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");
}
}
}
}
}
This question is related to
java
multithreading
if-statement
string-comparison
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.
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.
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.
String
s 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.
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>
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))
Source: Stackoverflow.com