Java

Question 1

List is Child of

Answer 1

Collection

Question 2

How do you convert a long to a String

Answer 2

Long.toString()

Question 3

how do you make a two-dimensional array

Answer 3

int [][] hi=new int[m][n];

Question 4

How do you reverse a list

Answer 4

Collections.reverse(list)

it reverses the input for you! it

Question 5

List is an interface, so you cant make a new list, who are the children?

Answer 5

ArrayList,

Question 6

how do you compare two strings lexicographically?

Answer 6

public int compareTo(String str)

Question 7

How to convert a character to an integer in java?

Answer 7

Character.getNumericValue(c);

Question 8

How to convert an integer to a character in java?

Answer 8

int REDIX = 10;

char c = Character.forDigit( a, REDIX );

Question 9

How to sort with a comparable?

Answer 9

Collections.sort(persons, new Comparator() {
            public int compare(Person person, Person person1) {
                int name = person.getName().compareTo(person1.getName());
                if(name == 0){
                    return name;
                }
                return person.getAge() > person1.getAge() ? 1 : person.getAge() < person1.getAge() ? -1 : 0;
        }
});

Question 10

How do you read input in Java, only tell me 3 methods and their advantages:

Answer 10

1.Using Buffered Reader Class: efficient reading
2. Using Scanner Class: Convenient methods for parsing primitives (nextInt(), nextFloat(), …) from the tokenized input.
Regular expressions can be used to find tokens.
3. Using Console Class, for not echoing password and stuff, and it’s synchronized.

Question 11

How do you read from a Buffered Reader Class?

Answer 11

BufferedReader reader =  
                   new BufferedReader(new InputStreamReader(System.in)); 

    String name = reader.readLine();

Question 12

String vs StringBuilder vs StringBuffer in Java, when to use which?string,

Answer 12

If a string is going to remain constant throughout the program, then use String class object because a String object is immutable.
If a string can change (example: lots of logic and operations in the construction of the string) and will only be accessed from a single thread, using a StringBuilder is good enough.
If a string can change and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous so you have thread-safety.

Question 13

how do you concatenate with a String buffer or a String Builder?

Answer 13

.append();

.toString() at the end;

Question 14

can you append an integer to string buffer or String Builder?

Answer 14

yes you can append, long, double, float, int, boolean,

String and

Question 15

What is the easiest way to reverse a string in java?

Answer 15

using string buffer or String Builder!
String s="khello";
StringBuffer sb=new StringBuffer(s);
String rev=sb.reverse.toString();

Question 16

How do we check strings equality?

Answer 16

equals

Question 17

how do you get a substring of a string?

Answer 17

String substring(begIndex, endIndex)
The substring begins with the character at the specified index and extends to the end of this string or up to endIndex – 1, if the second argument is given.

Question 18

How can we know a character is alphanumeric?

Answer 18

Character.isLetterOrDigit

Question 19

get the highest and the lowest element in a treeSet?

Answer 19

1. pollFIrst()

2. pollLast() -> highest

Question 20

how to get the min and max value?

Answer 20

Integer.MIN_VALUE

Integer.MAX_VALUE

Question 21

how do you use max or min-heap in Java?

Answer 21

We use PriorityQueue class to implement Heaps in Java. By default Min Heap is implemented by this class. To implement Max Heap, we use Collections.reverseOrder()

Question 22

How to convert a set into an Arraylist in java?

Answer 22

we make a new ArrayList(set);