Method Chaining and Method Recursion in Java

This tutorial explains method chaining and method recursion concept. These are basically a mechanism related with method calling in java programs. Sometimes it is quite useful to use these concepts in our programs. We will see these concepts with program examples in this tutorial.

Method Chaining in Java

Method chaining is a technique of calling multiple methods using dot(.) operator in a single statement. As the name itself suggests, it's a chain of method calls in a single statement. For example you might have already seen or used code like below :

   StringBuilder str = new StringBuilder("java");
   str.insert(0,"refresh ").append("tutorial ").append("online");  // Method chaining

As you can see we are calling a series of StringBuilder method in a single statement. This is called method chaining. Each method except the last method in a method chaining syntax must return an object to allow the calls to be chained together, since you need an object to call a method.

Method chaining is possible on these method because each of these methods returns an object, which is then used to call the next method. For example the definition of append method of StringBuilder class is like below :

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    } 

The method is returning this, that is why we are able to invoke it multiple time in same line. In java this is a reference to the same object on which the method is being called. We will see the details of this keyword in later tutorial.

The program below demonstrates method chaining using the methods of String and StringBuffer classes. Methods like insert, append, substring etc allows us to use the technique of method chaining.

Java program of method chaining using String and StringBuilder class methods

 class MethodChainingProgram {    
    public static void main(String args[]) {   
       StringBuilder sb = new StringBuilder("Java"); 
       // Below line uses method chaining technique
       sb.insert(0,"Learn ").append(" on refresh java").append(" tutorial");   
       System.out.println(sb);       
       // Below line also uses method chaining technique
       String str = sb.substring(14).replace(" tutorial", "").toUpperCase(); 
       System.out.println(str);  
    }  
 }  

Output:

Now let's create our own methods that can be used in method chaining approach. As mentioned above the method must return an object if it needs to be used in method chaining. The program below contains method setName and setAge that we can use in method chaining.

Method chaining program in Java

 class Person {      
    String name;
    int age;

    // This method can be used in chaining
    public Person setName(String name) {
        this.name = name;
        return this;
      }
    // This method can also be used in chaining
    public Person setAge(int age) {
        this.age = age;
        return this;
      }
    public void getPersonDetails() {
        System.out.println("Person name :  " + name + ", age : " +age+" year.");
     } 
  }

 class MethodChainingDemo { 
    public static void main(String[] args) {
       Person person= new Person();
       // Below line uses method chaining technique   
       person.setName("Rahul").setAge(22).getPersonDetails();
     }  
  }  

Output:

Using method chaining has couple of benefits like it improves the readability and reduces the amount of source code. But it also has some disadvantages as well like It's hard to debug if any of the method fails while execution. Exception handling also becomes tough, sometime it's confusing as well.

Recursion in Java

Recursion is a process in which a method call itself from within it's body. The method that calls itself is known as recursive method. The code below shows an example of recursive method :

 int calculate()
   {
     .....
     calculate(); //calling the same method
     .....
   } 

As you can see method calculate is calling itself, we can say calculate is a recursive method. Generally programmer uses some conditional/looping statements to call the same method. If we don't use any condition, the execution of method keep repeating itself again and again which results in an infinite recursion.

Recursion in computer science is a method of solving a problem. For example the program below calculates the factorial of a number using method recursion. Factorial of a number 5 is calculated as 5*4*3*2*1=120.

Factorial of a number using Recursion in Java

 class FactorialProgram {  
  
   static int factorial(int n) {    
      if(n == 0)    
         return 1;    
      else    
         return(n * factorial(n-1));  // Calling the same method  
     }   
  
   public static void main(String args[]) {   
      int number=5;  
      int fact = factorial(number);   
      System.out.println("Factorial of number "+number+" is : "+fact);    
    }  
 }  

Output:

Factorial of number 5 is : 120

How recursion works

As you know once a method is called, a new stack(memory) is allocated for method. This stack is used by method to store it's variables and performs it's operations and finally return the result to caller. In recursion every time the method is called, a new stack is allocated on the top of last stack. Once the last call happens, the method on top most stack evaluates it's result and returns the output to it's previous stack. The same process goes on until the method at first stack evaluates it's result and returns the result to it's caller.

For example in factorial program, once the main method calls the findFactoria(5), it calls findFactorial(4) and it goes on until findFactorial(0) is called which returns 1 to findFactorial(1) which further returns it's result to findFactorial(2) and so on until findFactorial(5) is reached which evaluates the final result and returns to main method. The image below shows a typical representation of above factorial program.

Programmer have to be careful while using method recursion, incorrect condition or logic may result in an infinite recursion. In an infinite recursion the method keeps calling itself again and again which means method call never ends. For example the program below results in an infinite recursion.

Infinite recursion program in Java

 class InfiniteRecursion {     
   
    static void printNumber(int num) {          
       if(num <= 10) { 
          System.out.println(num); 
          num = num-1;  
          printNumber(num);    
        }   
      }
    public static void main(String args[]) {   
        printNumber(10);    
     }   
 }  

Output:

10
9
8
.
.
Exception in thread "main" java.lang.StackOverflowError.

The condition statement that we have used in this program will always be true, as we are decreasing the number in each iteration. This will result in calling the method again and again. So you have to use the recursion carefully to avoid the infinite looping. If you change the conditions as if(num >= 1) the program will run successfully.