Wrapper Classes: Revision Notes (Computer Science A 4.7)

Full explanation and traced examples: Topic 4.7 study guide.

Recap

Integer and Double are wrapper classes in java.lang: each object holds one int or one double. They are immutable. The compiler converts automatically between a primitive and its wrapper. Autoboxing goes from primitive to wrapper (on assignment or when passed to a method expecting the wrapper). Unboxing goes from wrapper to primitive (on assignment or when passed to a method expecting the primitive). Arithmetic on wrapper objects unboxes them first.

Integer a = 5;          // autoboxing
int b = a + 3;          // unboxing, then int arithmetic: 8
double r = Double.parseDouble("3.5");   // 3.5
int n = Integer.parseInt("42");         // 42

Key relationships

Idea	Rule	Example
Autoboxing	int to Integer, double to Double only	Integer i = 4;
Unboxing	Integer to int, Double to double	int j = i;
No cross-boxing	int cannot box to Double	Double d = 7; does not compile
parseInt	static int parseInt(String s)	Integer.parseInt("007") is 7
parseDouble	static double parseDouble(String s)	Double.parseDouble("4") is 4.0
Bad text	NumberFormatException	"12a", "4.0" for parseInt, " 5"
Division	unboxes first, so two Integers use integer division	Integer 10 / 3 is 3
Immutable	n++ makes n refer to a new object	other references keep the old value
null	unboxing null throws NullPointerException	Integer z = null; int w = z;

Assumptions

• The String passed to a parse method holds exactly one valid number, with no spaces.
• Wrapper variables used in arithmetic are not null.
• equals (not ==) is used to compare the values of two wrapper objects.

Mistakes to avoid

1. Writing Double d = 7;. Use 7.0, or declare the variable as double.
2. Calling parseInt on a String such as "3.5"; use parseDouble.
3. Expecting decimals from Integer / Integer.
4. Comparing wrapper objects with ==.
5. Joining Strings with + and expecting addition of numbers.
6. Forgetting that Integer and Double need no import.

A mini-example to rehearse

Integer a = 5;           // autoboxing: a refers to an Integer holding 5
int b = a + 3;           // unboxing a, int addition: b is 8
Double d = 2.5;          // autoboxing
double e = d * b;        // unbox d; b widened to 8.0; e is 20.0
Integer c = b;           // autoboxing: c holds 8

Output of printing a, b, d, e, c with spaces: 5 8 2.5 20.0 8. Printing a wrapper shows its value.

For text data: "3.5,4.25,2".split(",") gives three Strings; Double.parseDouble on each gives 3.5, 4.25 and 2.0, whose mean is 3.25. Integer.parseInt("3.5") would throw NumberFormatException.

How this topic is usually tested

• Predict the type and value of a mixed expression (Integer with double, Integer / Integer).
• Find the line that does not compile (Double d = 7;).
• Convert text to numbers inside a loop and total or average the results.
• Explain a run-time failure: NumberFormatException from bad text, NullPointerException from unboxing null.

When a question shows Integer or Double variables in arithmetic, mentally replace each with its primitive first, then apply the normal rules for int and double arithmetic.

Quick self-check

1. Integer a = 4; int b = a * 2; What is b? 8, after unboxing a.
2. Does Double d = 5; compile? No, an int cannot box to Double.
3. What is Integer.parseInt("42") + Integer.parseInt("8")? 50.
4. What is Double.parseDouble("2")? 2.0.
5. Integer z = null; int w = z; happens at run time. What is thrown? NullPointerException.