Answer.

The toString() method is a standard method of the Object class in Java that allows you to obtain a string representation of an object. Historically, it was introduced to aid in debugging and logging, so instead of outputting technical information about the object (ClassName@hashCode), you could get a meaningful, readable description.

Background

Initially, if toString() is not overridden, the method returns a string of the form class_name@hash_code. This is inconvenient for understanding or displaying the object's state. Therefore, overriding toString() has become a good practice.

The Problem

Without an explicit implementation of toString(), debugging and logging objects become complicated. It is difficult to understand what exactly is contained in the object without knowing its internal structure or the current values of its fields.

The Solution

Correctly overriding toString() allows you to:

Display the values of meaningful fields.
Maintain readability, avoiding unnecessary/sensitive data.
Aid in debugging, logging, and testing.

Example code:

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{name='" + name + "', age=" + age + "}";
    }
}

Key features:

Maintain informativeness and readability.
Do not expose sensitive data.
Ensure correct operation when fields of the class change.

Tricky Questions.

Is it always necessary to override the toString() method in all classes?

No, it is not mandatory, but it is highly recommended to override it in classes whose objects are often logged or used for debugging.

Can the toString() method throw exceptions?

Technically — yes, if, for example, a NullPointerException occurs when accessing a field without checking for null. But good practice is to avoid this to ensure that toString() always returns a valid string.

Is an explicit call to toString() mandatory?

No, the compiler calls it implicitly when concatenating an object with a string or when outputting an object via System.out.println.

Person p = new Person("Ivan", 25);
System.out.println(p); // Calls p.toString() automatically

Common Mistakes and Anti-Patterns

toString() is not implemented in classes where it would be useful.
Outputs sensitive data (e.g., password).
Returns too concise or uninformative a string.
Generates exceptions due to uninitialized fields.

Real-life Example

Negative Case

A programmer does not override toString() in the User class and writes to the log:

log.info(user);

Pros:

No time is spent on overriding the method.

Cons:

The result is a string like User@1a2b3c, which provides no information about the user.
Makes debugging and investigating problems harder.

Positive Case

A programmer overrides toString():

@Override
public String toString() {
    return "User{name='" + name + "', id=" + id + "}";
}