CODEWITHSUNDEEP
javastringbooleanequals
Raj
Raj

Reputation: 2493

strange behavior of == in Java

I observed a strange behavior == operator in java. I am trying to print the out put as follows

String str1 = "Rajesh";
String str2 = "Rajesh";
System.out.println("Using equals() str1 and str2 Equals :"
            + str1.equals(str2));
System.out.println("Using == str1 and str2 Equals :" 
            + str1 == str2);

The first SOP statement printing

Using equals() str1 and str2 Equals :true

and the next SOP printing only false .

I tried compiling in both eclipse and Net Beans but result is the same . I am so confused why

Using == str1 and str2 Equals :

is not printing

Help me out in this

Thanks in advance,

Raj

Upvotes: 5

Views: 1565

Answers (10)

Suresh
Suresh

Reputation: 1504

Because + has higher priority compare to = and if you use bracket(str1 == str2) then this result give true because highest priority is (. So First it checks bracket inside data.

String str1 = "Rajesh";
        String str2 = "Rajesh";
        System.out.println("Using equals() str1 and str2 Equals :"
                + str1.equals(str2));
        System.out.println("Using == str1 and str2 Equals :" 
                + (str1 == str2));

Output:

Using equals() str1 and str2 Equals :true
Using == str1 and str2 Equals :true

Upvotes: 4

Eshan Sudharaka
Eshan Sudharaka

Reputation: 607

equals method returns true if and only if x and y refer to the same object.Follwoing is the Object class implementation of equals method.

public boolean equals(Object obj) {
    return (this == obj);
    }

In String class this method has overridden as following.

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = count;
        if (n == anotherString.count) {
        char v1[] = value;
        char v2[] = anotherString.value;
        int i = offset;
        int j = anotherString.offset;
        while (n-- != 0) {
            if (v1[i++] != v2[j++])
            return false;
        }
        return true;
        }
    }
    return false;
    }

And if you use == operator it just check both references are having same object. Similar to the Object class equals method.

Upvotes: 0

Kamran Ali
Kamran Ali

Reputation: 5954

In Java == operator matches the two objects i.e their address while .equals() method mathces the values of both objects, thats why you are getting true for equals() and false for == as both are different objects.

Upvotes: -1

Aaron Johnson
Aaron Johnson

Reputation: 815

See http://bmanolov.free.fr/javaoperators.php for a table of operator precedence in Java.

The + operator is higher precedence than the == operator.

So, in effect, your code is equivalent to the following:

System.out.println( ("Using == str1 and str2 Equals :" + str1) == str2);

Note the placement of the parentheses that I added. It evaluates to this:

System.out.println( (str_x + str1) == str2);

And then to this:

System.out.println( str_y == str2 );

And then to this:

System.out.println( false );

In order to get the result you want, you must use parentheses to specify that you want the == operator to be resolved BEFORE the + operator:

System.out.println( "Using == str1 and str2 Equals :" + (str1 == str2));

Notice the new placement of the parentheses.

Upvotes: 7

Sunil Kumar B M
Sunil Kumar B M

Reputation: 2795

str1.equals(str2) returns true because the equals() function compares the content of the string variables, where as == operator compares the instances. Since str1 and str2 are two differences of instances of String class, it returns false

Upvotes: -1

Kuldeep Jain
Kuldeep Jain

Reputation: 8598

Try surrounding it with () like this:

System.out.println("Using == str1 and str2 Equals :" + (str1 == str2));

Upvotes: 2

shift66
shift66

Reputation: 11958

it's the same as ("Using == str1 and str2 Equals :" + str1) == str2 and this is false, of course. Expression is parsed from left to right and so at first it concatenates "Using == str1 and str2 Equals :" and str1, then applies == operator.

Upvotes: 12

Abrixas2
Abrixas2

Reputation: 3295

The reason is that you cannot compare strings in Java using ==.

In C++ or C# (or other languages supporting operator redefinition), you can overwrite the == operator to provide that functionality. Java does not support that.

Upvotes: -1

Husain Basrawala
Husain Basrawala

Reputation: 1751

== can only be used to compare primitive datatypes. To compare objects you need to use equals method. Using a == operator on objects actually compares there addresses instead of values.

Upvotes: -2

Oleksi
Oleksi

Reputation: 13097

Maybe an order of operations thing? Try:

System.out.println("Using == str1 and str2 Equals :" + (str1 == str2));

Upvotes: 2

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