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Tip of the Day
Language: Java Language
Expertise: Beginner
Feb 1, 1999

Testing String Equality

Question:
This is a section from a program I am writing. It goes through the first if statement fine, but when it gets to the second if statement it doesn't go into it. I have checked to make sure that the command line is args[1] and that it equals "ptok."
if (args.length == 2) {
    if (args[1] == "ptok") {
       System.out.println("Hey it worked");
    }		
}

Answer:
This is a common beginner's mistake. To test the equality of two strings, you will want to modify your test to:

if (args[1].equals("ptok")) {
...
}
Java strings are references to immutable objects. The == operator will test if two strings reference the same object, not whether two strings are equal. To test if two objects are equal in Java, you should almost always use the equals(Object) method defined in java.lang.Object. This method will test if two objects are equal, rather than whether they just reference the same object. This is similar to the way pointers work in C. Two strings can be equal, but reside in two different locations in memory. If you use the == method to test the equality of the strings, you will only be testing whether the two pointers are equal. In Java, there are no pointers in the sense that you cannot access raw memory through an address. References are object identifiers that reference data structures. Testing their equality with the == only tells you if they reference the same object. The equals(Object) method will tell you if the objects they reference are equivalent.
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