If declared string using new keyword and same string be assigned another value using assignment oper-CodePudding

String newString=new String("JAVA");
newString="JAVA8";

WHAT will happen here if we print newString we know output will be JAVA8 only but 2nd line created new object in heap or constant pool. How its behave internally

CodePudding user response：

In Java Strings are immutable.

What that means is that once a String object is created it cannot be updated.

Also Java has something called Java String Pool, a memory location where Java stores Strings.

To minimize the memory space used by this String Pool, Java performs Interning when a new String literal is create using the following statement.

String s = "apple";

What Interning means is that JVM looks in the String Pool to see if the string literal "apple" exists, if it does it just passes its reference to new variable, no new object is created, if it does not exist it creates a new object passes its reference and also can pass the same reference if another string is created with same value. For Example:

String t= "apple";

If you check if the both strings created above are reference to the same string literal in string pool, you can do that as follows:

System.out.println(s == t); // True

When a string is created using new keyword it always creates a new Object in the String pool, even if the string literal exists in the string pool.

String u = new String("apple");

You can check that the new object is created by following statement:

System.out.println(s == u); // False
System.out.println(t == u); // False

In yoru case the first statement created a new object in String pool and passed its reference. The second line created another object and passed its reference. The first object is still in String pool but not being referenced by any variable and eventually the memory occupied by it will be released by Garbage collection.

Hope this answers your question, you can read about Java String pool to know more.

CodePudding user response：

First, newString is not a String!!!

newString is a reference to an instance of the class String! A C programmer would name it pointer and write String* newString.

With

String newString = new String( "JAVA" );

you declare the variable newString of type "reference to String" and assign the reference to (the address for) the new object, created by a call to the constructor of the class java.lang.String from the compile time constant "JAVA".

newString = "JAVA8";

now assigns the reference to the compile time constant "JAVA8" to that previously created variable. At the same time, the previously reference object gets abandoned (no more references to that object were hold) and therefore it is now ready to be garbage collected.

Most Java implementations do store the compile time constants not on the heap, and strings are usually stored in the string pool. Also it is possible that the call to new String() will never be executed; instead the compiler will directly use the reference to the compile time constant. Or it will be executed and the resulting String object is stored on the Heap, with or without referencing the string value in the string pool.

The details can be found in the Java Language Specification and the JVM Specification for the respective version, and if that says "implementation dependent", in the documentation for the respective JVM/Java implementation.