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Java Forum / General / February 2007Two identical Strings stored in two different object
Hi, I know java ensures that only one String object exists when there are two identicals Strings, i.e: String s1 = "Hello"; String s2 = "Hello"; System.out.println(s1==s2); It prints "true" However: String s1 = "Hello"; String s2 = new String("Hello"); System.out.println(s1==s2); prints "false" I don't understand why, since both Strings hold the same data. Any ideas? TIA > Hi, I know java ensures that only one String object exists when there > are two identicals Strings, i.e: [quoted text clipped - 15 lines] > I don't understand why, since both Strings hold the same data. Any > ideas? "Hello" and "Hello" becomes one object. new String("Hello") means create a new object and allocate memory for that and copy the input string to it. I am not sure that Java require it to be another object, but from a practical implementation point of view I consider it very natural that it is. Try consider writing the Java code for that constructor. Arne Neroku wrote: >> Hi, I know java ensures that only one String object exists when there >> are two identicals Strings, i.e: Only for literals and one other case, mentioned below. >> String s1 = "Hello"; >> String s2 = "Hello"; [quoted text clipped - 12 lines] >> I don't understand why, since both Strings hold the same data. Any >> ideas? > "Hello" and "Hello" becomes one object. Because both are literals. > new String("Hello") means create a new object and > allocate memory for that and copy the input string > to it. Because it isn't a literal. > I am not sure that Java require it to be another object, > but from a practical implementation point of view > I consider it very natural that it is. > > Try consider writing the Java code for that constructor. Read about the String.intern() method: <http://java.sun.com/javase/6/docs/api/java/lang/String.html#intern()> - Lew > Neroku wrote: >>> Hi, I know java ensures that only one String object exists when there [quoted text clipped - 39 lines] > > - Lew I think they use the same character buffer though? [on the topic of "Hello" and new String("Hello")] > I think they use the same character buffer though? Perhaps they do, but the == operator doesn't compare character buffers. It compares object references, and the two references refer to different objects. - Oliver > Hi, I know java ensures that only one String object exists when there > are two identicals Strings, i.e: [quoted text clipped - 17 lines] > > TIA Java is a reference language so when you call the '==' it compares addresses not values. If you want to compare Strings use the equals function (i.e. System.out.println(s1.equals(s2)) should print true) "Neroku" <n37...@gmail.com> wrote: >> Hi, I know java ensures that only one String object exists when there >> are two identicals Strings, i.e: [quoted text clipped - 17 lines] >> >> TIA > Java is a reference language so when you call the '==' it compares > addresses not values. If you want to compare Strings use the equals > function (i.e. System.out.println(s1.equals(s2)) should print true) That's not the issue. The OP was discussing the interning of Strings. At issue is object equality, not value equality, hence == is what is needed. - Lew >> Hi, I know java ensures that only one String object exists when there >> are two identicals Strings, i.e: [quoted text clipped - 19 lines] > addresses not values. If you want to compare Strings use the equals > function (i.e. System.out.println(s1.equals(s2)) should print true) That is not really what the original poster is asking about. Arne Java has a special optimize mechanism for declaring a new String Object using the format 'String sx = "xxx"', it will try to find whether already has a String Object represents "xxx", if there then return that Object's reference or create a new one. But the format 'new String("xxx")' will always create a new String Object. so when you type: String s1 = "Hello"; Stting s2 = new String("Hello"); There will be two String objects. String s1 = new String("Hello"); String s2 = "Hello"; You will get the same String Ojbect. > Hi, I know java ensures that only one String object exists when there > are two identicals Strings, i.e: [quoted text clipped - 17 lines] > > TIA Please do not top post. > Java has a special optimize mechanism for declaring a new String > Object using the format 'String sx = "xxx"', it will try to find > whether already has a String Object represents "xxx", if there then > return that Object's reference or create a new one. But the format > 'new String("xxx")' will always create a new String Object. Correct. Where "it" looks for these strings is in a private store within the String class. > so when you type: > String s1 = "Hello"; [quoted text clipped - 4 lines] > String s2 = "Hello"; > You will get the same String Ojbect. You were right the first time, but wrong the second. Both examples will yield distinct String objects for s1 and s2. - Lew > Java has a special optimize mechanism for declaring a new String > Object using the format 'String sx = "xxx"', it will try to find [quoted text clipped - 6 lines] > Stting s2 = new String("Hello"); > There will be two String objects. Yep. > String s1 = new String("Hello"); > String s2 = "Hello"; > You will get the same String Ojbect. Are you sure ? Arne >> Java has a special optimize mechanism for declaring a new String >> Object using the format 'String sx = "xxx"', it will try to find >> whether already has a String Object represents "xxx", if there then >> return that Object's reference or create a new one. But the format >> 'new String("xxx")' will always create a new String Object. [...] >> String s1 = new String("Hello"); >> String s2 = "Hello"; >> You will get the same String Ojbect. > > Are you sure ? <SSCCE> public class Test { public static void main(String[] args) { String s1 = new String("Hello"); String s2 = "Hello"; System.out.println(s1 == s2); } } </SSCCE> <output> false </output> The "special optimization mechanism" happens at compile time. The compiler adds a "constant pool" to every .class file it generates, storing any constants (string literals, among other things) that class file needs, and only stores one copy for constants that appear repeatedly in the source. Allocating the string, via the "new" operator, happens at runtime. - Oliver > Hi, I know java ensures that only one String object exists when there > are two identicals Strings No, Java doesn't ensure that for all strings. It does have a collections of unique Strings, but not all Strings belong to it. Literal Strings do, and the return value from String.intern() does. > , i.e: > [quoted text clipped - 3 lines] > > It prints "true" Yes, these are literal Strings. > However: > [quoted text clipped - 3 lines] > > prints "false" s2 is not a literal String. Note that "new" is guaranteed to create a brand-new object, so it would be quite incorrect for this to be true. But String s3 = s2.intern(); System.out.println(s1==s3) will once again be "true". Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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