5.3. Special Methods

Every class has a few special methods that belong to it, regardless of whether or not we define them. Exactly how every class obtains these methods will be explored in a future lesson. For now, let’s look at two important examples of these methods.

5.3.1. toString

The toString method returns a string representation of a class. Calling toString on a class that you’ve written will result in something like this:

Example

1
2
Student person = new Student("Violet");
System.out.println(person.toString());

Console Output

Here, we called toString on a Student object. The default toString implementation is generally not very useful. Most of the time, you’ll want to write your own toString method to override the default and provide better results.

Here’s how we might do it for Student to produce a much more friendly message:

Example

1
2
3
4
5
6
public String toString() {
   return name + " (Credits: " + numberOfCredits + ", GPA: " + gpa + ")";
}

Student person = new Student("Violet");
System.out.println(person.toString());

Console Output

Violet (Credits: 0, GPA: 0.0)

In the example, we define the toString method to return a string that reports the class fields name, numberOfCredits, and gpa in a clear manner.

Note that toString is often implicitly called for you. For example, the output above could have been generated by the following code, which calls toString on person within System.out.println.

1
2
Student person = new Student("Violet");
System.out.println(person);

5.3.2. equals

Suppose we have two objects of type Student, say student1 and student2, and we want to determine if they are equal. If we try to compare them using ==, we will likely get a result we did not expect. This is because student1 and student2 are reference variables, which means they hold a reference to, or the address of, the actual Student objects. student1 and student2 evaluate as equal only when they have the same memory address.

To state that again: student1 and student2 will be equal (==) only when they refer to, or point at, the exact same object. Consider the example below, which creates two Student objects:

Example

1
2
3
4
5
6
Student student1 = new Student("Maria", 1234);
Student student2 = new Student("Maria", 1234);

System.out.println(student1.name + ", " + student1.id + ": " + student1);
System.out.println(student2.name + ", " + student2.id + ": " + student2);
System.out.println(student1 == student2);

Console Output

Maria, 1234: [email protected]
Maria, 1234: [email protected]
false

Even though the objects have the exact same keys and values, student1 and student2 point to different memory locations. Therefore, the == check returns false.

This is not usually how we want to compare objects. For example, we might want to consider two Student objects equal if they have the same name, email, or student ID.

The equals() method determines if one object is equal to another in this sense. We introduced the method when discussing strings, but it also applies to all other classes.

The code below shows how to use equals() to compare two students. Note that they have different names but the same student ID, indicating they are actually the same person.

1
2
3
4
5
6
7
Student bono1 = new Student("Paul David Hewson", 4);
Student bono2 = new Student("Bono", 4);

if (bono1.equals(bono2)) {
   System.out.println(bono1.getName() +
      " is the same as " + bono2.getName());
}

If we don’t provide our own equals() method, the default option only considers two objects equal if they are the exact same object, which means they point to the same memory address. This is identical to the behavior we see when using the == operator: bono1 == bono2.

In the example above we created two different Student objects, so the expression bono1.equals(bono2) evaluates to false. In order to compare two objects based on their properties, rather than their memory references, we need to define our own equals() method.

The difference between the comparison carried out by the default equals() method (and by the == operator), and how we would like to compare our classes, is the difference between identity and equality.

  1. Two objects are identical if they both point to the same memory address. In essence, they are the same object. If object1 and object2 are identical, then changing one property value in object1 also changes that value for object2.
  2. Two objects are equal if the values they store are the same at the time of comparison. student1 and student2 point to different memory addresses, but their values are all the same. Thus, we can consider them equal, even though they are not identical.

The default equals() method and the == operator test for identity, whereas we want to test for equality instead. We can do so by overriding the equals() method. We will discuss overriding in more detail later, but for now just recognize that it involves defining different behavior for an existing method.

Two things can be considered equal even if they do NOT have all the same values. In the case of the Student class, we might specify that two Student objects are equal if they have the same ID numbers. We would then write a new method definition for equals() as follows:

1
2
3
4
public boolean equals(Object toBeCompared) {
   Student theStudent = (Student) toBeCompared;
   return theStudent.getStudentId() == getStudentId();
}

Now if we evaluate bono1.equals(bono2) we will get a result of true, since the student IDs match.

One catch of working with equals() is that its input parameter must be of type Object, even if we’re working in a class like Student. The reason why will become more clear in the next lesson, when we introduce the Object class. For now, the practical implication is that we must convert, or cast, the input toBeCompared to be of type Student with the syntax (Student) toBeCompared. Then we compare the converted student’s ID (bono2.id) to that of the current student (bono1.id).

Here’s a visualization of the concepts of equality and identity:

Equality

Equality

When you test for equality, you look at two different objects and compare some aspect of them to each other.

Identity

Identity

When you test for identity, you look at two variables to see if they reference the exact same object.

5.3.2.1. Coding a New equals Method

You’ll often want to implement equals() yourself. When you do, be sure you understand the best practices around how the method should behave. These are a little more involved compared to coding a new toString method.

In fact, the equals() method we defined above isn’t very good by most Java programmers’ standards. Let’s improve it.

5.3.2.1.1. Problem #1

The method argument cannot be converted to a Student instance.

When we attempt to cast the argument toBeCompared to type Student, we’ll get an exception if toBeCompared can’t be properly converted. This happens if something other than a Student object gets passed into equals(). To prevent this from happening, we’ll return false if toBeCompared was not created from the Student class. To check this, we use the getClass method, which is available to every object (similarly to toString).

1
2
3
4
5
6
7
8
9
public boolean equals(Object toBeCompared) {

   if (toBeCompared.getClass() != getClass()) {
      return false;
   }

   Student theStudent = (Student) toBeCompared;
   return theStudent.getStudentId() == getStudentId();
}

Lines 3 - 5 ensure that the two objects that we want to compare were created from the same class.

5.3.2.1.2. Problem #2

toBeCompared might be null.

If toBeCompared is null then toBeCompared.getClass() throws an exception. This is an easy issue to fix—just compare the object to null. If the comparison evaluates to true, then we know the object is null and equals() should return false.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
public boolean equals(Object toBeCompared) {

   if (toBeCompared == null) {
      return false;
   }

   if (toBeCompared.getClass() != getClass()) {
      return false;
   }

   Student theStudent = (Student) toBeCompared;
   return theStudent.getStudentId() == getStudentId();
}

Line 3 checks toBeCompared for null, preventing an error in line 7. Line 7 checks the class of toBeCompared, preventing an error in line 11.

5.3.2.1.3. Problem #3

The two objects to compare are the same object (identical).

This is less of a problem and more of a way to improve our equals() method. If toBeCompared is the same literal object that we are comparing it to, then we can make a quick determination and save a few checks.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
public boolean equals(Object toBeCompared) {

   if (toBeCompared == this) {
      return true;
   }

   if (toBeCompared == null) {
      return false;
   }

   if (toBeCompared.getClass() != getClass()) {
      return false;
   }

   Student theStudent = (Student) toBeCompared;
   return theStudent.getStudentId() == getStudentId();
}

Line 3 checks for identity. If true, then the remaining checks become unnecessary.

5.3.2.2. Components of equals

Almost every equals method you write will look similar to the last example above. It will contain the following segments in this order:

  1. Reference check: If the two objects are the same, return true right away.
  2. Null check: If the argument is null, return false.
  3. Class check: Compare the classes of the two objects to ensure a safe cast. Return false if the classes are different.
  4. Cast: Convert the argument to the type of our class, so getters and other methods can be called.
  5. Custom comparison: Use custom logic to determine whether or not the two objects should be considered equal. This will usually be a comparison of properties or fields.

5.3.2.3. Characteristics of equals

Now that we know how to write an equals() method, let’s look at some characteristics that every such method should have. Following the general outline above makes it easier to ensure that your equals() method has these characteristics.

  1. Reflexivity: For any non-null reference value x, x.equals(x) should return true.
  2. Symmetry: For any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) also returns true.
  3. Transitivity: For any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  4. Consistency: As long as x and y do not change x.equals(y) should always return the same result.
  5. Non-null: For any non-null reference value x, x.equals(null) should return false.

If you think back to what your math classes had to say about equality, then these concepts make sense.

Using the general approach outlined above to implement equals() will make it easier to meet these characteristics. However, always check your method! Missing one or more characteristics can be disastrous for your Java applications.

Tip

Seasoned Java developers will tell you that every time you implement your own version of equals() you should also implement your own version of hashCode(). hashCode() is another special method that every class has. Understanding hashCode() would take us a bit far afield at this point, but we would be remiss to not mention it. If you want to read more, check out this article and this stack overflow.

5.3.2.4. Take Away

You may not need to write your own equals() method for every class you create. However, as a new Java programmer, remember the following:

Always use equals() to compare objects.

This is especially true when working with objects of types provided by Java, such as String. A class that is part of Java or a third-party library will have implemented equals() in a way appropriate for the particular class, whereas == will only check to see if two variables refer to the same literal object.

5.3.4. Check Your Understanding

Question

Given the code:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
public class Pet {

   private String name;

   Pet(String name) {
      this.name = name;
   }

   public String getName() {
      return name;
   }
}

String firstPet = "Fluffy";
Pet secondPet = new Pet("Fluffy");
Pet thirdPet = new Pet("Fluffy");

Which of the following statements evaluates to true?

  1. firstPet == secondPet;
  2. secondPet == thirdPet;
  3. thirdPet.equals(secondPet);
  4. thirdPet.getName().equals(firstPet);
  5. thirdPet.equals(firstPet);

Question

We add the following code inside the Pet class:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
public boolean equals(Object petToCheck) {

   if (petToCheck == this) {
      return true;
   }

   if (petToCheck == null) {
      return false;
   }

   if (petToCheck.getClass() != getClass()) {
      return false;
   }

   Pet thePet = (Pet) petToCheck;
   return thePet.getName() == getName();
}

Which of the following statements evaluated to false before, but now evaluates to true?

  1. firstPet == secondPet;
  2. secondPet == thirdPet;
  3. thirdPet.equals(secondPet);
  4. thirdPet.getName().equals(firstPet);
  5. thirdPet.equals(firstPet);