freeCodeCamp/guide/english/java/equality/index.md

title
Checking for Equality

Checking for Equality

In Java, there are two ways to check if two variables are the "same": == and .equals(). These two methods do not work the same, however.

The == Operator

The basic equality operation in Java, == as in var1 == var2, checks whether var1 and var2 point to the same object reference. That is, if var1 is the same instance of a class in memory as var2, then var1 == var2 is true.

However, if var1 and var2 were created as two separate instances of a class (i.e. with the new keyword), then var1 == var2 will be false. Even if both objects happen to contain the exact same properties and values, the == comparison would not pass because they are not pointing to the same object in memory.

For primitive variable types, such as int and double, the == operator can always be used to check for equality, as their values are stored directly with the variable (rather than as a reference to another slot in memory).

int var1 = 1;
int var2 = 1;
System.out.println(var1 == var2) // true

MyObject obj1 = new MyObject();
MyObject obj2 = obj1;
MyObject obj3 = new MyObject();

System.out.println(obj1 == obj2) // true
System.out.println(obj1 == obj3) // false
System.out.println(obj2 == obj3) // false

The .equals() Method

The built-in Object class in Java, which all other classes automatically extend, contains a number of helpful built-in methods. One such method is equals(), which takes another object as its argument and returns whether the two objects should be considered "equal" according to the relevant logic for that class.

The 'String' class is one of the most common examples of a class that overrides the 'equals()' method. When comparing two 'String's for equality, you need to use the 'equals()' method, as '==' won't work as you expect.

String s1 = "Bob";
String s2 = "ob";
s2 = "B" + s2; //s2 now is also "Bob"
System.out.println(s1 == s2); //false
System.out.println(s1.equals(s2)); //true

When you create a new class in Java, you will often want to override the equals() method in order to provide a more meaningful way to compare two objects of the same class. How this method is implemented is completely up to the developer's judgement.

For example, you may decide that two Persons should be considered "equal" if their name and dateOfBirth are the same. This logic would be implemented in your Person class's equals() method:

public class Person {
    public String name;
    public Date dateOfBirth;
    
    public boolean equals(Person person) {
        return this.name.equals(person.name) && this.dateOfBirth.equals(person.dateOfBirth);
    }
}

Most of the built-in classes in Java, as well as classes provided by popular libraries, will implement the equals() method in a meaningful way.

For example, the java.util.Set interface specifies that a Set's equals() method will return true if "the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set".

However, if a class does not override the default equals() implementation, the default implementation will apply, which simply uses the == operator to compare the two objects.