String

Question 1

Is String a keyword in java?

Answer 1

No. String is not a keyword in java. String is a final class in java.lang package which is used to represent the set of characters in java.

Question 2

Is String a primitive type or derived type?

Answer 2

String is a derived type.

Question 3

In how many ways you can create string objects in java?

Answer 3

String s1 = new String(“abc”); //Creating string object using new operator

String s2 = “abc”; //Creating string object using string literal

Question 4

What is string constant pool?

Answer 4

String objects are most used data objects in Java. Hence, java has a special arrangement to store the string objects. String Constant Pool is one such arrangement. String Constant Pool is the memory space in heap memory specially allocated to store the string objects created using string literals. In String Constant Pool, there will be no two string objects having the same content.

Question 5

What is special about string objects as compared to objects of other derived types?

Answer 5

One special thing about string objects is that you can create string objects without using new operator i.e using string literals. This is not possible with other derived types (except wrapper classes). One more special thing about strings is that you can concatenate two string objects using ‘+’. This is the relaxation java gives to string objects as they will be used most of the time while coding. And also java provides string constant pool to store the string objects.

Question 6

What do you mean by mutable and immutable objects?

Answer 6

Immutable objects are like constants. You can’t modify them once they are created. They are final in nature. Where as mutable objects are concerned, you can perform modifications to them.

Question 7

Which is the final class in these three classes – String, StringBuffer and StringBuilder?

Answer 7

All three are final. (Interviewer will ask this type of questions to confuse you)

Question 8

Why StringBuffer and StringBuilder classes are introduced in java when there already exist String class to represent the set of characters?

Answer 8

The objects of String class are immutable in nature. i.e you can’t modify them once they are created. If you try to modify them, a new object will be created with modified content. This may cause memory and performance issues if you are performing lots of string modifications in your code. To overcome these issues, StingBuffer and StringBuilder classes are introduced in java.

Question 9

How many objects will be created in the following code and where they will be stored in the memory?
String s1 = “abc”;

String s2 = “abc”;

Answer 9

Only one object will be created and this object will be stored in the string constant pool.

Question 10

How do you create mutable string objects?

Answer 10

Using StringBuffer and StringBuilder classes. These classes provide mutable string objects.

Question 11

Which one will you prefer among “==” and equals() method to compare two string objects?

Answer 11

I prefer equals() method because it compares two string objects based on their content. That provides more logical comparison of two string objects. If you use “==” operator, it checks only references of two objects are equal or not. It may not be suitable in all situations. So, rather stick to equals() method to compare two string objects.

Question 12

Which class will you recommend among String, StringBuffer and StringBuilder classes if I want mutable and thread safe objects?

Answer 12

StringBuffer

Question 13

How do you convert given string to char array?

Answer 13

Using toCharArray() method.

Question 14

How many objects will be created in the following code and where they will be stored?
String s1 = new String("abc");

String s2 = “abc”;

Answer 14

Here, two string objects will be created. Object created using new operator(s1) will be stored in the heap memory. The object created using string literal(s2) is stored in the string constant pool.

Question 15

Where exactly string constant pool is located in the memory?

Answer 15

Inside the heap memory. JVM reserves some part of the heap memory to store string objects created using string literals. [more]

Question 16

I am performing lots of string concatenation and string modification in my code. which class among string, StringBuffer and StringBuilder improves the performance of my code. Remember I also want thread safe code?

Answer 16

StringBuffer class gives better performance in this scenario. As String class is immutable, if you use this class, a new object will be created after every string concatenation or string modification. This will lower the performance of the code. You can use StringBuilder also, but it is not thread safe. So, StringBuffer will be optimal choice here.

Question 17

What is string intern?

Answer 17

String object in the string constant pool is called as String Intern. You can create an exact copy of heap memory string object in string constant pool. This process of creating an exact copy of heap memory string object in the string constant pool is called interning. intern() method is used for interning.

Question 18

How many objects will be created in the following code and where they will be stored?
String s1 = new String("abc");

String s2 = new String(“abc”);

Answer 18

Two objects will be created and they will be stored in the heap memory.

Question 19

Can we call String class methods using string literals?

Answer 19

Yes, we can call String class methods using string literals. Here are some examples,

“abc”.charAt(0)

“abc”.compareTo(“abc”)

“abc”.indexOf(‘c’)

Question 20

do you have any idea why strings have been made immutable in java?

Answer 20

a) Immutable strings increase security. As they can’t be modified once they are created, so we can use them to store sensitive data like username, password etc.
b) Immutable strings are thread safe. So, we can use them in a multi threaded code without synchronization.
c) String objects are used in class loading. If strings are mutable, it is possible that wrong class is being loaded as mutable objects are modifiable.

Question 21

What is the similarity and difference between String and StringBuffer class?

Answer 21

The main similarity between String and StringBuffer class is that both are thread safe. The main difference between them is that String objects are immutable where as StringBuffer objects are mutable.

Question 22

What is the similarity and difference between StringBuffer and StringBuilder class?

Answer 22

The main similarity between StringBuffer and StringBuilder class is that both produces mutable string objects. The main difference between them is that StringBuffer class is thread safe where as StringBuilder class is not thread safe.

Question 23

What is the difference between equals() method and == operator?

Answer 23

The equals() method matches content of the strings whereas == operator matches object or reference of the strings.

Question 24

Does String is thread-safe in Java?

Answer 24

Strings are immutable, so we can’t change its value in the program. Hence it’s thread-safe and can be safely used in the multi-threaded environment.

Question 25

Why String is popular HashMap key in Java?

Answer 25

Since String is immutable, its hashcode is cached at the time of creation and it doesn’t need to be calculated again. This makes it a great candidate for key in a Map and it’s processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

Question 26

Syntax

How to Returns the character at the specified index?

Answer 26

char charAt(int index)

ex:
String s = “Strings are immutable”;
char result = s.charAt(8);

Question 27

Syntax

How to Compares this String to another Object?

Answer 27

int compareTo(Object o)

boolean equals(Object anObject)

Question 28

Syntax

How to Compares two strings lexicographically?

Answer 28

int compareTo(String anotherString)

ex:
result = str1.compareTo( str2 );

Question 29

Syntax

How to Concatenates the specified string to the end of this string?

Answer 29

public String concat(String s)

ex:
String s = “Strings are immutable”;
s = s.concat(“ all the time”);

Question 30

Syntax

Returns true if and only if this String represents the same sequence of characters as the specified StringBuffer.

Answer 30

boolean contentEquals(StringBuffer sb)

ex:
String str1 = "Not immutable";
      String str2 = "Strings are immutable";
      StringBuffer str3 = new StringBuffer( "Not immutable");

  boolean  result = str1.contentEquals( str3 );
  System.out.println(result);

Question 31

Syntax

Returns a String that represents the character sequence in the array specified.

Answer 31

static String copyValueOf(char[] data)

ex:
char[] Str1 = {‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘ ‘, ‘w’, ‘o’, ‘r’, ‘l’, ‘d’};
String Str2 = “”;
Str2 = Str2.copyValueOf( Str1 );
System.out.println(“Returned String: “ + Str2);

Question 32

Syntax

How to Tests if this string ends with the specified suffix?

Answer 32

boolean endsWith(String suffix)

Question 33

Syntax

How to Compares this String to another String, ignoring case considerations?

Answer 33

boolean equalsIgnoreCase(String anotherString)

Question 34

Syntax

How to Returns the index within this string of the first occurrence of the specified character?

Answer 34

int indexOf(int ch)

int indexOf(String str)

Question 35

Syntax

How to Returns the index within this string of the last occurrence of the specified character?

Answer 35

int lastIndexOf(int ch)

int lastIndexOf(String str)

Question 36

Syntax

How to Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar?

Answer 36

String replace(char oldChar, char newChar)

Question 37

Syntax

How to Replaces each substring of this string that matches the given regular expression with the given replacement?

Answer 37

String replaceAll(String regex, String replacement)

Question 38

Syntax

How to Replaces the first substring of this string that matches the given regular expression with the given replacement?

Answer 38

String replaceFirst(String regex, String replacement)

Question 39

Syntax

How to Splits this string around matches of the given regular expression?

Answer 39

String[] split(String regex)

Question 40

Syntax

How to Tests if this string starts with the specified prefix?

Answer 40

boolean startsWith(String prefix)

Question 41

Syntax

How to Returns a new string that is a substring of this string?

Answer 41

String substring(int beginIndex)

Question 42

Syntax

How to Converts this string to a new character array?

Answer 42

char[] toCharArray()

Question 43

Syntax

How to Converts all of the characters in this String to lower case using the rules of the default locale?

Answer 43

String toLowerCase()

Question 44

Syntax

How to This object (which is already a string!) is itself returned?

Answer 44

String toString()

Question 45

Syntax

How to Converts all of the characters in this String to upper case using the rules of the default locale?

Answer 45

String toUpperCase()

Question 46

Syntax

How to Returns a copy of the string, with leading and trailing whitespace omitted.

Answer 46

String trim()

Question 47

Syntax

How to Returns the string representation of the passed data type argument.

Answer 47

static String valueOf(primitive data type x)