Can any one solve this == &(==)

[Jetendra Ivaturi]

String s="abc";
String s1="abc;
System.out.println(s==s1); false
System.out.println((s==s1); true

Why is it so?

I some where read like jvm maintains constant pooling as there are many objects created for string every time.

Can any one help in this?

[Premak Rajan]

No. Answer for both that is true only

[Andy Morris]

That code would not even compile.

As the guys running this have asked a billion times, please quote source examples and copy them correctly!

[Raj Kumar Bindal]

If i do
String s=new String("abc");
then in both cases it returns false,i am not clear

[Tom Johnson]

   
String s = "abc";
String s1 = "abc";
String s2 = new String("abc");
System.out.println(s == s1);
System.out.println(s == s2);
System.out.println(s.equals(s1));
System.out.println(s.equals(s2));

produces :

true
false
true
true

Reason: Strings are handled differently that other variables. They are immutable and once created they are stored in a string pool for reuse (note, the strings themselves NOT the references). If a reference is created to a string that already exists ("abc") then that string is reused from the string pool, as it can never change - i.e there is only one string "abc" created above.
However, there are 3 string references created s, s1 and s2. The first thing to understand is the the == checks if the references refer to the same object in memory, it does not check if their contents are equal. Here, s and s1 refer to the same object on the heap and hence s == s1 returns true. However, s2 is created using the new operator meaning that s2 refers to a new object on the heap meaning that s == s2 will be false (they do NOT refer to the same object).

To check if they are meaningfully equal (using their values) you use the .equals() method. This has been overridden by the String class to check if the strings that the string reference variables refer to are equal. Hence both s.equals(s1) and s.equals(s2) return true.

You should always use the .equals() method to check if objects are meaningfully equal. This method (along with hashcode()) should be overridden in a class that you write if you need to know what makes the object equivalent. For example:

public class Car {
    // Used to determine if cars are "equal"
    private final String registration;
 
    public Car(final String reg) {
        this.registration = reg;
    }
 
    @Override
    public boolean equals(final Object otherObj) {
        if(!(otherObj instanceof Car)) {
        // For null and objects that arent Cars
        return false;
        }
        
        // Now know its safe to cast
        final Car otherCar = (Car)otherObj;
        
        if(this == otherCar) {
        // Refers to same object on heap, must be equal
        return true;
        }
        else {
        // Different objects but can be meaningfully equivalent
        return this.registration.equals(otherCar.registration);
        }
    }
}
 
and to test:
 final Car c1 = new Car("car1");
 final Car c2 = new Car("car2");
 final Car c3 = new Car("car1");
 System.out.println(c1 == c2); // False
 System.out.println(c1.equals(c2)); // False
 System.out.println(c1.equals(c3)); // True

Hope this helps
/Tom

[Tom Johnson]

Forgot, also :

System.out.println(c1 == c3); // False