Constructors And Instantiation

Constructors And Instantiation

Constructor Basics • Every class must have a constructor • Even abstract classes!! • No return types • Their names must exactly match the class names.

Ex1: • Given: • publicclass A { • void A() { • System.out.println("Class A"); • } • publicstaticvoid main(String[] args) { • new A(); • } • } • What is the result? • A. Class A • B. Compilation fails. • C. An exception is thrown at line 2. • D. An exception is thrown at line 6. • E. The code executes with no output.

Constructor chaining LivingBeings Mammal Cat

Ex2: • class Base { • Base() { System.out.print("Base"); } • } • publicclass A extends Base { • publicstaticvoid main( String[] args ) { • new A(); • new Base(); • } • } • What is the result? • A. Base • B. BaseBase • C. Compilation fails. • D. The code runs with no output. • E. An exception is thrown at runtime.

Rules for Constructors • Access modifiers • private • public • protected • default

Private constructors • class TestSuper { • privateinta; • private TestSuper(int i) • { • this.a=i; • } • publicvoid test() • { • System.out.println("inside test method"); • } • publicstaticvoid main(String args[]) • { • TestSuper s=new TestSuper(5); • System.out.println(s.a); • } • } • class Test • { • TestSuper t=new TestSuper(10); //error • publicvoid callMethod() • { • test(); //error • } • }

Default constructor • compiler- ALWAYS no args • class A { • A() { } • } • class B extends A { • } • Which two statements are true? (Choose two) • A. Class B’s constructor is public. • B. Class B’s constructor has no arguments. • C. Class B’s constructor includes a call to this(). • D. Class B’s constructor includes a call to super().

Contd.. • Features of default constructors • Same access modifier as class • No arguments • No arg call to super constructor

Ex3: • public class Test { } • What is the prototype of the default constructor? • A. Test() • B. Test(void) • C. public Test() • D. public Test(void) • E. public void Test()

Ex4: • Which three statements are true? (Choose three) • A.The default constructor initializes method variables. • B. The default constructor has the same access as its class. • C. The default constructor invokes the no-arg constructor of the superclass. • D. If a class lacks a no-arg constructor, the compiler always creates a default constructor. • E. The compiler creates a default constructor only when there are no other constructors for the class.

Ex5: • In which two cases does the compiler supply a default constructor for class A? (Choose • two) • A. class A { • } • B. class A { • public A() {} • } • C. class A { • public A(int x) {} • } • D. class Z {} • class A extends Z { • void A() {} • }

Ex6: which would generate compiler error??? • A. class A { • public A (int x) { } • } • B. class A { • } • class B extends A { • B() { } • } • C. class A { • A() {} • } • class B { • public B() { } • } • D. class Z { • public Z (int) { } • } • class A extends Z { • }

Ex7: • First statement in a constructor can either • super()- call to super class constructor • class A1 { • public A1() { • System.out.println("hello from a"); • } • } • class B extends A1 { • public B () { • System.out.println("hello from b"); • Super(); • } • } • publicclass A { • publicstaticvoid main(String args[]) { • A1 a = new B(); • } • } What is the result when main is executed? • A. Compilation fails. B. hello from a C. hello from b • D. hello from b E. hello from a • hello from a hello from b

this()- call to overloaded constructor • class E { • intm_x; • intm_y; • public E(int x, int y) { • m_x = x; • m_y = y; • System.out.println("inside parameterised constructor"); • } • public E() • { • this(0, 0); // Calls other constructor. • System.out.println("inside no parameterised constructor"); • } • } • publicclass Ex • { • publicstaticvoid main(String args[]) • { • E e=new E(); • } • }

Ex8: • Compiler will insert a no arg call to super() as the very first statement by default • class TestSuper { • TestSuper(int i) { } • } • class TestSubextends TestSuper{ • } • publicclass A{ • publicstaticvoid main (String [] args) { • new TestSub(); • } • } • Which is true? • A. Compilation fails. • B. The code runs without exception. • C. An exception is thrown at line 7. • D. An exception is thrown at line 2.

Contd… • A call to super() may or may not contain arguments • class Super { • publicinti = 0; • public Super(String text) { • i = 1; • } • } • publicclass Sub extends Super { • public Sub(String text) { • super(text); • i = 2; • } • publicstaticvoid main(String args[]) { • Sub sub = new Sub("Hello"); • System.out.println(sub.i); • } • } • What is the result? • A. 0 B. 1 C. 2 D. Compilation fails.

Contd.. • No args constructor does not necessarily mean a default constructor. • publicclass Ex • { • Ex() //this is not a default constructor • { • } • publicstaticvoid main(String args[]) • { • Ex e=new Ex(); • } • }

Contd… • Cannot call an instance method or variable until the super class constructor runs. • Only static variables and methods can be accessed as a part of the call to super • class E{ • int a; • E(int i){ } • } • publicclass Ex extends E • { Ex() • {super(E.a); //this will generate an error, but if int a is declared • //as static it will not generate an error • } • publicstaticvoid main(String args[]){ • Ex e=new Ex(); • } • }

Contd… • when invoking a constructor explicitly from another constructor, only static methods can be invoked. No instance methods can be invoked. • publicclass Ex { • publicstaticvoid main (String [] args) { • E e1 = new E(); • System.out.println(e1.val); • E e2= new E(4); • System.out.println(e2.val); • } • } • class E { • intval; • E(int n) { • this.val = n; • } • E() { • this(getValue()); • } • staticint getValue() { • int i=10; • return i; • } • }

Contd.. • When concrete class is instantiated, abstract class constructors are called • Interfaces do not have constructors • Constructors can only be called from within a constructor not from within a method.

Overloaded constructors • publicclass Ex { • intvar; • Ex(int var) { • this("hello"); • } • Ex(String s) { • this(); • System.out.println(s); • } • Ex() { • System.out.println("good-bye"); • } • publicstaticvoid main(String[] args) { • Ex t = new Ex(5); • } • }

Benefits of constructor overloading • Flexible ways to instantiate objects • Avoid code duplication- one constructor calling another overloaded constructor

Recursive call to constructors • class A { • A() { • this("foo"); • } • A(String s) { • this(); • } • } • Compiler error

Important points • Constructors are never inherited • They are not methods • They cannot be overridden • They can be overloaded

contd…

THANK YOU

Constructors And Instantiation

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