Oca Java Se 7 Exam Chapter 4 (Strings)

Front 1

Why are String objects immutable?

Back 1

Because the value is stored as private final array of characters.

Front 2

When is a new objects created for a String (describe 2 scenarios)

Back 2

When a String is created using keyword new and when a String is created by other method and the passed value is not present in String Constant Pool

Front 3

What are the 3 ways to create a String

Back 3

1) Using keyword new; 2) using = sign; 3) using double quote marks around the value, for example passed to a method or in value comparison statements.

Front 4

What is String Constant Pool and when is it populated

Back 4

String Constant Pool (or String Pool) helps save memory resources as it stores all String objects created using assignment symbol and double quotes, without repeating them. So when a new String is created using those methods Java first checks whether an object with that value exists in String Pool already, and if it doesn't , the object is created and variable refers to it, otherwise the variable will point to an existing object with passed value.

Front 5

Name at least 4 possible argument types for String() constructor

Back 5

1) Text in double quotes 2) array of characters 3) StringBuffer object, 4) StringBuilder object 5) No arguments (which creates an empty string)

Front 6

What String methods modify the String value?

Back 6

None. String methods always return a new object with modified value of original String.

Front 7

What type of argument does charAt() take in String class and what's a possible exception thrown?

Back 7

charAt() takes any integer values (or values resolvable to int, e.g. char) and throws run-time exception if the index passed is greater than length of String minus 1

Front 8

What type of argument does indexOf() in String class take and what's the output when the value is not found

Back 8

indexOf() takes char and String values as arguments, and returns negative 1 if not found

Front 9

What versions of substring() method exist and what's the possible exception thrown in String class?

Back 9

substring() can take 1 integer for starting point and returns the rest of the string starting from that point; substring() can take 2 integers for starting and ending positions, and will return the characters not including the end position. Remember the length of returned element is end integer minus start integer. Run time exception will occur if start or end is out of bounds.

Front 10

What does trim() method do in String class?

Back 10

Trim() returns a String without leading and trailing spaces, including new lines and tabs

Front 11

What versions of replace() method exist in String class?

Back 11

replace() method takes 2 character arguments, or 2 String arguments, but not a mix of those two.

Front 12

What arguments do startsWith() and endsWith() methods take in String class?

Back 12

startsWith() and endsWith() take String arguments. Remember String startsWith() a String, and endsWith() a String. The value returned is Boolean.

Front 13

How does method chaining work for Strings?

Back 13

String methods chained are read from left to write.

Front 14

Can you call methods on String constructor used with keyword new? If so, can that constructor have no arguments?

Back 14

Yes to both. You can call new String() with or without arguments and call methods on those objects. If the argument is not passed to constructor, the code will still compile and no error will be thrown during runtime.

Front 15

How does concatenation work for Strings with plus sign?

Back 15

If at least one operand is a String, plus sign concatenates the values into a String. The evaluation of the statement starts on the left, so if the first operand is a String, all the other values will be concatenated together.

Front 16

When does equals() method return true for comparison of Strings?

Back 16

When a) it's the same object or b) when it's a String object and the sequence of characters is the same.

Front 17

When does == (double equals) operator return true for comparison of Strings?

Back 17

When the variables and/or literals compared refer to the same object. For example they were initialized using assignment operator or double quotes and the value is on String pool

Front 18

What's the recommended way to compare String objects?

Back 18

Use equals() method. the double = sign is not recommended due to its limitations.

Front 19

What is the benefit of using StringBuilder objects instead of String?

Back 19

StringBuilder objects are mutable. The char array is not final, so it can be modified, which saves on the amount of created objects.

Front 20

Describe 4 StringBuilder constructors and objects they create

Back 20

1) No arguments - empty array with default 16 capacity. 2) StringBuilder argument - same array and capacity as passed object. 3) Integer - sets initial capacity to empty array of characters. 4) String or String literal - sets value to value of String and capacity equals length of string + 16 of buffer.

Front 21

What argument does StringBuilder indexOf() method take?

Back 21

Only string and optionally a second argument - integer. Char throws compilation error.

Front 22

What arguments does StringBuilder append() method take

Back 22

All 8 primitive types, array of characters, String, StringBuffer and any Java API object

Front 23

What is the default implementation of toString() method in Java objects

Back 23

Name of the class followed by @ (at) sign followed by hexadecimal representation of the hash of the object.

Front 24

What arguments does StringBuilder insert() method take?

Back 24

The first argument is always the offset integer, the second argument could be a primitive value to insert, a String, StringBuffer, array of Characters, CharSequence object, or any other Java object. For array of Characters the optional following arguments are startPosition and length of subarray.

Front 25

What methods delete characters from StringBuilder objects and how do they work?

Back 25

delete() method takes 2 integers and works like substring() method, not removing the endPosition character. deleteCharAt() method takes a single integer and removes the character at that position.

Front 26

What does trim() method do for StringBuilder objects?

Back 26

There is no method trim() defined for StrinbBuilder class

Front 27

What is the difference between functionality of most String methods and StringBuilder methods?

Back 27

String methods do not modify the String object they are called on, however, calling a method on a StringBuilder object, will modify that particular object if they return StringBuilder type.

Front 28

How is StringBuilder replace() method different from the same method in String class?

Back 28

It takes 2 integers for start and end position of replacement with the provided String as third argument. The String can be longer than the removed characters between positions. The end position index is not touched.

Front 29

How does StringBuilder subSequence() method work?

Back 29

It takes start and end position integers as subString(), but returns an object of type CharSequence. It doesn't modify the original object.

Front 30

What is the difference between StringBuffer and StringBuilder and when you should use either?

Back 30

StringBuffer is defined with the same methods as StringBuilder, but they are synchronized, which means they can be accessed by multiple threads, but that leads to overhead. If your code needs access from multiple threads, use StringBuffer, otherwise, it's simpler to stick to StringBuilder.

Front 31

Can a character object be assigned to a String variable?

Back 31

No. Causes compilation error.

Front 32

Is it correct to say that "all string literals are automatically instantiated into a string object"

Back 32

Yes, according to tests online.

Front 33

If you print the String object that hasn't been initialized or assigned null, what will show up?

Back 33

The literal "null"