Java String Interview Questions - JournalDev

Question 1

What is String in Java? Is it a data type?

Answer 1

String is a Class in java and defined in java.lang package. It’s not a primitive data type like int and long. String class represents character Strings. String is used in almost all the Java applications. String is immutable and final in Java, and JVM uses String Pool to store all the String objects.
We can instantiate a String object using double quotes and overloading of “+” operator for concatenation.

Question 2

What are the different ways to create a String Object?

Answer 2

We can create String object using new operator like any normal java class or we can use double quotes to create a String object. There are several constructors available in String class to get String from char array, byte array, StringBuffer and StringBuilder.

When we create a String using double quotes, JVM looks in the String pool to find if any other String is stored with the same value. If found, it just returns the reference to that String object else it creates a new String object with given value and stores it in the String pool.
When we use the new operator, JVM creates the String object but don’t store it into the String Pool. We can use intern() method to store the String object into String pool or return the reference if there is already a String with equal value present in the pool.

Question 3

Write a method to check if input String is Palindrome.

Answer 3

A String is said to be Palindrome if it’s value is same when reversed. For example “aba” is a Palindrome String.
String class doesn’t provide any method to reverse the String but StringBuffer and StringBuilder class has reverse method that we can use to check if String is palindrome or not.

Sometimes interviewer asks not to use any other class to check this, in that case, we can compare characters in the String from both ends to find out if it’s palindrome or not.

Question 4

How can we make String uppercase or lowercase?

Answer 4

We can use String class toUpperCase and toLowerCase methods to get the String in all upper case or lower case. These methods have a variant that accepts Locale argument and use that locale rules to convert String to upper or lower case.

Question 5

How to compare two Strings in Java?

Answer 5

Java String implements Comparable interface and it has two variants of compareTo() methods.

compareTo(String anotherString) method compares the String object with the String argument passed lexicographically. If String object precedes the argument passed, it returns negative integer and if String object follows the argument String passed, it returns a positive integer. It returns zero when both the String have the same value, in this case equals(String str) method will also return true.

compareToIgnoreCase(String str): This method is similar to the first one, except that it ignores the case. It uses String CASE_INSENSITIVE_ORDER Comparator for case insensitive comparison. If the value is zero then equalsIgnoreCase(String str) will also return true.

Question 6

How to convert String to char and vice versa?

Answer 6

This is a tricky question because String is a sequence of characters, so we can’t convert it to a single character. We can use use charAt method to get the character at given index or we can use toCharArray() method to convert String to character array.