48

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 < 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 + "> "+ s); if (s == "/quit") { System.out.println("quitted"); } if(! s.equals("")) { for(int j = 0; j < connectionList.size(); j++) { Connection c2 = connectionList.elementAt(j); c2.out.println(s); } } } } catch(Exception e){ System.out.println("reading error"); } } } } }
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  • 2
    Too bad we can't give +1 for edits. Commented Mar 18, 2009 at 16:23
  • == means; is this the same object. It doesn't match objects which contain the same data. Commented Apr 28, 2009 at 20:51
  • how come it's tagged 'multithreading'? Commented Feb 14, 2010 at 9:58

6 Answers 6

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104

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))
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6 Comments

I prefer s.equals("/quit"). It is cosmetically but I want to have the important current data (s) to be visible easily.
s.equales("/quit") will throw a NullPointerException is s is null, "/quit".equals(s) will never throw an NPE
So "/quit".equals(s) will mask bugs where s is unintentionally null.
@starblue good point, always something else to consider
@ReneS whatever string/context s is I'd still do "<data>".equals(s) for I know <data> won't be NULL whereas s might. But if s.equals(s2) then you are right, but then again you'll have to surround it with if(s!= null){ block.
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39

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 > t) // ILLEGAL if (s.compareTo(t) > 0) // also CORRECT>
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Checks to see that the references are the same.
12

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.

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6

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.

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5

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.

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5

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.

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