usage of String.split()!";
    System.out.println(java.util.Arrays.toString(
        testString.split("(?<=[!])")
    ));

    // output : [RealHowto!, java-0438.html!, usage of String.split()!]
  }