[java] How to use the toString method in Java?

Can anybody explain to me the concept of the toString() method, defined in the Object class? How is it used, and what is its purpose?

It may optionally have uses within the context of an application but far more often it is used for debugging purposes. For example, when you hit a breakpoint in an IDE, it's far easier to read a meaningful toString() of objects than it is to inspect their members.

There is no set requirement for what a toString() method should do. By convention, most often it will tell you the name of the class and the value of pertinent data members. More often than not, toString() methods are auto-generated in IDEs.

Relying on particular output from a toString() method or parsing it within a program is a bad idea. Whatever you do, don't go down that route.

If you learn Python first and then Java. I think it plays the same role as __str__() method in Python, it is a magic method like __dict__() and __init__() but to refer to a string representing the the object.

Whenever you access an Object (not being a String) in a String context then the toString() is called under the covers by the compiler.

This is why

Map map = new HashMap();
System.out.println("map=" + map);

works, and by overriding the standard toString() from Object in your own classes, you can make your objects useful in String contexts too.

(and consider it a black box! Never, ever use the contents for anything else than presenting to a human)

/**
 * This toString-Method works for every Class, where you want to display all the fields and its values
 */
public String toString() {

    StringBuffer sb = new StringBuffer();

    Field[] fields = getClass().getDeclaredFields(); //Get all fields incl. private ones

    for (Field field : fields){

        try {

            field.setAccessible(true);
            String key=field.getName();
            String value;

            try{
                value = (String) field.get(this);
            } catch (ClassCastException e){
                value="";
            }

            sb.append(key).append(": ").append(value).append("\n");

        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

    }

    return sb.toString();
}

Apart from what cletus answered with regards to debugging, it is used whenever you output an object, like when you use

 System.out.println(myObject);

or

System.out.println("text " + myObject);

the toString() converts the specified object to a string value.

The main purpose of toString is to generate a String representation of an object, means the return value is always a String. In most cases this simply is the object's class and package name, but on some cases like StringBuilder you will got actually a String-text.

Correctly overridden toString method can help in logging and debugging of Java.

Use of the String.toString:

Whenever you require to explore the constructor called value in the String form, you can simply use String.toString... for an example...

package pack1;

import java.util.*;

class Bank {

    String n;
    String add;
    int an;
    int bal;
    int dep;

    public Bank(String n, String add, int an, int bal) {

        this.add = add;
        this.bal = bal;
        this.an = an;
        this.n = n;

    }

    public String toString() {
        return "Name of the customer.:" + this.n + ",, "
                + "Address of the customer.:" + this.add + ",, " + "A/c no..:"
                + this.an + ",, " + "Balance in A/c..:" + this.bal;
    }
}

public class Demo2 {

    public static void main(String[] args) {

        List<Bank> l = new LinkedList<Bank>();

        Bank b1 = new Bank("naseem1", "Darbhanga,bihar", 123, 1000);
        Bank b2 = new Bank("naseem2", "patna,bihar", 124, 1500);
        Bank b3 = new Bank("naseem3", "madhubani,bihar", 125, 1600);
        Bank b4 = new Bank("naseem4", "samastipur,bihar", 126, 1700);
        Bank b5 = new Bank("naseem5", "muzafferpur,bihar", 127, 1800);
        l.add(b1);
        l.add(b2);
        l.add(b3);
        l.add(b4);
        l.add(b5);
        Iterator<Bank> i = l.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }

}

... copy this program into your Eclipse, and run it... you will get the ideas about String.toString...

Coding:

public class Test {

    public static void main(String args[]) {

        ArrayList<Student> a = new ArrayList<Student>();
        a.add(new Student("Steve", 12, "Daniel"));
        a.add(new Student("Sachin", 10, "Tendulkar"));

        System.out.println(a);

        display(a);

    }

    static void display(ArrayList<Student> stu) {

        stu.add(new Student("Yuvi", 12, "Bhajji"));

        System.out.println(stu);

    }

}

Student.java:

public class Student {

    public String name;

    public int id;

    public String email;

    Student() {

    }

    Student(String name, int id, String email) {

        this.name = name;
        this.id = id;
        this.email = email;

    }

     public String toString(){           //using these toString to avoid the output like this [com.steve.test.Student@6e1408, com.steve.test.Student@e53108]
          return name+" "+id+" "+email;     
         }  


    public String getName(){

        return name;
    }

    public void setName(String name){

        this.name=name;
    }

    public int getId(){

        return id;
    }

    public void setId(int id){

        this.id=id;
    }

    public String getEmail(){

        return email;

    }

    public void setEmail(String email){

        this.email=email;
    }
}

Output:

[Steve 12 Daniel, Sachin 10 Tendulkar]

[Steve 12 Daniel, Sachin 10 Tendulkar, Yuvi 12 Bhajji]

If you are not used toString() in Pojo(Student.java) class,you will get an output like [com.steve.test.Student@6e1408, com.steve.test.Student@e53108].To avoid these kind of issue we are using toString() method.

The toString() method returns a textual representation of an object. A basic implementation is already included in java.lang.Object and so because all objects inherit from java.lang.Object it is guaranteed that every object in Java has this method.

Overriding the method is always a good idea, especially when it comes to debugging, because debuggers often show objects by the result of the toString() method. So use a meaningful implementation but use it for technical purposes. The application logic should use getters:

public class Contact {
  private String firstName;
  private String lastName;
  public Contact (String firstName, String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  public String getFirstName() {return firstName;}
  public String getLastName() {return lastName;}

  public String getContact() {
    return firstName + " " + lastName;
  }

  @Override
  public String toString() {
    return "["+getContact()+"]";
  }
}

toString() returns a string/textual representation of the object. Commonly used for diagnostic purposes like debugging, logging etc., the toString() method is used to read meaningful details about the object.

It is automatically invoked when the object is passed to println, print, printf, String.format(), assert or the string concatenation operator.

The default implementation of toString() in class Object returns a string consisting of the class name of this object followed by @ sign and the unsigned hexadecimal representation of the hash code of this object using the following logic,

getClass().getName() + "@" + Integer.toHexString(hashCode())

For example, the following

public final class Coordinates {

    private final double x;
    private final double y;

    public Coordinates(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public static void main(String[] args) {
        Coordinates coordinates = new Coordinates(1, 2);
        System.out.println("Bourne's current location - " + coordinates);
    }
}

prints

Bourne's current location - Coordinates@addbf1 //concise, but not really useful to the reader

Now, overriding toString() in the Coordinates class as below,

@Override
public String toString() {
    return "(" + x + ", " + y + ")";
}

results in

Bourne's current location - (1.0, 2.0) //concise and informative

The usefulness of overriding toString() becomes even more when the method is invoked on collections containing references to these objects. For example, the following

public static void main(String[] args) {
    Coordinates bourneLocation = new Coordinates(90, 0);
    Coordinates bondLocation = new Coordinates(45, 90);
    Map<String, Coordinates> locations = new HashMap<String, Coordinates>();
    locations.put("Jason Bourne", bourneLocation);
    locations.put("James Bond", bondLocation);
    System.out.println(locations);
}

prints

{James Bond=(45.0, 90.0), Jason Bourne=(90.0, 0.0)}

instead of this,

{James Bond=Coordinates@addbf1, Jason Bourne=Coordinates@42e816}

Few implementation pointers,

1. You should almost always override the toString() method. One of the cases where the override wouldn't be required is utility classes that group static utility methods, in the manner of java.util.Math. The case of override being not required is pretty intuitive; almost always you would know.
2. The string returned should be concise and informative, ideally self-explanatory.
3. At least, the fields used to establish equivalence between two different objects i.e. the fields used in the equals() method implementation should be spit out by the toString() method.

4. Provide accessors/getters for all of the instance fields that are contained in the string returned. For example, in the Coordinates class,

   public double getX() {
       return x;
   }
   public double getY() {
       return y;
   }
   

A comprehensive coverage of the toString() method is in Item 10 of the book, Effective Java™, Second Edition, By Josh Bloch.