`StringBuffer`/`StringBuilder` constructors should not be passed characters as the first argumentJAVA-E1023

StringBuffer/StringBuilder constructors should not be passed character values, as this will not yield the expected result (an initialized object with an initial single character string). It will instead create an empty StringBuffer/StringBuilder with a capacity value set to the ASCII value of the character passed as an argument.

StringBuffer and StringBuilder both have a constructor that allows one to set the initial capacity of the contained string. They also have a constructor which can be used to set the initial string to build from.

Due to Java's numeric type promotion rules, char values will automatically be promoted to a numeric type such as int if a char is passed where a number was expected.

For example:

Passing 'a' (ASCII code 97) to a method expecting an int value will first convert 'a' to the int value 97 before passing it to the method.

Consider this code now:

StringBuffer sb = new StringBuffer('a');

While the expectation may be that sb will be initialized with a single character string ("a"), sb will instead be initialized as an empty StringBuffer with an initial capacity of 97 characters.

It is possible that the developer may accidentally pass a character value (like 'a' for example) instead of passing a string (like "a") to the constructor of either of StringBuffer/StringBuilder. This may be because of the assumption that StringBuffer or StringBuilder can accept a single character as an argument, or because of a mistake when typing quotes for a single character string.

If this was intentional, it makes for very abstruse code and is better off replaced.

Bad Practice

// Equivalent to creating a StringBuilder with an initial capacity of 97 characters.
StringBuilder sb = new StringBuilder('a');

Recommended

StringBuilder sb = new StringBuilder("a");

If you wish to create the StringBuilder with a specific initial capacity, Just specify it directly:

StringBuilder sb = new StringBuilder(100);

References

• Oracle Java 8 Language Specification - Numeric promotion ruleshttps://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.2)
• Oracle Java 8 JavaDocs - java.lang.StringBuilder
• Oracle Java 8 JavaDocs - java.lang.StringBuffer