Java equal vs == (String Instance Variable)

Question

As I know == basically compares memory location, So I totally understand that line 1 is returning true. But we have not overridden the equal method in class , then why line 2 is returning true?

private String category = "procedura1";
public static void main(String[] args) {
    Lang obj1 = new Lang();
    Lang obj2 = new Lang();
    if (obj1.category == obj2.category) {  ///  Line 1
        System.out.println("Equal");
    } else {
        System.out.println("Not equal");
    }
    if (obj1.category.equals(obj2.category)) {  /// Line 2
        System.out.println("Equal");
    } else {
        System.out.println("Not equal");
    }
}

And Why line 3 of following returning false?

    String a1 = new String("String 1");
    String a2 = new String("String 1");

    if (a1 == a2) {      // Line 3
        System.out.println("True");
    } else {
        System.out.println("False");

    }

    if (a1.equals(a2)) {
        System.out.println("True");
    } else {
        System.out.println("False");

    }

You are comparing the Strings so you are using String.equals. — Peter Lawrey, Dec 14 '14 at 10:39
https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#equals(java.lang.Object) — le3th4x0rbot, Dec 14 '14 at 10:43

nitishagar · Accepted Answer · 2014-12-14T10:41:30.457

2

The category is of type String which uses the String equals() implementation.

edited Dec 14 '14 at 10:41

answered Dec 14 '14 at 10:35

nitishagar

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So, string data type automatically calling String class? – Robin Paul Dec 14 '14 at 10:40
1

@RobinPaul String is not a primitive data type, it is an object. So yes. – Rudi Kershaw Dec 14 '14 at 10:40
I have added a new code, why line 3 is returning false? – Robin Paul Dec 14 '14 at 11:06
As they point to different locations in memory. – nitishagar Dec 14 '14 at 11:09

score 0 · Answer 2 · answered Dec 14 '14 at 10:38

0

This is because you're overlooking the custom object entirely and going to a field that has an equals method defined.

If you were doing something like this instead:

if(obj1.equals(obj2))

...you would have an issue if equals isn't defined on Lang. But, as we know, equals is indeed defined on String, so you would see it working just fine.

This is, of course assuming that category actually is String; I'm only inferring it for now.

answered Dec 14 '14 at 10:38

Makoto

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category is listed in the post as a String. – JamesB Dec 14 '14 at 10:56

Bohemian · Answer 3 · 2014-12-14T11:30:00.277

0

The value of the category field is a String literal, which is interned, so every instance's category refers to the same String object, hence they are equal according to ==.

The contract for equals() is that it return true when called on itself, and you can rely on classes from the JDK honouring this contract.

edited Dec 14 '14 at 11:30

answered Dec 14 '14 at 10:40

Bohemian

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score 0 · Answer 4 · answered Dec 14 '14 at 10:52

It's true 'cause you're basically comparing the two thing properties.. That's equivalent to:

public static void main(String[] args) {
    String str1 = "procedura1";
    String str2 = "procedura1";

    if (str1.equals(str2)) {
        System.out.print("Both String objects represent the same sequence of characters");
    }
}

But if you may compare it this was:

public static void main(String[] args) {
    Lang obj1 = new Lang();
    Lang obj2 = new Lang();

    if (obj1.equals(obj2)) {
        System.out.println("Both Lang objects are equal.");
    }
}

You won't get any message.. hope this helps ;)

Java equal vs == (String Instance Variable)

4 Answers4