Java Error Iii - Using == Instead Of Equals() To Compare Objects Inwards Java

In this usage of Java programming mistakes, nosotros volition receive got a hold off at some other mutual pattern, where programmers tend to usage "==" operator to compare Objects, similar to comparison primitives. Since equality of object tin endure really dissimilar inwards the physical in addition to logical sense, in addition to inwards the illustration of domain objects it's by in addition to large driven yesteryear trouble organisation rules, comparison objects amongst "==" operator, introduces subtle bugs, which are difficult to find. The departure betwixt equals() in addition to == operator,  one of the Java classics is too asked to honor out if the developer is familiar amongst this of import concept or not. Using == operator exclusively brand feel when comparison primitives similar int, or in conclusion constants similar Enum. Though at that topographic point is to a greater extent than involved inwards comparing ii Enum, which y'all larn yesteryear next that link.

One of the most mutual patterns of this error is comparison ii Strings amongst the == operator, which nosotros volition run into inwards this article. By the way, this is the 3rd article inwards serial of common Java programming mistakes, in addition to if y'all haven't read the previous two, y'all tin read them hither :

• Java Mistake 1 : Using double in addition to Float for monetary Calculation
• Java Mistake 2 : Mixing upward static in addition to non-static synchronized methods

Comparing Strings amongst == instead of equals() method

String literal if compared amongst equality functioning (==) or non equality operator (!=) behaves perfectly, which makes programmers intend that, this is the correct agency of comparison Strings inwards Java, which inwards fact is not. 

In reality, the  "==" operator returns true if both operands indicate to the same object. In the illustration of String literal, it’s truthful because String pool returns same String object if created every bit literal, simply it breaks when y'all compare ii String which is non inwards the puddle or exclusively 1 of them is literal. 

To avoid this subtle bug, ever use equals() method of String to compare equality of  two String inwards Java, which returns truthful if both string objects comprise exact same characters. 


You tin too run into Core Java Volume 1 - Fundamentals yesteryear Cay S. Horstmann to larn most both equality operator in addition to how it industrial plant amongst primitive in addition to object types inwards Java. It is too 1 of the meliorate books most practical Java programming inwards the market. 

Anyway, hither is an illustration of comparison String which makes this Java error clear:

public class UseEqualsToCompareString {        public static void main(String args[]) throws IOException {         String mention = "abc";         String surname = "abc";                 //this volition provide truthful because mention in addition to surname points to same string object         if(name == surname){             System.out.println("Two Strings are equal yesteryear == because they cached inwards string pool");         }                 surname = new String("abc");                 //this volition trial inwards imitation every bit forthwith surname is non a string literal in addition to points to dissimilar object         if(name == surname){             System.out.println("String literal in addition to String created amongst new() are equal using ==");         }else{             System.out.println("String literal in addition to String created amongst new() are non equal using ==");         }                 //both strings are equal because at that topographic point content is same         if(name.equals(surname)){             System.out.println("Two Strings are equal inwards Java using equals method because content is same");         }else{             System.out.println("Two Strings are non equal inwards Java using equals method because content is same");         }          }        } Output: Two Strings are equal yesteryear == because they cached inwards string puddle String literal in addition to String created amongst new() are non equal using == Two Strings are equal inwards Java using equals method because content is same

So ever usage equals() method to compare ii strings inwards Java, that's too the correct agency to compare Strings inwards Java.  This is too truthful for comparison an instance of Integer cast inwards Java 5. 

The JDK v release, too known every bit Tiger introduces autoboxing in addition to unboxing which converts int to Integer automatically. the worst usage is that autoboxing creates Integer object yesteryear calling Integer.valueOf() method which maintains an internal Integer cache of default hit -128 to 127. 

So, anytime y'all machine box an int value which is from -128 to 127 it volition provide same Integer object which if compared amongst equality operator "==" volition provide true, simply every bit presently every bit y'all moved out of that hit autoboxed integer compared amongst "==" volition provide imitation despite having same numerical value. See here for to a greater extent than details on this subtle issue.  

This creates a lot of confusion approximately Java programmers. The bottom trace is avoid using "==" for comparison object until y'all desire it explicitly, ever usage equals() method for checking equality of ii objects . 

Btw, If y'all receive got come upward hither every bit usage of your Java Interview training in addition to then y'all tin too banking concern jibe out Java Programming Interview Exposed, 1 of the practiced majority for Java guys

Anyway, next Java code illustration volition brand these things clear:

public class IntegerAutoBoxingTest {        public static void main(String args[]) throws IOException {         Integer number1 = 12;         Integer number2 = 12;                 //two integers volition endure equal because Java maintains cache of Integers for values -128 to 128         if(number1 == number2){             System.out.println("Two Integers objects are equal amongst == because they created "                     + "using machine boxing in addition to at that topographic point value is betwixt -128 to 128");         }                 number1 = new Integer(12);                 //two integers volition non endure equal because 1 novel Integer() creates split object         if(number1 == number2){             System.out.println("Integer created using auto-boxing in addition to created using new() "                     + "are equal using == ");         }else{             System.out.println("Integer created using auto-boxing in addition to crated using new() "                     + "are non equal using == ");         }                 //both Integers volition endure equals because at that topographic point numeric value is same         if(number1.equals(number2)){             System.out.println("Two Integers inwards Java are equal using equals method "                     + "because of same numeric value");         }else{             System.out.println("Integers inwards Java are non equal using equals method");         }                 number1 = 150;         number2 = 150;                 //Integers volition non endure equal every bit at that topographic point are out of cache hit -128 in addition to 128.         if(number1 == number2){             System.out.println("Two Integer objects created amongst machine boxing amongst value "                     + "outside -128 to 128 are equal using == ");         }else{             System.out.println("Two Integer objects created amongst machine boxing amongst value "                     + "outside -128 to 128 are non equal using == ");         }          }        }

That's all on this usage of common Java mistakes. Remember, ever prefer equals() method over == operator for checking object’s equality. Let us know if y'all similar this serial in addition to receive got whatsoever other mutual Java error or pattern, which y'all would similar to portion amongst Java community. 

Further Learning
Complete Java Masterclass
Java Fundamentals: The Java Language
Java In-Depth: Become a Complete Java Engineer!

Share this post