Application Program Interface (API) and Libraries: Revision Notes (Computer Science A 1.7)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

• A library is a collection of classes. An application programming interface (API) specification informs the programmer how to use them.
• Classes in the APIs and libraries are grouped into packages.
• A class defines a specific reference type. Existing classes and class libraries can be used to create objects.
• Attributes are the data related to the class, stored in variables.
• Behaviors are what instances of the class can do (or what can be done with them), defined by methods.
• The documentation in API specifications is essential for understanding the attributes and behaviors of a class.
• The Java Quick Reference, provided on the exam, lists accessible library methods and explains them.

Key relationships

Idea	Example
Default package	java.lang: String, Math, Integer
Needs an import	java.util: Scanner, ArrayList
Method entry	static double sqrt(double x): return type double, name sqrt, one double parameter
Return type position	before the method name
Parameter list	after the name, in brackets, with types in order
String attribute	the sequence of characters
String behaviors	length(), substring(...), indexOf(...), equals(...)
indexOf not found	returns -1
substring(from, to)	characters from index from up to but not including to
int passed to a double parameter	converted automatically

Assumptions and conventions

• You are expected to read documentation, not to memorise it.
• A method entry gives return type, name and parameters, plus a description of what it does.
• A class method such as Math.sqrt is called with the class name and a dot (Topic 1.10).
• A missing import produces a compile-time “cannot find symbol” error.

A four-step routine for any API entry

1. Decide how the method is reached: through an object, or through the class name (a static method).
2. Match the parameters: the number, the types (an int fits a double; the reverse needs a cast) and the order.
3. Use the return type: the call is an expression of that type, and void means there is no value to use.
4. Read the description for special cases, such as “returns -1 if not found” or “ending at index to - 1”.

Mistakes to avoid

1. Confusing the library with its documentation.
2. Expecting every class to be available without an import.
3. Mixing up attributes (data) and behaviors (methods).
4. Ignoring the return type, for example storing a double result in an int variable.
5. Passing arguments in the wrong order or number.
6. Forgetting that substring(from, to) excludes index to.
7. Forgetting the special return value -1 for “not found”.

Quick self-check

1. Which package holds String and Math? (java.lang, available by default.)
2. In String substring(int from, int to) what is the return type? (String)
3. Is length() an attribute or a behavior of String? (A behavior: it is a method.)
4. What does "banana".indexOf("x") return? (-1)
5. What does Math.pow(2, 3) return and of what type? (8.0, a double)

Next: practice questions.