Java – Throw exception when second instance is created

In this article, we will discuss how to prevent and throw exception while creating 2nd instance/object of a Class

1. Problem statement :

  • Given Class A, Class B extends A and Class C extends B
  • If we create A a1 = new A() for the very first time then one instance/object of Class A should be created but when we try create another instance of Class A for the 2nd time then program should throw Exception
  • Similarly, the next Class B which extends A should throw Exception, when 2nd time we try to instantiate Class B
  • Likewise, for third Class C which extends B should throw Exception when we instantiate Class C for the 2nd time

2. Solution/Implementation :

  • We defined top-most Class A
  • Then we have defined Class B which extends A and another Class C which extends B
  • Inside constructor of Class A, we are trying to add corresponding Class (A, B, C) while instantiating using add() method of HashSet
  • If add() method returns false which means there is already one object is instantiated and stored of same-type and in this case we will throw Exception for that particular Class
  • As Class B extends A and Class C extends B it will also exhibit same behavior like that of Class A via super-constructor call ( or constructor-chaining)
  • Note: Class B & C has constructor with simple sysout to understand the flow/behavior of the program and super() constructor call is optional

A.java ( B.java and C.java )

package interview.program;

import java.util.HashSet;
import java.util.Set;

// Inheritance : top-most class A
public class A {

	// create HashSet instance to store object
	private static Set<String> createdInstanceList = new HashSet<String>();

	// default constructor for A()
	A() throws Exception {

		// while adding same class, return false
		if(!createdInstanceList.add(this.getClass().toString())) {

			// throw exception for 2nd time object creation
			throw new Exception("Trying to create 2nd time - " 
					+ this.getClass().toString());
		} 
		else {

			// this will print only for the 1st time object creation
			System.out.println("A() constructor invoked ...\n");
		}

	}
}

// Inheritance : 2nd level class B which extends A
class B extends A {

	// default constructor for B()
	B() throws Exception {
		super(); // super call, optional
		System.out.println("B() constructor invoked ...\n");
	}
}

// Inheritance : 3rd level class C which extends B
class C extends B {

	// default constructor for C()
	C() throws Exception {
		super(); // super call, optional
		System.out.println("C() constructor invoked ...\n");
	}
}

3. Testing different scenarios :

3.1 Testing scenario 1 – success

  • We are trying to create instance of Classes A, B and C for the very first time (just once)

TestingMain1.java

package interview.program;

public class TestingMain1 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time A :- \n");
			A a = new A();

			System.out.println("Trying to instantiate 1st time B :- \n");
			B b = new B();

			System.out.println("Trying to instantiate 1st time C :- \n");
			C c = new C();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are creating instance of Classes A, B and C once, it will create successfully as shown in the console output below
Trying to instantiate 1st time A :- 

A() constructor invoked ...

Trying to instantiate 1st time B :- 

A() constructor invoked ...

B() constructor invoked ...

Trying to instantiate 1st time C :- 

A() constructor invoked ...

B() constructor invoked ...

C() constructor invoked ...

3.2 Testing scenario 2 – trying to instantiate Class A twice

  • We are trying to create instance of Class A twice

TestingMain2.java

package interview.program;

public class TestingMain2 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time A :- \n");
			A a = new A();

			System.out.println("Trying to instantiate 2nd time A :- \n");
			A a2 = new A();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are trying to create instance of Class A twice therefore an Exception is thrown as shown in the console output below
Trying to instantiate 1st time A :- 

A() constructor invoked ...

Trying to instantiate 2nd time A :- 

java.lang.Exception: Trying to create 2nd time - class interview.program.A
	at interview.program.A.<init>(A.java:19)
	at interview.program.TestingMain2.main(TestingMain2.java:13)

3.3 Testing scenario 3 – trying to instantiate Class B twice

  • We are trying to create instance of Class B twice

TestingMain3.java

package interview.program;

public class TestingMain3 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time B :- \n");
			B b = new B();

			System.out.println("Trying to instantiate 2nd time B :- \n");
			B b2 = new B();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are trying to create instance of Class B twice therefore an Exception is thrown as shown in the console output below
Trying to instantiate 1st time B :- 

A() constructor invoked ...

B() constructor invoked ...

Trying to instantiate 2nd time B :- 

java.lang.Exception: Trying to create 2nd time - class interview.program.B
	at interview.program.A.<init>(A.java:19)
	at interview.program.B.<init>(A.java:36)
	at interview.program.TestingMain3.main(TestingMain3.java:13)

3.4 Testing scenario 4 – trying to instantiate Class C twice

  • We are trying to create instance of Class C twice

TestingMain4.java

package interview.program;

public class TestingMain4 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time C :- \n");
			C c = new C();

			System.out.println("Trying to instantiate 2nd time C :- \n");
			C c2 = new C();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are trying to create instance of Class C twice therefore an Exception is thrown as shown in the console output below
Trying to instantiate 1st time C :- 

A() constructor invoked ...

B() constructor invoked ...

C() constructor invoked ...

Trying to instantiate 2nd time C :- 

java.lang.Exception: Trying to create 2nd time - class interview.program.C
	at interview.program.A.<init>(A.java:19)
	at interview.program.B.<init>(A.java:36)
	at interview.program.C.<init>(A.java:46)
	at interview.program.TestingMain4.main(TestingMain4.java:13)

3.5 Testing scenario 5 – trying to instantiate Class A 2nd time

  • We are trying to create instance of Class A for 2nd time after instantiating Classes A, B and C once already

TestingMain5.java

package interview.program;

public class TestingMain5 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time A :- \n");
			A a = new A();

			System.out.println("Trying to instantiate 1st time B :- \n");
			B b = new B();

			System.out.println("Trying to instantiate 1st time C :- \n");
			C c = new C();

			System.out.println("Trying to instantiate 2nd time A :- \n");
			A a2 = new A();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are trying to create instance of Class A twice therefore an Exception is thrown after instantiating Classes A, B and C once successfully, as shown in the console output below
Trying to instantiate 1st time A :- 

A() constructor invoked ...

Trying to instantiate 1st time B :- 

A() constructor invoked ...

B() constructor invoked ...

Trying to instantiate 1st time C :- 

A() constructor invoked ...

B() constructor invoked ...

C() constructor invoked ...

Trying to instantiate 2nd time A :- 

java.lang.Exception: Trying to create 2nd time - class interview.program.A
	at interview.program.A.<init>(A.java:19)
	at interview.program.TestingMain5.main(TestingMain5.java:19)

3.6 Testing scenario 6 – trying to instantiate Class B 2nd time

  • We are trying to create instance of Class B for 2nd time after instantiating Classes A, B and C once already

TestingMain6.java

package interview.program;

public class TestingMain6 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time A :- \n");
			A a = new A();

			System.out.println("Trying to instantiate 1st time B :- \n");
			B b = new B();

			System.out.println("Trying to instantiate 1st time C :- \n");
			C c = new C();

			System.out.println("Trying to instantiate 2nd time B :- \n");
			B b2 = new B();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are trying to create instance of Class B twice therefore an Exception is thrown after instantiating Classes A, B and C once successfully, as shown in the console output below
Trying to instantiate 1st time A :- 

A() constructor invoked ...

Trying to instantiate 1st time B :- 

A() constructor invoked ...

B() constructor invoked ...

Trying to instantiate 1st time C :- 

A() constructor invoked ...

B() constructor invoked ...

C() constructor invoked ...

Trying to instantiate 2nd time B :- 

java.lang.Exception: Trying to create 2nd time - class interview.program.B
	at interview.program.A.<init>(A.java:19)
	at interview.program.B.<init>(A.java:36)
	at interview.program.TestingMain6.main(TestingMain6.java:19)

3.7 Testing scenario 7 – trying to instantiate Class C 2nd time

  • We are trying to create instance of Class C for 2nd time after instantiating Classes A, B and C once already

TestingMain7.java

package interview.program;

public class TestingMain7 {

	public static void main(String[] args) {

		try {

			System.out.println("Trying to instantiate 1st time A :- \n");
			A a = new A();

			System.out.println("Trying to instantiate 1st time B :- \n");
			B b = new B();

			System.out.println("Trying to instantiate 1st time C :- \n");
			C c = new C();

			System.out.println("Trying to instantiate 2nd time C :- \n");
			C c2 = new C();
		} 
		catch (Exception e) {
			e.printStackTrace();
		}
	}
}

Output:

  • As we are trying to create instance of Class C twice therefore an Exception is thrown after instantiating Classes A, B and C once successfully, as shown in the console output below
Trying to instantiate 1st time A :- 

A() constructor invoked ...

Trying to instantiate 1st time B :- 

A() constructor invoked ...

B() constructor invoked ...

Trying to instantiate 1st time C :- 

A() constructor invoked ...

B() constructor invoked ...

C() constructor invoked ...

Trying to instantiate 2nd time C :- 

java.lang.Exception: Trying to create 2nd time - class interview.program.C
	at interview.program.A.<init>(A.java:19)
	at interview.program.B.<init>(A.java:36)
	at interview.program.C.<init>(A.java:46)
	at interview.program.TestingMain7.main(TestingMain7.java:19)

Post credits : Shivam Agarwal and Sanket Chalke

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