String comparison and String interning in Java

Question

When should one compare Strings as objects and when should one use their equals method? To make sure, I always use equals, but that doesn't seem very efficient. In what situations can I be certain that string1 == string2 is a safe to use?

Thanks!

Answer 1

From what I know of Java, string1==string2 will only be true if the references to those objects are the same. Take a look at the following case

String string1 = new String("Bob");
String string2 = new String("Bob");

string1 == string2; // false, they are seperate objects
string1 = string2;  // asigning string1 to string2 object
string1 == string2; // true, they both refer to the same object

Answer 2

You can only use the == for comparison if you are sure the objects are the same.

For example, this could occur if you had a final static String variable. You could be certain that a comparison would be between the same object.

Stick with the equals for string comparison.

Answer 3

You should almost always use equals. You can be certain that string1 == string2 will work if:

• You've already made sure you've got distinct values in some other way (e.g. you're using string values fetched from a set, but comparing them for some other reason)
• You know you're dealing with compile-time string constants
• You've manually interned the strings yourself

It really doesn't happen very often, in my experience.