All Flashcards
Why use wrapper classes?
To use primitive types in contexts that require objects, such as collections or methods expecting objects.
Can Integer objects be null?
Yes, unlike the primitive int, an Integer object can be assigned a null value.
What is the purpose of the intValue() method?
It returns the int value of an Integer object.
What is the purpose of the doubleValue() method?
It returns the double value of a Double object.
Why are wrapper classes useful with collections?
Collections like ArrayList can only store objects, not primitive types. Wrapper classes allow you to store primitive data in collections.
What is the difference between int and Integer?
int is a primitive type; Integer is a class that wraps the int type. Integer can be null, int cannot.
What is the difference between double and Double?
double is a primitive type; Double is a class that wraps the double type. Double can be null, double cannot.
What are the differences between Integer.parseInt() and the Integer constructor?
Integer.parseInt() parses a String to an int. The Integer constructor creates an Integer object from an int or String.
What are the differences between Double.parseDouble() and the Double constructor?
Double.parseDouble() parses a String to a double. The Double constructor creates a Double object from a double or String.
What does the following code output?
Integer num = new Integer(10); int val = num.intValue(); System.out.println(val);
10
What does the following code output?
Double num = new Double(3.14); double val = num.doubleValue(); System.out.println(val);
3.14
What does the following code output?
Integer num = 5; int val = num; System.out.println(val);
5
What does the following code output?
Double num = 2.718; double val = num; System.out.println(val);
2.718
What does the following code output?
Integer num = null; int val = 0; try { val = num; } catch (NullPointerException e) { System.out.println("NullPointerException caught!"); }
NullPointerException caught!