If statement with String comparison fails

Question

I really don t know why the if statement below is not executing   if  s      quit         System out println  quitted        Below is the whole class   It is probably a really stupid logic problem but I have been pulling my hair out over here not being able to figure this out   Thanks for looking     class TextParser extends Thread       public void run             while  true                for int i   0  i  lt  connectionList size    i                      try                                      System out println  reading     i                       Connection c   connectionList elementAt i                        Thread sleep 200                        System out println  reading     i                        String s                            if  c in ready      true                            s   c in readLine                              System out println i     gt     s                            if  s      quit                                 System out println  quitted                                                       if   s equals                                    for int j   0  j  lt  connectionList size    j                                      Connection c2   connectionList elementAt j                                   c2 out println s                                                                                                   catch Exception e                       System out println  reading error

User · Answer

To compare Strings for equality  don t use     The    operator checks to see if two objects are exactly the same object   In Java there are many string comparisons   String s    something   t    maybe something else   if  s    t          Legal  but usually WRONG  if  s equals t      RIGHT if  s  gt  t        ILLEGAL if  s compareTo t   gt  0     also CORRECT gt

User · Answer

You shouldn t do string comparisons with      That operator will only check to see if it is the same instance  not the same value   Use the  equals method to check for the same value

User · Answer

In your example you are comparing the string objects  not their content   Your comparison should be    if  s equals   quit      Or if s string nullity doesn t mind   or you really don t like NPEs   if    quit  equals s

User · Answer

If you code in C   as well as Java  it is better to remember that in C    the string class has the    operator overloaded  But not so in Java  you need to use equals   or equalsIgnoreCase   for that

User · Answer

Strings in java are objects  so when comparing with     you are comparing references  rather than values  The correct way is to use equals     However  there is a way  If you want to compare String objects using the    operator  you can make use of the way the JVM copes with strings  For example   String a    aaa   String b    aaa   boolean b   a    b    b would be true  Why   Because the JVM has a table of String constants  So whenever you use string literals  quotes     the virtual machine returns the same objects  and therefore    returns true   You can use the same  table  even with non-literal strings by using the intern   method  It returns the object that corresponds to the current string value from that table  or puts it there  if it is not   So   String a   new String  aa    String b   new String  aa    boolean check1   a    b     false boolean check1   a intern      b intern       true      It follows that for any two strings s and t  s intern      t intern   is true  if and only if s equals t  is true

User · Answer

You can use   if   quit  equals s            or   if   quit  compareTo s     0             The latter makes a lexicographic comparison  and will return 0 if the two strings are the same

[java] If statement with String comparison fails

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