3 Strings

Question 1

Is the new operator required for Strings?

Answer 1

no, it’s optional

Question 2

In a string, When does + concat and when does it perform addition?

Answer 2

If either operand involved in the + expression is a String, concatenation is used; otherwise, addition is used.

Question 3

What are 13 methods of the String class?

Answer 3

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charAt(),
concat(),
endsWith(),
 equals(),
 equalsIgnoreCase(),
 indexOf(),
length(),
 replace(), 
startsWith(),
 substring(), 
toLowerCase(), toUpperCase(),
 and
trim()

Question 4

What are 10 methods of the StringBuilder class?

Answer 4

append(),
 charAt(),
 delete(),
 deleteCharAt(),
indexOf(),
 insert(),
 length(),
 reverse(),
 substring(),
 toString().

Question 5

What is the difference between StringBuilder & StringBuffer?

Answer 5

StringBuffer is the same as StringBuilder except that it is thread safe

Question 6

What does == do with two Strings?

Answer 6

Calling == on String objects will check whether they point to the same object in the pool

Question 7

What does == do with two StringsBuilder references?

Answer 7

Calling == on StringBuilder references will check whether they are pointing to the same StringBuilder object

Question 8

What does equals() do on String objects?

Answer 8

Calling equals() on String objects will check whether the sequence of characters is the same.

Question 9

What does equals() do on StringBuilder objects?

Answer 9

Calling equals() on StringBuilder objects will check
whether they are pointing to the same object rather than looking at the values inside.

Question 10

Is array size fixed on creation?

Answer 10

yes

Question 11

What does Arrays.sort(myArray) do?

Answer 11

sorts an array

Question 12

Arrays.binarySearch()

Answer 12

searches a sorted array and returns the
index of a match. If no match is found, it negates the position where the element would need to be inserted and subtracts 1.

Question 13

T or F Methods that are passed varargs (…) can be used as

if a normal array was passed in.

Answer 13

T

Question 14

Can an ArrayList contain primitives?

Answer 14

No, primitives must be in wrapper classes. Java will autobox parameters passed in to the proper wrapper type.

Question 15

What is the method to sort an ArrayList?

Answer 15

Collections.sort()

Question 16

x

Answer 16

A LocalDate contains just a date, a LocalTime contains just a time, and a LocalDateTime contains both a date and time. All three have private constructors and are created using LocalDate.now() or LocalDate.of() The Period class represents a number of days, months, or years to add or subtract from a LocalDate or LocalDateTime. DateTimeFormatter is used to output dates and times in the desired format. The date and
time classes are all immutable, which means the return value must be used.

Question 17

Is an array measured by size or length?

Answer 17

Arrays use length and it is a property not a method Example twoD.length is correct. Do NOT use twoD.length();

Question 18

What's the difference? 
String name = "Fluffy";
String name = new String("Fluffy");

Answer 18

The first one is in the string pool. second is a new object.

Question 19

What are the rules for concatenation vs addition?

Answer 19

1. If both operands are numeric, + means numeric addition.
2. If either operand is a String, + means concatenation.
3. The expression is evaluated left to right.
   Now let’s look at some examples:
   System.out.println(1 + 2); // 3
   System.out.println(“a” + “b”); // ab
   System.out.println(“a” + “b” + 3); // ab3
   System.out.println(1 + 2 + “c”); // 3c

Question 20

What are the two method signatures for String’s replace() method?

Answer 20

• It replaces ALL instances.
  System.out.println(“abcabc”.replace(‘a’, ‘A’)); // AbcAbc
  System.out.println(“abcabc”.replace(“a”, “A”)); // AbcAbc

It can replace 1 character or a whole sequence but it will replace ALL instances of a match.

animalsssss. replace(‘s’,’z’) –> animalzzzzz
animalsssss. replace(“ss”,”z”) –> animalzzzzs

Question 21

6 facts on Strings

Answer 21

These are the six facts on Strings:

1. Literal strings within the same class in the same package represent references to the same String object.
2. Literal strings within different classes in the same package represent references to the same String object.
3. Literal strings within different classes in different packages likewise represent references to the same String object.
4. Strings computed by constant expressions are computed at compile time and then treated as if they were literals.
5. Strings computed at run time are newly created and therefore are distinct. (So line 4 prints false.)
6. The result of explicitly interning a computed string is the same string as any pre-existing literal string with the same contents. (So line 5 prints true.)

When a variable and literal are concatenated at run time it is not the same object.

Consider following classes:
//In File Other.java
package other;
public class Other { public static String hello = "Hello"; }

//In File Test.java
package testPackage;
import other.*;
class Test{
   public static void main(String[] args){
      String hello = "Hello", lo = "lo";
      System.out.print((testPackage.Other.hello == hello) + " ");    //line 1
      System.out.print((other.Other.hello == hello) + " ");   //line 2
      System.out.print((hello == ("Hel"+"lo")) + " ");           //line 3
      System.out.print((hello == ("Hel"+lo)) + " ");              //line 4
      System.out.println(hello == ("Hel"+lo).intern());          //line 5
   }
}
class Other { static String hello = "Hello"; }
What will be the output of running class Test?

Question 22

Is String a final class?

Answer 22

Yes and so are these: StringBuffer and StringBuilder are also final. All Primitive wrappers are also final (i.e. Boolean, Integer, Byte etc).

Question 23

Can string be subclassed? Is this legal?

class MyString extends String{
   MyString(){ super(); }
}

Answer 23

No, String is a final class. It cannot be subclassed (extended)

Question 24

Why is this false?

“String”.replace(‘g’,’G’)==”String”.replace(‘g’,’G’);

Answer 24

replace creates a new String object

Question 25

Why is this true?

“String”.replace(‘g’,’g’)==”String”.replace(‘g’,’g’);

Answer 25

If there is no change, replace returns the same object