Operator "new" in java, creating String with

Question

What is the difference between creating Object, for example in my code, type Obj with operator "new" and Object type String?

public class Objs {
    int a;

    public Objs(int a)
    {
        this.a = a;
    }

    public static void main(String[] args)
    {
        String str = new String("Hello");
        String str1 = str; // (str1 == str) == true
        str += ", world!!"; // after this (str1 == str) == false - Why?

        Objs o = new Objs(4);
        Objs o1 = o;        //(o == o1) == true
        o.a += 9;           // after this (o == o1) == true also
    }
}

Why after I'm changing value of "str", references "str" and "str1" become not equal, but if I'm doing the same with class Obj references stay equal?

Strings are immutable in Java. The assignment operator is putting a new string instance into the `str` variable, not modifying the original. Meanwhile, you never change the value referred to by the reference `o`. You simply update a property on that object instance. — nbrooks, Mar 11 '19 at 20:41
you are not doing the same: adding something to an object (`str +=`) is not the same as adding something to a field of an object(`o.a +=`) {{your title is totally misleading - not related to `new`}} — user85421, Mar 11 '19 at 20:43
Besides what @nbrooks said, important thing here is that: equals() compares the content and == checks the reference. — alexandrum, Mar 11 '19 at 20:44

Andronicus · Accepted Answer · 2019-03-11T21:05:25.067

5

Here:

str += ", world!!"

you're creating a new object with changed value (that's how + operator works for Strings in java). Here:

o.a += 9;

you're modifying the field of the object, but it's still the same reference.

edited Mar 11 '19 at 21:05

answered Mar 11 '19 at 20:43

Andronicus

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it means, that operator "+" creates a new string, and free memory of the previous? – A.Starshov Mar 11 '19 at 20:46
Yes, but only if the other is not used – Andronicus Mar 11 '19 at 20:54
String is an immutable class so its instances can not be modified. += operator creates new String instance. – GaripTipici Mar 11 '19 at 21:00
The "field of the object" (or, instance variable), not the "parameter on the object." – David Conrad Mar 11 '19 at 21:03
@GaripTipici yes, that's correct! – Andronicus Mar 11 '19 at 21:08

score 0 · Answer 2 · answered Mar 11 '19 at 21:03

Because a String type in Java is immutable that means a string object can not be modified, instead, a new string object is created with the given expression.

str += ", world!!"; // at this line, a new str object is created with given concatenation, hence str loses reference to the previous object and now points to the new one - that is the reason.

Operator "new" in java, creating String with

2 Answers2