Working with Java data types

Question 1

Which of the following statements are true:

1. method length() of String class is a final method.
2. You can make mutable subclasses of the String class.
3. StringBuilder extends String.
4. StringBuilder is a final class.
5. String class is not final.

Answer 1

1, 4

• StringBuilder extends Object.
• String, StringBuilder and StringBuffer are all final classes

Question 2

Which of these expressions will return true:

1. “hello world”.equals(“hello world”)
2. “hello world”.equals(“hello world”)
3. “hello”.concat(“ world”).trim().equals(“hello world”)
4. “hello”.concat(“ world”).trim().equals(“hello world”)
5. “Hello world”.toLowerCase( ).equals(“hello world”)

Answer 2

1, 2, 3, 5

Question 3

True/false:
Wrapper objects are always immutable:
Integer dataWrapper = new Integer(5);

Answer 3

True

Question 4

True/false: The following is valid:

unsigned byte b = 0;

Answer 4

False

- There is no unsigned keyword in Java

Question 5

True/false: The following is valid:

int 2ndArgument = Integer.parseInt(args[2]); //3

Answer 5

False

- an identifier name can’t start with a digit. Note: it may start with an underscore.

Question 6

Which of these are not part of the StringBuilder class:

1. trim( )
2. ensureCapacity(int )
3. append(boolean)
4. reverse( )
5. setLength(int)

Answer 6

1.

Question 7

Which of the following is not a primitive data value in Java:

1. “x”’
2. ‘x’
3. 10.2F
4. Object
5. false

Answer 7

1, 4

Question 8

True/false: Constructors can’t be marked as final

Answer 8

True

Question 9

True/false: Any class can be declared abstract even if it does not have any abstract method.

Answer 9

True

Question 10

True/false: Variables cannot be declared synchronized.

Answer 10

True

- Only methods can be declared synchronized.

Question 11

True/false: Static methods can always refer to non-static/instance members

Answer 11

False.

- (this includes fields and methods).

Question 12

True/false: An integer can be assigned to a double but not vice versa.

Answer 12

True

Question 13

True/false: The charAt( ) method can take a char value as an argument.

Answer 13

True

Question 14

True/false: The charAt( ) method returns a Character object.

Answer 14

False

- it returns char

Question 15

The following compiles:
class MyString extends String{ 
 MyString(){ super(); }
}

Answer 15

False
- String is a final class and can't be extended

Question 16

True/false: the following is valid:
char c = s;
s = c;

Answer 16

False.
- Not all short values are valid char values, and neither are all char values valid short values, Both lines won’t compile.