Best Java Interview Questions & Answers
1. Explain with example to describe when to use abstract class and interface?
Consider a scenario where all Cars will have 4 tyres and other features can be different.
In this case any subclass of Car has to have 4 tyres. This is a case where abstract class will be used and a default implementation for tyres will be provided.
public abstract class Car{
public abstract String getCarName();
public final int getNoOfTyres(){
return 4;
}
}
Consider a scenario where Cars can have any number of tyres and other features can also be different. In this case interface will be created.
public interface Car{
public abstract String getCarName();
public abstract int getNoOfTyres();
}
2. What are the different types of references in java?
Java has a more expressive system of reference than most other garbage-collected programming languages, which allows for special behavior for garbage collection. A normal reference in Java is known as a strong reference. The java.lang.ref package defines three other types of references--soft, weak, and phantom references. Each type of reference is designed for a specific use.
A SoftReference can be used to implement a cache. An object that is not reachable by a strong reference (that is, not strongly reachable), but is referenced by a soft reference is called softly reachable. A softly reachable object may be garbage collected at the discretion of the garbage collector. This generally means that softly reachable objects will only be garbage collected when free memory is low, but again, it is at the discretion of the garbage collector. Semantically, a soft reference means "keep this object unless the memory is needed."
A WeakReference is used to implement weak maps. An object that is not strongly or softly reachable, but is referenced by a weak reference is called weakly reachable. A weakly reachable object will be garbage collected during the next collection cycle. This behavior is used in the class java.util.WeakHashMap. A weak map allows the programmer to put key/value pairs in the map and not worry about the objects taking up memory when the key is no longer reachable anywhere else. Another possible application of weak references is the string intern pool. Semantically, a weak reference means "get rid of this object when nothing else references it."
A PhantomReference is used to reference objects that have been marked for garbage collection and have been finalized, but have not yet been reclaimed. An object that is not strongly, softly or weakly reachable, but is referenced by a phantom reference is called phantom reachable. This allows for more flexible cleanup than is possible with the finalization mechanism alone. Semantically, a phantom reference means "this object is no longer needed and has been finalized in preparation for being collected."
3. How to change the heap size of a JVM?
The old generation's default heap size can be overridden by using the -Xms and -Xmx switches to specify the initial and maximum sizes respectively:
java -Xms <initial size> -Xmx <maximum size> program
for example:
java -Xms64m -Xmx128m program
4. What is difference between instanceof and isInstance(Object obj)?
Differences are as follows:
a) instanceof is a reserved word of Java, but isInstance(Object obj) is a method of java.lang.Class.
if (obj instanceof MyType) {
...
}else if (MyType.class.isInstance(obj)) {
...
}
b) instanceof is used of identify whether the object is type of a particular class or its subclass but isInstance(obj) is used to identify object of a particular class.
5. Java supports pass by value or pass by reference?
Java supports only pass by value. The arguments passed as a parameter to a method is mainly primitive data types or objects. For the data type the actual value is passed.
Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references.Java copies and passes the reference by value, not the object. Thus, method manipulation will alter the objects, since the references point to the original objects.
Consider a scenario where all Cars will have 4 tyres and other features can be different.
In this case any subclass of Car has to have 4 tyres. This is a case where abstract class will be used and a default implementation for tyres will be provided.
public abstract class Car{
public abstract String getCarName();
public final int getNoOfTyres(){
return 4;
}
}
Consider a scenario where Cars can have any number of tyres and other features can also be different. In this case interface will be created.
public interface Car{
public abstract String getCarName();
public abstract int getNoOfTyres();
}
2. What are the different types of references in java?
Java has a more expressive system of reference than most other garbage-collected programming languages, which allows for special behavior for garbage collection. A normal reference in Java is known as a strong reference. The java.lang.ref package defines three other types of references--soft, weak, and phantom references. Each type of reference is designed for a specific use.
A SoftReference can be used to implement a cache. An object that is not reachable by a strong reference (that is, not strongly reachable), but is referenced by a soft reference is called softly reachable. A softly reachable object may be garbage collected at the discretion of the garbage collector. This generally means that softly reachable objects will only be garbage collected when free memory is low, but again, it is at the discretion of the garbage collector. Semantically, a soft reference means "keep this object unless the memory is needed."
A WeakReference is used to implement weak maps. An object that is not strongly or softly reachable, but is referenced by a weak reference is called weakly reachable. A weakly reachable object will be garbage collected during the next collection cycle. This behavior is used in the class java.util.WeakHashMap. A weak map allows the programmer to put key/value pairs in the map and not worry about the objects taking up memory when the key is no longer reachable anywhere else. Another possible application of weak references is the string intern pool. Semantically, a weak reference means "get rid of this object when nothing else references it."
A PhantomReference is used to reference objects that have been marked for garbage collection and have been finalized, but have not yet been reclaimed. An object that is not strongly, softly or weakly reachable, but is referenced by a phantom reference is called phantom reachable. This allows for more flexible cleanup than is possible with the finalization mechanism alone. Semantically, a phantom reference means "this object is no longer needed and has been finalized in preparation for being collected."
3. How to change the heap size of a JVM?
The old generation's default heap size can be overridden by using the -Xms and -Xmx switches to specify the initial and maximum sizes respectively:
java -Xms <initial size> -Xmx <maximum size> program
for example:
java -Xms64m -Xmx128m program
4. What is difference between instanceof and isInstance(Object obj)?
Differences are as follows:
a) instanceof is a reserved word of Java, but isInstance(Object obj) is a method of java.lang.Class.
if (obj instanceof MyType) {
...
}else if (MyType.class.isInstance(obj)) {
...
}
b) instanceof is used of identify whether the object is type of a particular class or its subclass but isInstance(obj) is used to identify object of a particular class.
5. Java supports pass by value or pass by reference?
Java supports only pass by value. The arguments passed as a parameter to a method is mainly primitive data types or objects. For the data type the actual value is passed.
Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references.Java copies and passes the reference by value, not the object. Thus, method manipulation will alter the objects, since the references point to the original objects.