IF strings are immutable why is the variable s appending and changing its obect

class StringTest
{
public static void main(String args[])
{
String s="java";
s=s+" hi";
System.out.println(s);
}
}

Remember that a variable is not an object; it points to an object. You can have more than one variable all pointing to the same object.

The expression 's + "hi"' creates a new String, and the variable "s" is set to point to it. The original String object that "s" pointed to is unchanged.

you mean to the s variable is pointing towards teo objects? "java" and "java hi"

Yes. First it points to "java" and then it is moved to point to another completely separate String containing "java hi".

is this diagram correct

"java" doesn't get destroyed; it's just not referenced anymore. For most objects, that means the garbage collector will eventually delete it. For this particular String, since it's a literal (a String constant that appears in the source code) it's held in the string pool, so it won't be GCd.

try this program.

public class StringConstruction {
	static String str1 = "You cannot change me!"; // Interned

public static void main(String[] args) {
		String emptyStr = new String(); // ""
		System.out.println("emptyStr: " + emptyStr);
		String str2 = "You cannot change me!"; // Interned
		String str3 = "You cannot" + " change me!"; // Interned
		String str4 = new String("You cannot change me!"); // New String object
		String words = " change me!";
		String str5 = "You cannot" + words; // New String object
		System.out.println("str1 == str2: " + (str1 == str2)); // (1) true
		System.out.println("str1.equals(str2): " + str1.equals(str2)); // (2)
																		// true
		System.out.println("str1 == str3: " + (str1 == str3)); // (3) true
		System.out.println("str1.equals(str3): " + str1.equals(str3)); // (4)
																		// true
		System.out.println("str1 == str4: " + (str1 == str4)); // (5) false
		System.out.println("str1.equals(str4): " + str1.equals(str4)); // (6)
																		// true
		System.out.println("str1 == str5: " + (str1 == str5)); // (7) false
		System.out.println("str1.equals(str5): " + str1.equals(str5)); // (8)
																		// true
		System.out.println("str1 == Auxiliary.str1: "
				+ (str1 == Auxiliary.str1)); // (9) true
		System.out.println("str1.equals(Auxiliary.str1): "
				+ str1.equals(Auxiliary.str1)); // (10) true
		System.out.println("\"You cannot change me!\".length(): "
				+ "You cannot change me!".length());// (11) 21
	}
}

class Auxiliary {
	static String str1 = "You cannot change me!"; // Interned
}

so will java object be garbage collected

yes

If it is no longer necessary, yes. But as Ernest said, that may happen later than regular objects, if the String pool decides to hang onto it.

Hi friends, can anybody tell me why st1 == st4 is false and st1 == str5 is false.

Thank you all in advance. Have a nice day.

String s4 is a new object...so calling == gives false as they are different objects...since s5 is "change me" + words....this creates a new string in the pool...so s1 and s5 refers to different objects in the string pool...s5 is not a constant because of this

Hi Ted can you be more specific, really I haven't understood.

As you said str1 and str4 are different objects, I got a false. Then str1 and str2 are also different objects, but they are returning true. Why?

krishna anusha wrote: Then str1 and str2 are also different objects, but they are returning true. Why?

Because they are not different objects. The strings assigned to str1 and str2 are compile time constants, so the compiler uses the string pool, and hence, they refer to the same string object.

Henry

Hi Henry, I understood now, thanks for the quick reply.

yup s1 and s2 are the objects...