[java] How to compare two strings are equal in value, what is the best method?

Always confused about this stuff. Anyone can help?

string1.equals(string2) is right way to do it.

String s = "something", t = "maybe something else";
 if (s == t)      // Legal, but usually results WRONG.
 if (s.equals(t)) // RIGHT way to check the two strings
  /* == will fail in following case:*/
 String s1 = new String("abc");
 String s2 = new String("abc");

 if(s1==s2) //it will return false

You should use some form of the String#equals(Object) method. However, there is some subtlety in how you should do it:

If you have a string literal then you should use it like this:

"Hello".equals(someString);

This is because the string literal "Hello" can never be null, so you will never run into a NullPointerException.

If you have a string and another object then you should use:

myString.equals(myObject);

You can make sure you are actually getting string equality by doing this. For all you know, myObject could be of a class that always returns true in its equals method!

Start with the object less likely to be null because this:

String foo = null;
String bar = "hello";
foo.equals(bar);

will throw a NullPointerException, but this:

String foo = null;
String bar = "hello";
bar.equals(foo);

will not. String#equals(Object) will correctly handle the case when its parameter is null, so you only need to worry about the object you are dereferencing--the first object.

Not forgetting

.equalsIgnoreCase(String)

if you're not worried about that sort of thing...

== checks to see if they are actually the same object in memory (which confusingly sometimes is true, since they may both be from the pool), where as equals() is overridden by java.lang.String to check each character to ensure true equality. So therefore, equals() is what you want.

You can either use the == operator or the Object.equals(Object) method.

The == operator checks whether the two subjects are the same object, whereas the equals method checks for equal contents (length and characters).

if(objectA == objectB) {
   // objects are the same instance. i.e. compare two memory addresses against each other.
}

if(objectA.equals(objectB)) {
   // objects have the same contents. i.e. compare all characters to each other 
}

Which you choose depends on your logic - use == if you can and equals if you do not care about performance, it is quite fast anyhow.

String.intern() If you have two strings, you can internate them, i.e. make the JVM create a String pool and returning to you the instance equal to the pool instance (by calling String.intern()). This means that if you have two Strings, you can call String.intern() on both and then use the == operator. String.intern() is however expensive, and should only be used as an optimalization - it only pays off for multiple comparisons.

All in-code Strings are however already internated, so if you are not creating new Strings, you are free to use the == operator. In general, you are pretty safe (and fast) with

if(objectA == objectB || objectA.equals(objectB)) {

}

if you have a mix of the two scenarios. The inline

if(objectA == null ? objectB == null : objectA.equals(objectB)) {

}

can also be quite useful, it also handles null values since String.equals(..) checks for null.