This prints true (even though we don't use equals method: correct way to compare strings)
String s = "a" + "bc";
String t = "ab" + "c";
System.out.println(s == t);
When compiler optimizes your string literals, it sees that both s and t have same value and thus you need only one string object. It's safe because String is immutable in Java.
As result, both s and t point to the same object and some little memory saved.
Name 'string pool' comes from the idea that all already defined string are stored in some 'pool' and before creating new String object compiler checks if such string is already defined.
This prints true (even though we don't use equals method: correct way to compare strings)
String s = "a" + "bc";
String t = "ab" + "c";
System.out.println(s == t);
When compiler optimizes your string literals, it sees that both s and t have same value and thus you need only one string object. It's safe because String is immutable in Java.
As result, both s and t point to the same object and some little memory saved.
Name 'string pool' comes from the idea that all already defined string are stored in some 'pool' and before creating new String object compiler checks if such string is already defined.
I don't think it actually does much, it looks like it's just a cache for string literals. If you have multiple Strings who's values are the same, they'll all point to the same string literal in the string pool.
String s1 = "Arul"; //case 1
String s2 = "Arul"; //case 2
In case 1, literal s1 is created newly and kept in the pool. But in case 2, literal s2 refer the s1, it will not create new one instead.
if(s1 == s2) System.out.println("equal"); //Prints equal.
String n1 = new String("Arul");
String n2 = new String("Arul");
if(n1 == n2) System.out.println("equal"); //No output.
http://p2p.wrox.com/java-espanol/29312-string-pooling.html
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The string pool is the JVM's particular implementation of the concept of string interning:
In computer science, string interning is a method of storing only one copy of each distinct string value, which must be immutable. Interning strings makes some string processing tasks more time- or space-efficient at the cost of requiring more time when the string is created or interned. The distinct values are stored in a string intern pool.
Basically, a string intern pool allows a runtime to save memory by preserving immutable strings in a pool so that areas of the application can reuse instances of common strings instead of creating multiple instances of it.
As an interesting side note, string interning is an example of the flyweight design pattern:
Flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory.
The string pool allows string constants to be reused, which is possible because strings in Java are immutable. If you repeat the same string constant all over the place in your Java code, you can actually have only one copy of that string in your system, which is one of the advantages of this mechanism.
When you use String s = "string constant"; you get the copy that is in the string pool. However, when you do String s = new String("string constant"); you force a copy to be allocated.