Question about references in java

Question

I want to know what the difference between :

String s = "text";

and :

String s = new String("text");

Answer 1

The latter explicitly creates a new and referentially distinct instance of a String object; the former may reuse an instance from the string pool if one is available.

You very rarely would ever want to use the new String(anotherString) constructor. From the API:

String(String original) : Initializes a newly created String object so that it represents the same sequence of characters as the argument; in other words, the newly created string is a copy of the argument string. Unless an explicit copy of original is needed, use of this constructor is unnecessary since strings are immutable.

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• Strings are objects in Java, so why don’t we use ‘new’ to create them?

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What referential distinction means

Examine the following snippet:

    String s1 = "foobar";
    String s2 = "foobar";

    System.out.println(s1 == s2);      // true

    s2 = new String("foobar");
    System.out.println(s1 == s2);      // false
    System.out.println(s1.equals(s2)); // true

== on two reference types is a reference identity comparison. Two objects that are equals are not necessarily ==. It is usually wrong to use == on reference types; most of the time equals need to be used instead.

Nonetheless, if for whatever reason you need to create two equals but not == string, you can use the new String(anotherString) constructor. It needs to be said again, however, that this is very peculiar, and is rarely the intention.

References

• JLS 15.21.3 Reference Equality Operators == and !=
• class Object - boolean Object(equals)

Related issues

• Java String.equals versus ==
• How do I compare strings in Java?

Answer 2

One creates a String in the String Constant Pool

String s = "text";

the other one creates a string in the constant pool ("text") and another string in normal heap space (s). Both strings will have the same value, that of "text".

String s = new String("text");

s is then lost (eligible for GC) if later unused.

String literals on the other hand are reused. If you use "text" in multiple places of your class it will in fact be one and only one String (i.e. multiple references to the same string in the pool).

Answer 3

Think of "bla" being a magic factory like Strings.createString("bla") (pseudo). The factory holds a pool of all strings yet created this way.

If it gets invoked, it checks if there is already string in the pool with this value. If true, it returns this string object, hence to strings obtained this way are indeed the same object.

If not, it creates a new string object internally, saves it in the pool and then returns it. Thus, when the same string value is queried the next time, it returns the same instance.

Manually creating new String("") overrides this behaviour by bypassing the string literal pool. So equality should always be checked using equals() which compares the character sequence instead of the object reference equality.

Answer 4

One simple way to understand the difference is below:-

String s ="abc";
String s1= "abc";
String s2=new String("abc");

        if(s==s1){
            System.out.println("s==s1 is true");
        }else{
            System.out.println("s==s1 is false");
        }
        if(s==s2){
            System.out.println("s==s2 is true");
        }else{
            System.out.println("s==s2 is false");
        }

output is

s==s1 is true
s==s2 is false

Thus new String() will always create a new instance.

Answer 5

Although it looks the same from a programmers point of view, it has big performance impact. You would want to use the first form almost always.