java: difference between == and equals() with examples

Both "==" and "equals()" are used to check if two things equal to each other.

The "==" is an operator which can be used to compare primitives as well as Objects. It checks if the two items reference to the same memory location. Since it is an operator, it cannot be overridden.

The "equals()" is a method in the Object class. The "equals()" method in the Object class works the same as the "==" operator. However, since it is a method, it can be overridden to compare based on object components or business logic. As in the case of ArrayList, the equals method compares if the two ArrayLists have the same sequence of objects.

Below are some examples.

1. The String class

The String class is peculiar for it can be initiated with a string constant or a new String instance. Two String constants with the same sequence of characters are always equal regardless of using "==" or "equals()". However, it is a good practice to use equals when comparing two Strings.

String strA = "Welcome!";
String strB = "Wel" + "come!";
String strC = new String(strA);
String strD = new String("Welcome!");

System.out.println("strA == strB? " + (strA == strB));
System.out.println("strA.equals(strB)? " + (strA.equals(strB)));
System.out.println("strA == strC? " + (strA == strC));
System.out.println("strA.equals(strC)? " + (strA.equals(strC)));
System.out.println("strC == strD? " + (strC == strD));
System.out.println("strC.equals(strD)? " + (strC.equals(strD)));

The output:
strA == strB? true
strA.equals(strB)? true
strA == strC? false
strA.equals(strC)? true
strC == strD? false
strC.equals(strD)? true

2. Without overriding the equals() method

public class ObjA {
   private ArrayList<String> array;
   private int numb;

   public ObjA() {
        array = new ArrayList<>();
        numb = 3;
   }
 
   public static void main(String[] args) {
        ObjA theFirst = new ObjA();
        ObjA theSecond = new ObjA();
        ObjA theThird = theFirst;

        System.out.println("Is theFirst == theSecond? " + (theFirst == theSecond));
        System.out.println("Is theFirst.equals(theSecond)? " + (theFirst.equals(theSecond)));
        System.out.println("Is theFirst == theThird? " + (theFirst == theThird));
        System.out.println("Is theFirst.equals(theThird)? " + (theFirst.equals(theThird)));
        System.out.println("Is theSecond == theThird? " + (theSecond == theThird));
        System.out.println("Is theSecond.equals(theThird)? " + (theSecond.equals(theThird)));
   }
}

The Outpput:
Is theFirst == theSecond? false
Is theFirst.equals(theSecond)? false
Is theFirst == theThird? true
Is theFirst.equals(theThird)? true
Is theSecond == theThird? false
Is theSecond.equals(theThird)? false

3. With overriding the equals() method

public class ObjB {
   private ArrayList<String> array;
   private int numb;

   public ObjB() {
        array = new ArrayList<>();
        numb = 3;
   }

   public ArrayList getArray() {
         return array;
   }

   public int getNumber() {
         return numb;
   }

   @Override
   public boolean equals(Object obj) {
         if (obj instancdof ObjB){
              ObjB ob = (ObjB)obj;
              if (getArray().equals(ob.getArray()) && getNumber() == ob.getNumber()){
                   return true;
              }
         }
         return false;
   }
 
   public static void main(String[] args) {
        ObjB theFirst = new ObjB();
        ObjB theSecond = new ObjB();
        ObjB theThird = theFirst;

        System.out.println("Is theFirst == theSecond? " + (theFirst == theSecond));
        System.out.println("Is theFirst.equals(theSecond)? " + (theFirst.equals(theSecond)));
        System.out.println("Is theFirst == theThird? " + (theFirst == theThird));
        System.out.println("Is theFirst.equals(theThird)? " + (theFirst.equals(theThird)));
        System.out.println("Is theSecond == theThird? " + (theSecond == theThird));
        System.out.println("Is theSecond.equals(theThird)? " + (theSecond.equals(theThird)));
   }
}

The Outpput:
Is theFirst == theSecond? false
Is theFirst.equals(theSecond)? true
Is theFirst == theThird? true
Is theFirst.equals(theThird)? true
Is theSecond == theThird? false
Is theSecond.equals(theThird)? true

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