String contains in Java

Question

String s1    The quick brown fox jumps over the lazy dog   String s2       boolean b   s1 contains s2   System out println b     I run the Java code above  the b return true  Since s2 is empty  why does s1 contains s2   I check the Java API  it write   Returns true if and only if this string contains the specified sequence of char values    Parameters   s - the sequence to search for   Returns   true if this string contains s  false otherwise

User · Answer

The obvious answer to this is  that s what the JLS says    Thinking about why that is  consider that this behavior can be useful in certain cases  Let s say you want to check a string against a set of other strings  but the number of other strings can vary   So you have something like this   for String s   myStrings       check aString contains s        where some s s are empty strings   If the empty string is interpreted as  no input   and if your purpose here is ensure that aString contains all the  inputs  in myStrings  then it is misleading for the empty string to return false  All strings contain it because it is nothing  To say they didn t contain it would imply that the empty string had some substance that was not captured in the string  which is false

User · Answer

I will answer your question using a math analogy   In this instance  the number 0 will represent no value  If you pick a random number  say 15  how many times can 0 be subtracted from 15  Infinite times because 0 has no value  thus you are taking nothing out of 15  Do you have difficulty accepting that 15 - 0   15 instead of ERROR  So if we switch this analogy back to Java coding  the String    represents no value  Pick a random string  say  hello world   how many times can    be subtracted from  hello world

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Thinking of a string as a set of characters  in mathematics the empty set is always a subset of any set

User · Answer

Empty is a subset of any string     Think of them as what is between every two characters   Kind of the way there are an infinite number of points on any sized line      Hmm    I wonder what I would get if I used calculus to concatenate an infinite number of empty strings   Note that    equals     only though

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no real explanation is given by Java  in either JavaDoc or much coveted code comments   but looking at the code  it seems that this is magic   calling stack   String indexOf char    int  int  char    int  int  int  line  1591   String indexOf String  int  line  1564   String indexOf String  line  1546    String contains CharSequence  line  1934       code          Code shared by String and StringBuffer to do searches  The    source is the character array being searched  and the target    is the string being searched for         param   source       the characters being searched      param   sourceOffset offset of the source string      param   sourceCount  count of the source string      param   target       the characters being searched for      param   targetOffset offset of the target string      param   targetCount  count of the target string      param   fromIndex    the index to begin searching from      static int indexOf char   source  int sourceOffset  int sourceCount                     char   target  int targetOffset  int targetCount                     int fromIndex      if  fromIndex  gt   sourceCount            return  targetCount    0   sourceCount   -1             if  fromIndex  lt  0            fromIndex   0            if  targetCount    0     my comment  this is where it returns  the size of the      return fromIndex        incoming string is 0   which is passed in as targetCount                             fromIndex is 0 as well  as the search starts from the                              start of the source string          the rest of the method

User · Answer

Similarly      contains             Returns true    Therefore  it appears that an empty string is contained in any String

[java] String.contains in Java

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