Java – Using ‘==’ When Comparing Objects

java

Recently in a job interview I was asked this following question (for Java):

Given:

String s1 = "abc";
String s2 = "abc";

What is the return value of

(s1 == s2)

I answered with it would return false because they are two different objects and == is a memory address comparison rather than a value comparison, and that one would need to use .equals() to compare String objects. I was however told that although the .equals(0 methodology was right, the statement nonetheless returns true. I was wondering if someone could explain this to me as to why it is true but why we are still taught in school to use equals()?

Best Answer

String constants are interned by your JVM (this is required by the spec as per here):

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification

This means that the compiler has already created an object representing the string "abc", and sets both s1 and s2 to point to the same interned object.

Related Solutions

Java – How to Compare Two Objects with .equals() and == Operator

== compares object references, it checks to see if the two operands point to the same object (not equivalent objects, the same object).

If you want to compare strings (to see if they contain the same characters), you need to compare the strings using equals.

In your case, if two instances of MyClass really are considered equal if the strings match, then:

public boolean equals(Object object2) {
    return object2 instanceof MyClass && a.equals(((MyClass)object2).a);
}

...but usually if you are defining a class, there's more to equivalency than the equivalency of a single field (a in this case).

Side note: If you override equals, you almost always need to override hashCode. As it says in the equals JavaDoc:

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

Best Answer

Related Solutions

Java – How to Compare Two Objects with .equals() and == Operator

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