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| 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
Note: a common first time coding error is to use a single equal sign instead of double equal signs. Using a single = will cause a compile time error. For example:
int var1 = 4;
int var2 = 4;
if(var1 = var2)
System.out.println("The variables are equal.");
else
System.out.println("The variables are not equal.");
The if statement will cause an error. It is important to remember that a single equal sign is used for assignment while the double equal sign is used for comparison.
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. Instead of returning 'true' when comparing objects, the '==' operator will continuously return 'false' unless the objects being compared are the same object.
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 Person objects should be considered equal if their name and dateOfBirth instance variables are the same. This logic would be implemented in your Person class's equals() method:
public class Person {
public String name;
public Date dateOfBirth;
//The two lines above are instance attributes of the Person object
public boolean equals(Person person) {
return this.name.equals(person.name) && this.dateOfBirth.equals(person.dateOfBirth);
//An edited equals() method can allow for the developer to choose what characteristics of an object should be compared in order to deem them equal
}
}
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.
The .equalsIgnoreCase() Method
This built-in function in java is used to compare the equality of 2 strings return true or false depending on the match but this function does not see if the characters are in upper case or in lower case. Example:
String s1="DEMO for Equality";
String s2="Demo for equality";
System.out.println(s1.equals(s2)); //false
System.out.println(s1.equalsIgnoreCase(s2)); //true
Object Equality
The java Object class provides two methods for comparing objects which are equals(Object obj) and hashcode().
As discussed above, the equals(Object obj) method indicates if the object passed in argument is equal to the current instance. The default implementation of this method interprets that two objects are equal if and only if the memory address of the objects are same.
The hashCode() method returns an integer that represents the memory location of the object and this will be unique for each and every new instance.
However, the default implementation of the methods will have limitations as we cannot compare the custom objects on our own attributes.
Consider the following example for the limitation.
public static void main (String[]args) {
Employee emp1 = new Employee(1,"foo");
Employee emp2 = new Employee(1,"foo");
System.out.println(emp1.equals(emp2)); //output :- false
}
The example prints false because both the employee objects (though they have same attribute id which is 1) are stored in different memory locations. Hence, they are not considered to be equal.
The above limitation can be resolved by overriding the equals() method of the Employee object.
@Override
public boolean equals(Object obj) {
// If the object is null return false.
if (obj == null) return false;
// If the object is not an instance of Employee object return false.
if (!(obj instanceof Employee))
return false;
// If the objects are stored in same memory address return true.
if (obj == this)
return true;
// If the objects have same employee Id return true.
return this.getId() == ((Employee) obj).getId();
}
Now, the following example will print true as we are comparing the objects on the id attribute of the Employee object.
public static void main (String[]args) {
Employee emp1 = new Employee(1,"foo");
Employee emp2 = new Employee(1,"foo");
System.out.println(emp1.equals(emp2)); //output : true
}