Java inflexible String class substring

Written in ** JDK 1.8 **.

Substring of the String class

The String class has a substring method that cuts out part of a string. It will cut out two types according to the number of arguments.

When there is one argument

** Definition **

    substring(int beginIndex)

It cuts out the characters after beginIndex. Note that the string index counts from 0.

Example of use

    String str = ("Doraemon");
    String cutStr = str.substring(2);
    System.out.println("After cutting:" + cutStr);

result

After cutting: Emon

The character string after the character e in the specified index 2 is cut out.

When there are two arguments

** Definition **

    substring(int beginIndex, int endIndex)

It cuts out the characters after beginIndex including the characters of ʻendIndex-1`. Example of use

    String str = ("Doraemon");
    String cutStr = str.substring(2, 4);
    System.out.println("After cutting:" + cutStr);

result

After cutting: Emo

The character string Emo from the specified beginIndex = 2 to the character E at ʻendIndex --1 = 3` is cut out.

This is subtle

If there are two arguments and the string is less than ʻendIndex -1`, an exception will be thrown.

Use Case

Output the character string to be displayed on the screen. Since there is a limit that only 4 characters can be displayed on the screen, I want to limit the output character string to 4 characters. If the character string is less than 4 characters, I want to output it as it is.

code

    String str1 = ("Doraemon");
    String str2 = ("Nobita");
    String str3 = ("Suneo");

    List<String> sourceStrList = Arrays.asList(str1, str2, str3);
    List<String> outStrList = new ArrayList<>();

    for (String sourceStr : sourceStrList) {
        outStrList.add(sourceStr.substring(0, 4));
    }

    System.out.println("After cutting:" + outStrList);

result

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 4

Yes, an exception has occurred. Here, Nobita and Suneo have indexes up to 2, so they are exceptions because they are less than ʻendIndex -1 = 3`. It is troublesome because it is necessary to check the character length in advance to solve it.

Resolved using substring in StringUtils

If you use the substring provided in ʻorg.apache.commons.lang3.StringUtils`, you will be able to solve this problem.

    substring(String str, int start, int end)

The character string whose index of the character string to be cut out in str is start to ʻend -1` is output.

code

    String str1 = ("Doraemon");
    String str2 = ("Nobita");
    String str3 = ("Suneo");

    List<String> sourceStrList = Arrays.asList(str1, str2, str3);
    List<String> outStrList = new ArrayList<>();

    for (String sourceStr : sourceStrList) {
        outStrList.add(StringUtils.substring(sourceStr, 0, 4));
    }

    System.out.println("After cutting out:" + outStrList);

result

After cutting out[Doraemon,Nobita,Suneo]

Even if the character string is less than ʻend --1 = 3`, it will be output as it is.

reference

java.lang String substring org.apache.commons.lang3 Class StringUtils substring