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Thread: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

  1. #1
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    Default Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Hello,
    This is my very first post. I'm working on a codingBat problem and would like to know why the arguments in each if statement below produce different results. Can anyone help?

    if (str.substring(1,4) == "del") This if statement returns 'false'
    if (str.substring(1,4).equals("del")) This if statement returns 'true'

    here is an example with the line above where the if statement returns false...

    public class DelDel {
    	public static void main(String[] args) {
    		String str = "adel";
    		if (str.substring(1,4) == "del"){
    			System.out.println(str.substring(0, 1) + str.substring(4));
    	        } else {
    		System.out.println(str);
    	        }
    	}
    }


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    == compares the values in the variables
    equals() compares the values in the objects

    String x = "X";
    x is a variable that has the address of the object that contains the value: "X"
    If you don't understand my answer, don't ignore it, ask a question.

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    Kalagor (September 3rd, 2014)

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Look at the following code:
    String a = "abc";
    String b = "abc";
    boolean equal = a == b;
    Here you have 2 variables of type String, one of them is a and one of them is b.
    Both variables are referencing a different Object of Type String. The variable a is referencing a String with the contents "abc". The variable b is referencing a String with the contents "abc" as well. But both reference different String Objects.

    If you use the == operator in java you compare the references of variables. The statement "equal = a == b;" is evaluating whether the variables a and b are referencing the same Object or not. Since I said that both reference different objects which just happen to have the same contents it should be obvious that the variable equal will have the value false.

    If I was to write:
    String a = "abc";
    String b = a;
    boolean equal = a == b;
    Then the result of the evaluation would be true and the variable equal would have the value true assigned to it.


    If we use the method equals(Object other) then we call whatever code is defined in the particular class. For the String class the equals method will check whether the contents of 2 Objects are the same.

  5. The Following User Says Thank You to Cornix For This Useful Post:

    Kalagor (September 3rd, 2014)

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Wow... nice first experience. Thanks Norm and Cornix... both answers helped me understand better. Appreciate it.

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Quote Originally Posted by Cornix View Post
    the variable equal will have the value false.
    Have you tried to run that code? The result will be true due to the String Literal Pool.
    Improving the world one idiot at a time!

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    I am trying to teach theory, not compiler optimization.

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Hello Cornix,

    I just tried the code you mentioned (here below) and the variable equal actually give me a value of true instead of false. Can you help to see if I did something wrong.

    		String a = "abc";
    		String b = "abc";
    		boolean equal = a == b;
    		System.out.println(equal);

  10. #8
    Super Moderator Norm's Avatar
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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    The compiler does special magic with Strings defined at compile time to optimize. Do some research on the String Literal Pool. See post#5
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Quote Originally Posted by Kalagor View Post
    I just tried the code you mentioned (here below) and the variable equal actually give me a value of true instead of false. Can you help to see if I did something wrong.
    Post the _entire_ test program you used, and make sure you use a good text editor. The only reason that can be false is if the strings are different.

    In this case, the comparison is very related to hashCode(). Specifically, in this case, "abc".hashCode().

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Quote Originally Posted by Cornix View Post
    I am trying to teach theory, not compiler optimization.
    In that case use better examples. As reply #7 shows you just confused the poor bugger.
    Improving the world one idiot at a time!

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    Default Re: Why do these two lines of code not produce the same results? ("==" vs. ".equals()")

    Excellent. Thanks guys. It's ok with me because I can use it all for learning... probably a little deeper than needed at this point in my learning, but there is definitely a benefit. I'm so thankful you guys are willing to help out. I found a nice explanation based on your suggestion Norm. Thanks. Here it is in case anyone cares... What is String literal pool?

    I'll set this thread back to solved.

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