Introduction
• Referring to a method from another Java file is easy
• Benefits of having methods in different files
o Enables us to reuse code
o Enables modular programming → can develop libraries of static methods for use by any other
program; can independently develop, compile & debug parts of big programs one piece at a time
Using static methods in other programs
• To refer to static method in one class that’s defined in another
o Make both classes accessible to Java (e.g. in same directory in your computer)
o To call a method → prepend its class name & a period separator
• Modular programming = independently develop & debug methods for an application, and use them at any
later time
The public keyword Compile when necessary
• Identifies method as available for use by any • When compiling a program, Java compiles
other program with access to the file everything that needs to be compiled in order to
• Can also identify a method as private (but no run that program
reason to do so at this point) • Useful because all classes that call methods
will use the new version of your code if you fix
Each module is a class any bugs etc.
• Module = all code we keep in a single file
• Each module is a Java class that is kept in a file Multiple main() methods
with the same name as the file but has a .java • More than one class might have a main()
extension method
• For now → each class is a set of static methods • When you type java followed by a class name,
Java transfers control to the machine code
The .class file corresponding to the main() method defined in
that class
• When compiling a program, Java compiler
• Usually have a main() method in every class to
makes a file with the class name + a .class
test & debug its methods
extension that has the program’s code in a
language more suited to the computer Tips
• If you have a .class file, you can use the
• Think of each program you write as
module’s methods in another program without
something you might want to use later
having the source code in the corresponding
• Will allow you to have multiple tools to
.java file (but will be tricky if you find a bug)
use later & save time
1
, Libraries
• Module whose methods are mainly intended for use by many other
programs
• Important characteristic of Java → thousands of libraries have been
predefined for your use
• User-defined libraries = classes that contain a set of related methods for
use by other programs
• No Java library can contain all the methods we might need for a given
computation, so being able to build our own libraries is crucial to
address complex programs
Clients
• Program that calls a given library method
• When a class contains a method that is a client of a method in another
class → first class is a client of the second class
APIs
• Programmers think in terms of a contract between the client & the
implementation that’s a clear specification of what the method is to do
• When writing both clients & implementations, you are making contracts
with yourself → helpful when you are debugging & can reuse code
• API = Application Programming Interface
o Programs written that are clients of built-in Java classes
o Precise specification of signatures of methods available for use
Implementations
• Java code that implements the methods in an API, kept by
convention in a file with the library name & a .java extension
• Every Java program is an implementation of some API (no API is
of use without some implementation)
• Goal when developing an implementation → honour the terms of
the contract
• Separating client code from implementation code gives us
freedom to substitute new & improved implementations
• Limits to the amount of info we can productively include in an API
→ provide to client programmers the info they need & no more
• Implementations change quite frequently
Random numbers
• Code uses particular idioms that convert the random
double values between 0 and 1 that Math.random()
provides to the type of random numbers that we want to
use
• To effectively reuse code that implements these idioms,
we’ll now use the StdRandom library
• Summarize the methods in the StdRandom library with
an API
• Collecting methods with Math.random() to get random
numbers of different types in 1 file → focus on
generating random numbers to this one file instead of
spreading through all programs that use these methods
2
• Referring to a method from another Java file is easy
• Benefits of having methods in different files
o Enables us to reuse code
o Enables modular programming → can develop libraries of static methods for use by any other
program; can independently develop, compile & debug parts of big programs one piece at a time
Using static methods in other programs
• To refer to static method in one class that’s defined in another
o Make both classes accessible to Java (e.g. in same directory in your computer)
o To call a method → prepend its class name & a period separator
• Modular programming = independently develop & debug methods for an application, and use them at any
later time
The public keyword Compile when necessary
• Identifies method as available for use by any • When compiling a program, Java compiles
other program with access to the file everything that needs to be compiled in order to
• Can also identify a method as private (but no run that program
reason to do so at this point) • Useful because all classes that call methods
will use the new version of your code if you fix
Each module is a class any bugs etc.
• Module = all code we keep in a single file
• Each module is a Java class that is kept in a file Multiple main() methods
with the same name as the file but has a .java • More than one class might have a main()
extension method
• For now → each class is a set of static methods • When you type java followed by a class name,
Java transfers control to the machine code
The .class file corresponding to the main() method defined in
that class
• When compiling a program, Java compiler
• Usually have a main() method in every class to
makes a file with the class name + a .class
test & debug its methods
extension that has the program’s code in a
language more suited to the computer Tips
• If you have a .class file, you can use the
• Think of each program you write as
module’s methods in another program without
something you might want to use later
having the source code in the corresponding
• Will allow you to have multiple tools to
.java file (but will be tricky if you find a bug)
use later & save time
1
, Libraries
• Module whose methods are mainly intended for use by many other
programs
• Important characteristic of Java → thousands of libraries have been
predefined for your use
• User-defined libraries = classes that contain a set of related methods for
use by other programs
• No Java library can contain all the methods we might need for a given
computation, so being able to build our own libraries is crucial to
address complex programs
Clients
• Program that calls a given library method
• When a class contains a method that is a client of a method in another
class → first class is a client of the second class
APIs
• Programmers think in terms of a contract between the client & the
implementation that’s a clear specification of what the method is to do
• When writing both clients & implementations, you are making contracts
with yourself → helpful when you are debugging & can reuse code
• API = Application Programming Interface
o Programs written that are clients of built-in Java classes
o Precise specification of signatures of methods available for use
Implementations
• Java code that implements the methods in an API, kept by
convention in a file with the library name & a .java extension
• Every Java program is an implementation of some API (no API is
of use without some implementation)
• Goal when developing an implementation → honour the terms of
the contract
• Separating client code from implementation code gives us
freedom to substitute new & improved implementations
• Limits to the amount of info we can productively include in an API
→ provide to client programmers the info they need & no more
• Implementations change quite frequently
Random numbers
• Code uses particular idioms that convert the random
double values between 0 and 1 that Math.random()
provides to the type of random numbers that we want to
use
• To effectively reuse code that implements these idioms,
we’ll now use the StdRandom library
• Summarize the methods in the StdRandom library with
an API
• Collecting methods with Math.random() to get random
numbers of different types in 1 file → focus on
generating random numbers to this one file instead of
spreading through all programs that use these methods
2
