Core Java Interview Questions

61 Q What is numeric promotion?
A

Numeric promotion is the conversion of a smaller numeric type to a larger numeric type. In numerical promotion, byte, char, and short values are converted to int  values. The int, long and float values are converted to the desired types if required.

62 Q What is the difference between the prefix and postfix forms of the ++ operator?
A

The prefix form first performs the increment operation and then returns the value of the increment operation. The postfix form first returns the current value of the expression and then performs the increment operation on that value.

63 Q What are synchronized methods and synchronized statements?
A

Synchronized methods are methods that are declared with the keyword synchronized. A thread executes a synchronized method only after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. It is a block of code declared with synchronized keyword. A synchronized statement can be executed only after a thread has acquired the lock for the object or class referenced in the synchronized statement.

64 Q How can we create a thread?
  A

A thread can be created by extending Thread class or by implementing Runnable interface. Then we need to override the method public void run().

65 Q What is the difference between a switch statement and an if statement?
A

If statement is used to select from two alternatives. It uses a boolean expression to decide which alternative should be executed. The expression in if must be a boolean value.  The switch statement is used to select from multiple alternatives.  The case values must be promoted to an  to int value. 

66 Q What is hashCode?
A

The hashcode of a Java Object is simply a number (32-bit signed int) that allows an object to be managed by a hash-based data structure. A hashcode should be, equal for equal object (this is mandatory!) ,  fast to compute based on all or most of the internal state of an object, use all or most of the space of 32-bit integers in a fairly uniform way , and likely to be different even for objects that are very similar. If you are overriding hashCode you need to override equals method also.

67 Q What is an I/O filter?
A

An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

68 Q What is the difference between RandomAccessFile and File?
A

The File class contains information  the files and directories of the local file system. The RandomAccessFile class contains  the methods needed to directly access data contained in any part of a file.

69 Q What is final ?
A

A final is a keyword in java. If final keyword is applied to a variable, then the variable will become a constant. If it applied to method, sub classes cannot override the method. If final keyword is applied to a class we cannot extend from that class.

70 Q What is the difference among JVM Spec, JVM Implementation, JVM Runtime ?
A

The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a vendor and the JVM runtime is the actual running instance of a JVM implementation

71 Q How is the difference between thread and process?
A

A process runs in its own address space. No two processes share their address space. Threads will run in the same address space of the process that owns them.

72 Q What is the difference between Vector and ArrayList ?
A

Vector is synchronized, ArrayList is not. Vector is having a constructor to specify the incremental capacity. But ArrayList don't have. By default Vector grows by 100% but ArrayList grows by 50% only.

73 Q What is the difference between Hashtable and HashMap ?
A

Hashtable is synchronized . but HashMap is not synchronized.  Hashtable does not allow null values , but HashMap allows null values.

74 Q What are the access modifiers available in Java.
A

Access modifier specify where a method or attribute can be used. Public is accessible from anywhere. Protected is accessible from the same class and its subclasses. Package/Default are accessible from the same package. Private is only accessible from within the class.

75 Q Why java is said to be pass-by-value ?
A

When assigning an object to a variable, we are actually assigning the memory address of that object to the variable. So the value passed is actually the memory location of the object. This results in object aliasing, meaning you can have many variables referring to the same object on the heap.

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