Sunday, May 20, 2018

Java lang package interview questions

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What is the base class of all classes?
java.lang.Object

What do you think is the logic behind having a single base class for all classes?

1. casting
2. Hierarchial and object oriented structure.

Why most of the Thread functionality is specified in Object Class?

Basically for interthread communication.

What is the importance of == and equals() method with respect to String object?

== is used to check whether the references are of the same object.
.equals() is used to check whether the contents of the objects are the same.
But with respect to strings, object refernce with same content
will refer to the same object.


String str1=”Hello”;
String str2=”Hello”;


(str1==str2) and str1.equals(str2) both will be true.

If you take the same example with Stringbuffer, the results would be different.


Stringbuffer str1=”Hello”;
Stringbuffer str2=”Hello”;


str1.equals(str2) will be true.
str1==str2 will be false.

Is String a Wrapper Class or not?

No. String is not a Wrapper class.

How will you find length of a String object?

Using length() method of String class.

How many objects are in the memory after the exection of following code segment?

String str1 = “ABC”;
String str2 = “XYZ”;
String str1 = str1 + str2;

There are 3 Objects.

What is the difference between an object and object reference?

An object is an instance of a class. Object reference is a pointer to the object. There can be many refernces to the same object.

What will trim() method of String class do?

trim() eliminate spaces from both the ends of a string.***

What is the use of java.lang.Class class?

The java.lang.Class class is used to represent the classes and interfaces that are loaded by a java program.
What is the possible runtime exception thrown by substring() method?

ArrayIndexOutOfBoundsException.

What is the difference between String and Stringbuffer?

Object’s of String class is immutable and object’s of Stringbuffer class is mutable moreover stringbuffer is faster in concatenation.

What is the use of Math class?

Math class provide methods for mathametical functions.

Can you instantiate Math class?

No. It cannot be instantited. The class is final and its constructor is private. But all the methods are static, so we can use them without instantiating the Math class.

What will Math.abs() do?

It simply returns the absolute value of the value supplied to the method, i.e. gives you the same value. If you supply negative value it simply removes the sign.

What will Math.ceil() do?

This method returns always double, which is not less than the supplied value. It returns next available whole number

What will Math.floor() do?

This method returns always double, which is not greater than the supplied value.

What will Math.max() do?

The max() method returns greater value out of the supplied values.

What will Math.min() do?
The min() method returns smaller value out of the supplied values.

What will Math.random() do?

The random() method returns random number between 0.0 and 1.0. It always returns double.
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