I constructed a class with one String
field. Then I created two objects and I have to compare them using ==
operator and .equals()
too. Here's what I've done:
public class MyClass { String a; public MyClass(String ab) { a = ab; } public boolean equals(Object object2) { if(a == object2) { return true; } else return false; } public boolean equals2(Object object2) { if(a.equals(object2)) { return true; } else return false; } public static void main(String[] args) { MyClass object1 = new MyClass("test"); MyClass object2 = new MyClass("test"); object1.equals(object2); System.out.println(object1.equals(object2)); object1.equals2(object2); System.out.println(object1.equals2(object2)); } }
After compile it shows two times false as a result. Why is it false if the two objects have the same fields - "test"?
equals() method. The major difference between the == operator and . equals() method is that one is an operator, and the other is the method. Both these == operators and equals() are used to compare objects to mark equality.
equals() method in Java. Both equals() method and the == operator are used to compare two objects in Java. == is an operator and equals() is method. But == operator compares reference or memory location of objects in a heap, whether they point to the same location or not.
The == operator compares whether two object references point to the same object.
== should be used during reference comparison. == checks if both references points to same location or not. equals() method should be used for content comparison. equals() method evaluates the content to check the equality.
==
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 thehashCode
method, which states that equal objects must have equal hash codes.
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