String

Question 1

What are the three rules for concatenation?

Answer 1

1. If both operands are numeric, +1 means numeric addition
2. If either operand is a String, +1 means concatenation,
3. The expression is evaluated left to right

Question 2

What will the following print:
````

System.out.println(1+2);
System.out.pirntln(“a” + “b”);

System.out.println(“a”+”b” +3);

System.out.println(1+2+”c”);

~~~
`

Answer 2

3
ab
ab3
3c

Question 3

What does the following output?

````
String s = “1”;
s += “2”;
s += 3;
System.out.println(s);

~~~
`

Answer 3

123

Question 4

What’s the output?
int a =9;
String c = “hello”;
String b = c + a + 1;
System.out.println(b);

Answer 4

hello91

Question 5

What’s the output?
int a =9;
String b = a + 1;
System.out.println(b);

Answer 5

will not compile

Question 6

What’s the output?
int a =9;
String b = “”+ a + 1;
System.out.println(b);

Answer 6

91

Question 7

If an String object is created, can it be allowed to change?

Answer 7

No.
Strings are immutable which means they cannot be changed.

Question 8

Are String mutable or immutable?

Answer 8

Cannot be changed, Hence IMMUTABLE

Question 9

What is the output of the following?

````
String s1 = “1”;
String s2 = s1.concat(“2”);
s2.concat(“3”);
System.out.println(s2);

~~~

Answer 9

12

Question 10

Can you do this? What’s the difference?

````
String name = “Fluffy”;
String name = new String(“Fluffy”);

~~~
`

Answer 10

The string pool is like a cache. If you do this:

String s = “Fluffy”;
String p = “Fluffy”;
then the Java compiler is smart enough to make just one String object, and s and p will both be referring to that same String object. If you do this:

String s = new String(“Fluffy”);
then there will be one String object in the pool, the one that represents the literal “Fluffy”, and there will be a separate String object, not in the pool, that contains a copy of the content of the pooled object. Since String is immutable in Java, you’re not gaining anything by doing this; calling new String(“literal”) never makes sense in Java and is unnecessarily inefficient.

Question 11

What method length() does ?

````
String string = “0123456789”;
System.out.println(string.length());

~~~
`

Answer 11

The method length() returns the number of characters in the String.

````
System.out.println(string.length()); // 10

~~~
`

Question 12

String a=”hello”;
String b=”hello”;
if(a==b) returns what?

Answer 12

true
only a.equals returns true.

Question 13

What method chartAt() does ?

````
String string = “animals”;
System.out.println(string.charAt(0));

~~~
`

Answer 13

The method charAt() lets you query the string to find out what character is at a specific index. The method signature is as follows: char charAt(int index)

````
a

````

Question 14

StringBuilder a = “aa”;

Answer 14

Must know. DNC

Question 15

What method indexOf() does ?

````

String string = “animals”;
System.out.println(string.indexOf(‘a’));
System.out.println(string.indexOf(“al”));
System.out.println(string.indexOf(“al”, 5));

````

Answer 15

The method indexOf()looks at the characters in the string and finds the first index that matches the desired value. indexOf can work with an individual character or a whole String as input. It can also start from a requested position. indexOf() returns –1 when no match is found.

````

String string = “animals”;
System.out.println(string.indexOf(‘a’)); // 0
System.out.println(string.indexOf(“al”)); // 4
System.out.println(string.indexOf(“al”, 5)); // -1

````

Question 16

````
String four = “LMAO”;
System.out.println(four.substring(1));
System.out.println(four.substring(0));
System.out.println(four.substring(3));
System.out.println(four.substring(4)); //nothing
System.out.println(four.substring(-1)); //outofbounds
System.out.println(four.substring(5)); //outofbounds

System.out.println(four.substring(0, 1));
System.out.println(four.substring(0, 2));
System.out.println(four.substring(0, 3));
System.out.println(four.substring(0, 4));
System.out.println(four.substring(0, 5)); //out of bounds

System.out.println(four.substring(0, 0)); //empty

````

Answer 16

String four = “LMAO”;
System.out.println(four.substring(1)); //MAO
System.out.println(four.substring(0)); //LMAO
System.out.println(four.substring(3)); //O
System.out.println(four.substring(4)); //nothing
//System.out.println(four.substring(-1)); //outofbounds
//System.out.println(four.substring(5)); //outofbounds

	System.out.println(four.substring(0, 1)); //L
	System.out.println(four.substring(0, 2)); //LM
	System.out.println(four.substring(0, 3)); //LMA
	System.out.println(four.substring(0, 4)); //LMAO
	//System.out.println(four.substring(0, 5)); //out of bounds

	System.out.println(four.substring(0, 0)); //empty

Question 17

String a= “LMAO”;
String aa =null;
System.out.println(a+aa);

Answer 17

LMAOnull

Question 18

String a= “LMAO”;
String aa =null;
System.out.println(a+aa.toUpperCase());

Answer 18

nullpointerexception

Question 19

String a= “LMAO”;
boolean aa =true;
System.out.println(a+aa);

Answer 19

LMAOtrue

Question 20

What method substring() does ?

````
String string = “animals”;
System.out.println(string.substring(3));
System.out.println(string.substring(string.indexOf(‘m’)));
System.out.println(string.substring(3, 4));
System.out.println(string.substring(3, 7));

~~~
`

Answer 20

It returns parts of the string. The first parameter is the index to start with for the returned string. There is an optional second parameter, which is the end index you want to stop at.

String string = “animals”;
System.out.println(string.substring(3)); // mals
System.out.println(string.substring(string.indexOf(‘m’))); // mals
System.out.println(string.substring(3, 4)); // m
System.out.println(string.substring(3, 7)); // mals

Question 21

What toLowerCase() and toUpperCase() do ?

````
String string = “animals”;
System.out.println(string.toUpperCase());
System.out.println(“Abc123”.toLowerCase());
~~~
`

Answer 21

toUpperCase() converts any lowercase characters to uppercase in the returned string. toLowerCase() converts any uppercase characters to lowercase in the returned string. These methods leave alone any characters other than letters.

String string = “animals”;
System.out.println(string.toUpperCase()); // ANIMALS
System.out.println(“Abc123”.toLowerCase()); // abc123

Question 22

What equals() and equalsIgnoreCase() do ?

````
System.out.println(“abc”.equals(“ABC”));
System.out.println(“ABC”.equals(“ABC”));
System.out.println(“abc”.equalsIgnoreCase(“ABC”));

~~~
`

Answer 22

The equals() method checks whether two String objects contain exactly the same characters in the same order. The equalsIgnoreCase() method checks whether two String objects contain the same characters with the exception that it will convert the characters’ case if needed.

System.out.println(“abc”.equals(“ABC”)); // false
System.out.println(“ABC”.equals(“ABC”)); // true
System.out.println(“abc”.equalsIgnoreCase(“ABC”)); // true

Question 23

What startsWith() and endsWith() do ?

````
System.out.println(“abc”.startsWith(“a”));
System.out.println(“abc”.startsWith(“A”));
System.out.println(“abc”.endsWith(“c”));
System.out.println(“abc”.endsWith(“a”));

~~~
`

Answer 23

The startsWith() and endsWith() methods look at whether the provided value matches part of the String.

System.out.println(“abc”.startsWith(“a”)); // true
System.out.println(“abc”.startsWith(“A”)); // false
System.out.println(“abc”.endsWith(“c”)); // true
System.out.println(“abc”.endsWith(“a”)); // false

Question 24

What method contains() does ?

````
System.out.println(“abc”.contains(“b”));
System.out.println(“abc”.contains(“B”));

~~~
`

Answer 24

The contains() method also looks for matches in the String. It isn’t as particular as
startsWith() and endsWith()—the match can be anywhere in the String.

System.out.println(“abc”.contains(“b”)); // true
System.out.println(“abc”.contains(“B”)); // false

Question 25

What method replace() does ? ```` System.out.println("abcabc".replace('a', 'A')); System.out.println("abcabc".replace("a", "A")); ````

Answer 25

The replace() method does a simple search and replace on the string. There’s a version that takes char parameters as well as a version that takes CharSequence parameters. System.out.println("abcabc".replace('a', 'A')); // AbcAbc System.out.println("abcabc".replace("a", "A")); // AbcAbc

Question 26

What method trim() does ? ```` System.out.println("abc".trim()); System.out.println("\t a b c\n".trim()); ````

Answer 26

The trim() method removes whitespace from the beginning and end of a String. In terms of the exam, whitespace consists of spaces along with the \t (tab) and \n (newline) characters. Other characters, such as \r (carriage return), are also included in what gets trimmed. System.out.println("abc".trim()); // abc System.out.println("\t a b c\n".trim()); // a b c

Question 27

What is the main difference between StringBuilder and StringBuffer ?

Answer 27

StringBuilder is faster than StringBuffer because it's not synchronized.

Question 28

What equals() and '==' do with String / StringBuilder ?

Answer 28

Calling == on String objects will check whether they point to the same object in the pool. Calling == on StringBuilder references will check whether they are pointing to the same StringBuilder object. Calling equals() on String objects will check whether the sequence of characters is the same. Calling equals() on StringBuilder objects will check whether they are pointing to the same object rather than looking at the values inside.

Question 29

What Arrays.binarySearch() does ?

Answer 29

Arrays.binarySearch() 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 30

What does append () do? ```` StringBuilder sb = new StringBuilder("Luck"); sb.append("is needed").append(':').append(true); System.out.println(sb); ````

Answer 30

It's a method in StringBuilder class. Luckisneeded:true

Question 31

What is insert() used in?

Answer 31

StringBuilder class.

Question 32

StringBuilder sb = new StringBuilder("animals"); sb.insert(7, "."); sb.insert(0, 'A');

Answer 32

1. animals. 2. Aanimals.

Question 33

StringBuilder is mutable or immutable?

Answer 33

changeable/ Mutable

Question 34

What does delete() and deleteCharAt() do and used in? WAP to delete all the contents of sb WAP to delete the first char of sb

Answer 34

in StringBuilder. Simply deletes. ```` StringBuilder sb = new StringBuilder("hello"); sb.delete(0, sb.length()); sb.append("hhello"); sb.deleteCharAt(0); ````

Question 35

String methods memorize - length(); charAt(); indexOf(); subString(); toLowerCase(); toUpperCase(); equals(); equalsIgnoreCase(); startsWith(); endsWith(); contains(); replace(); trim();

Answer 35

Just refresh your memory

Question 36

String builder methods: length(); charAt(); indexOf(); subString(); -new append(),insert(), delete(), deleteChartAt() reverse(), toString()

Answer 36

Just to refresh your memory

Question 37

What happens when you do equals on StringBuilder?

Answer 37

Checks for reference equality

Question 38

What is the output? StringBuilder a = new StringBuilder("aa"); StringBuilder b = new StringBuilder("aa"); System.out.println(a.equals(b)); System.out.println(a==b);

Answer 38

false false

Question 39

String a = new String("aa"); String b = "aa"; System.out.println(a.equals(b)); System.out.println(a==b);

Answer 39

true false

Question 40

String a = "aa"; String b = "aa"; System.out.println(a.equals(b)); System.out.println(a==b);

Answer 40

true, true

Question 41

String a = "aa"; String b = "bb"; System.out.println(a=b);

Answer 41

bb cause you are assigning.

Question 42

if("string".toUpperCase() == "STRING") { System.out.println(true); } else { System.out.println(false); }

Answer 42

false

Question 43

if("string".toUpperCase().equals("STRING")) { System.out.println(true); } else { System.out.println(false); } }

Answer 43

true

Question 44

output? System.out.println("0123456789".substring(1, 6).substring(2,4).substring(1));

Answer 44

4

Question 45

String s1="java"; StringBuilder s2= new StringBuilder("java"); System.out.println(s1==s2));

Answer 45

you cant use String == StringBuilder gives you java: incomparable types:

Question 46

String a = "nelly"; a.concat("1").replace("1", "L").toUpperCase().toLowerCase();

Answer 46

It is evaulated left to right. so contact to