CS150: Schemer's Guide to Java

Schemer's Guide to Java

Java is an object-oriented, structured, imperative programming language invented in 1995 by James Gosling and associated with Bill Joy and Guy Steel (of inventing-Scheme fame). It was originally named Oak after a tree outside the window.

The purpose of this document is to give an undergraduate student who is familiar with Scheme and Python an introduction to reading Java.

Summary

Python is a universal programming language — this means every process we could describe using a Turing Machine (or using Scheme or Python), we can also describe in Java. The syntax of Java is very different from Scheme — it is more similar to Python.

Key differences with Java, for the purposes of this class:

Java has no lambda.
You must know and write down the type of every Java variable in advance.
List manipulation is difficult in Java. (No filter, no (list 1 2 3 4), etc.)
You must use object-oriented programming in Java, packaging state with methods.

More recent versions of Java lift some of these restrictions, but for standard Java you are likely to encounter they are all basically true. (If you already know enough about Java to suggest anonymous inner classes as a weak substitute for lambda, this guide is not for you.)

Glossy Comparison

Property	Scheme	Python	Java
Universal	Yes	Yes	Yes
Source Code	Succinct	Succinct	Verbose
Functional Programming	Easy	Easy	Harder
Object-Oriented Programming	If You Want	If You Want	Basically Required
State Updates	Discouraged `set!`	Encouraged =	Basically Required =
Write Types In Source	No	No	Yes
Catch Type Errors Early	No	No	Yes
Block Structured	If You Want (usually No)	If You Want (usually Yes)	Always Yes
Operation Ordering	Prefix	Infix	Infix

Expressions

Scheme	Python	Java
(+ age (sqrt 3))	age + sqrt(3)	age + sqrt(3)

Variable Declaration and Assignment

Scheme	Python	Java
(let ((x 1)) (set! x (+ x 5)) (print x) )	x = 1 x = x + 5 print x	{ int x = 1; x = x + 5; println(x); }

In the Java example, int is the type of the variable x. Note that Java requires a semicolon after every statement.

Blocks and Conditionals

Scheme	Python	Java
(if (eq? x 0) (begin (print x) (print y) ) (print z) )	if x == 0: print x print y else: print z	if (x == 0) { println(x); println(y); } else { println(z); }

Python uses tabbing to show block structure. Java uses { curly braces }.

Functions

Scheme	Python	Java
(define (fib n) (if (< n 2) n (+ (fib (- n 1)) (fib (- n 2)))))	def fib(n): if n < 2: return n else: return fib(n-1) + fib(n-2)	int fib(int n) { if (n < 2) return n; else return fib(n-1) + fib(n-2); }

In the Java example, int fib(...) means that fib is a function that returns something of type int. Similarly, int n means the argument must be of type int.

Credits: This guide was created by Westley Weimer in Spring 2009.

Numeric Iteration

Scheme	Python	Java
(map (lambda (x) (print (* x 3)) ) (intsto 10))	for x in range(1,10): print (x * 3)	for (int x=1; x <= 10; x=x+1) { print (x*3) }

The Java for loop construct has three parts: the initialization, the loop guard, and the increment.