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Ans. Interfaces don't have member elements and method definitions that could cause diamond problem. With Java 8, Interfaces have default method definitions. This could have created diamond problem but Java introduced a compile time check for "duplicate default methods" in case same method is derived from multiple interfaces and no definition is overridden by the class.
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Ans. At the beginning of an object's life, the Java virtual machine (JVM) allocates memory on the heap to accommodate the object's instance variables. When that memory is first allocated, however, the data it contains is unpredictable. If the memory were used as is, the behavior of the object would also be unpredictable. To guard against such a scenario, Java makes certain that memory is initialized, at least to predictable default values before it is used by any code.
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Ans. member variable are loaded into heap, so they are initialized with default values when an instance of a class is created. In case of local variables, they are stored in stack until they are being used.
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Ans. Compile time error as it won't find the constructor matching BuggyBread2().
Compiler won't provide default no argument constructor as programmer has already defined one constructor.
Compiler will treat user defined BuggyBread2() as a method, as return type ( void ) has been specified for that.
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Ans. With Java 8, We can provide method definitions in the Interfaces that gets carried down the classes implementing that interface in case they are not overridden by the Class. Keyword "default" is used to mark the default method.
Ans. Earlier any class implementing an interface was supposed to implement all methods declared in an interface. There was no place for optionally implementing all or subset of methods.Though we have abstract classes wherein we could have provided such a mechanism by declaring some methods as abstract while providing definition for some. But as Abstract classes have a body and are comparatively heavier than interfaces and interfaces associate closely to the concept of providing interfacing than abstract classes, Java might have though of providing optional implementation for default methods. This way same interface can be reused in variety of ways rather than making copies of an interface to suit different needs.
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Ans. No. The whole idea of default method is to provide a default definition in case the implementing class intend to provide definition for only some of the methods. Making any of the interface method private would restrict it from being implemented by the implementing class. So default method is an option to provide implementation and not restricting a new definition.
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Constructor provided by Java if no constructor is declared
Constructor with empty body
All of the above
Q14. Difference between Class#getInstance() and new operator ?
Ans. Class.getInstance doesn't call the constructor whereas if we create an object using new operator , we need to have a matching constructor or copiler should provide a default constructor.
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Ans. No. Every Class only needs to have one constructor - With parameters or without parameters. Compiler provides a default non parameterized constructor if no constructors is defined.
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Ans. Java expects the superclass ( Object Class ) constructor to be called while creation of any object. So super constructor is called in case there are no instance variables to initialize.
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Constructor provided by Java if no constructor is declared
Constructor with empty body
All of the above
Q19. What will be the output of following code ?
public static void main(String[] args) { int x = 10; int y; if (x < 100) y = x / 0; if (x >= 100) y = x * 0; System.out.println("The value of y is: " + y); }
Ans. The code will not compile raising an error that the local variable y might not have been initialized. Unlike member variables, local variables are not automatically initialized to the default values for their declared type.
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Q23. Can a class implement two Interfaces having default method with same name and signature ?
public interface DefaultMethodInterface { default public void defaultMethod(){ System.out.println("DefaultMethodInterface"); } }
public interface DefaultMethodInterface2 { default public void defaultMethod(){ System.out.println("DefaultMethodInterface2"); } }
public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 { public static void main(String[] args){ DefaultMethodInterface defMethIn = new HelloJava8(); defMethIn.defaultMethod(); } }
Ans. No. Compiler gives error saying "Duplicate Default Methods"
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Q24. What If we make the method as abstract in another Interface ?
public interface DefaultMethodInterface { default public void defaultMethod(){ System.out.println("DefaultMethodInterface"); } }
public interface DefaultMethodInterface2 { public void defaultMethod(){ System.out.println("DefaultMethodInterface2"); } }
public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 { public static void main(String[] args){ DefaultMethodInterface defMethIn = new HelloJava8(); defMethIn.defaultMethod(); } }
Ans. Even then the Compiler will give error saying that there is a conflict.
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Q25. What if we override the conflicting method in the Class ?
public interface DefaultMethodInterface { default public void defaultMethod(){ System.out.println("DefaultMethodInterface"); } }
public interface DefaultMethodInterface2 { default public void defaultMethod(){ System.out.println("DefaultMethodInterface2"); } }
public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 { public static void main(String[] args){ DefaultMethodInterface defMethIn = new HelloJava8(); defMethIn.defaultMethod(); }
public void defaultMethod(){ System.out.println("HelloJava8"); } }
Ans. There won't be any error and upon execution the overriding class method will be executed.
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Q26. What will happen if there is a default method conflict as mentioned above and we have specified the same signature method in the base class instead of overriding in the existing class ?
Ans. There won't be any problem as the Base class method will have precedence over the Interface Default methods.
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Q27. If a method definition has been specified in Class , its Base Class , and the interface which the class is implementing, Which definition will be picked if we try to access it using Interface Reference and Class object ?
Ans. Class method definition is overriding both the definitions and hence will be picked.
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Q28. If a method definition has been specified in the Base Class and the interface which the class is implementing, Which definition will be picked if we try to access it using Interface Reference and Class object ?
Ans. Base Class Definition will have precedence over the Interface Default method definition.
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Q29. Can we have default method with same name and signature in the derived Interface as the static method in base Interface and vice versa ?
Ans. Yes , we can do that as static methods are not accessible using references and hence cannot lead to conflict. We cannot do inverse as Default methods cannot be overridden with the static methods in derived interface.
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Ans. This is an informative annotation that specify that the interface is a functional interface. A Function Interface has only one abstract method and many default methods. Compiler generates an error if the interface specified with the annotation doesn't abide by the specifications for functional interface.
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Which access specifiers can be used with top level class ?
