User-controlled conversion of objects into String representation - toString()

Previously discussed: converting primitive types (int, double, ...) to String ( click here)

Java provides an automatic conversion feature when the + operator is used between:

a number typed data and a string typed data

Java's automatic conversion rule for number ⇒ string:

When the + operator is used between
Then:

Example:

public class String05 { public static void main(String[] args) { String a, b; int x; a = "abc"; x = -12; b = a + x; System.out.println(b); // Prints "abc-12" b = x + a; System.out.println(b); // Prints "-12abc" } }

Webpage: click here

Automatic conversion of variables of a user-definied type to String

Java also provides an automatic conversion feature when the + operator is used between:

a user-defined type variable (= object) and a string typed data

Java's automatic conversion rule for object ⇒ string:

When the + operator is used between

a object reference variable and
a string

Then:

the object referred to by the object reference variable is automatically converted to a string
The + operator is then applied on 2 strings (i.e., the + operator will be a string concatenation !)

Example:

public class Class10 { public static void main(String[] args) { String x; BankAccount stu1 = new BankAccount( 123, "John", 1000 ); x = "Test --- " + stu1 ; // First converts stu1 to a String System.out.println( x ); } }

Output:

Test --- BankAccount@130c19b

The string BankAccount@130c19b is the result of the conversion of the object stu1 into the String type

Example Program: (Demo above code)
- The BankAccount class Prog file: click here
- The Class10.java Prog file: click here
How to run the program:

Java's internal mechanism used to provide automatic object ⇒ String conversion

The automatic conversion from an object type to the String type is accomplished by

Translating an expression containing the + operator by the Java compiler:

The expression:

String + Object or Object + String

is translated into:

String + Object ⇒ String + Object.toString() or Object + String ⇒ Object.toString() + String

Furthermore:

Every user-defined type (= object) contains the method

public String toString() { .... }

Advanced Java programming note:

Exploiting (how to use) Java's mechanism for providing automatic object ⇒ String conversion

We can "exploit" (a better word is use) the Java's mechanism for providing automatic object ⇒ String conversion

Suppose we define our own toString() method inside the BankAccount class:

public class BankAccount { private int accNum; // Private access private String name; // -- limit to ONLY inside private double balance; // the class "BankAccount" /* ==================================================== Constructor 1: initialize all 3 instance variables ==================================================== */ public BankAccount(int a, String n, double amount) { accNum = a; name = n; balance = amount; } /* ==================================================== Default Constructor ==================================================== */ public BankAccount( ) { accNum = -1; name = "Nobody"; balance = 0.0; } /* ================================================== toString(): return a String containing information of BankAccount (BTW, this method used to be called: convToString) =================================================== */ public String toString( ) { return( "Account number: " + this.accNum + ", Name: " + this.name + ", Balance: " + this.balance); } /* ============================================== deposit(amount): Add "amount" to balance ============================================== */ public void deposit( double amount ) { this.balance += amount; } /* ====================================================== withdraw(amount): Subtract "amount" from balance ====================================================== */ public void withdraw( double amount ) { if ( this.balance >= amount ) this.balance -= amount; // Subtract "amount" from balance } }

Let's run the same program Class10.java with the new BankAccount class

public class Class10 { public static void main(String[] args) { String x; BankAccount stu1 = new BankAccount( 123, "John", 1000 ); x = "Test --- " + stu1 ; // Converted to: "Test --- " + stu1.toString() System.out.println( x ); } }

Output:

Test --- Account number: 123, Name: John, Balance: 1000.0

Explanation:

The relavant statement is:

x = "Test --- " + stu1 ; // String + (BankAccount) Object

Due to the automatic String conversion rule, The Java compiler will translate this statement into:

x = "Test --- " + stu1.toString() ; // use "toString()" to convert object to String

Our own toString() method defined inside the BankAccount class is now invoked, which returns the string:
This returned string is then concatenated with "Test --- "

Printing object variable

Programming practice in Java:

If you intend to print object variables in your Java program, then:
Because then you can use System.out.print( ) to print your object variables !!!

Example:

public class Class11 { public static void main(String[] args) { BankAccount stu1 = new BankAccount( 123, "John", 1000 ); System.out.print( "stu1 = " ); System.out.println( stu1 ); stu1.deposit(450); System.out.print( "After stu1.deposit(450)... stu1 = " ); System.out.println( stu1 ); BankAccount stu2 = new BankAccount( 456, "Mary", 3000 ); System.out.print( "stu2 = " ); System.out.println( stu2 ); } }

Output:

stu1 = Account number: 123, Name: John, Balance: 1000.0 After stu1.deposit(450)... stu1 = Account number: 123, Name: John, Balance: 1450.0 stu2 = Account number: 456, Name: Mary, Balance: 3000.0