Java Cheat

Appendix D: Java Programming Cheatsheet

Under construction.

This appendix summarizes the most commonly used Java language features in the textbook. Here are the APIs of the most common libraries.

Hello, World.

Editing, compiling, and executing.

Built-in data types.

Declaration and assignment statements.

Integers.

Floating point numbers.

Booleans.

Comparison operators.

Math library.

$Math library API$

The full Math API.

Command-line arguments.

int a = Integer.parseInt(args[0]); // read int from command-line
double b = Double.parseDouble(args[1]); // read double from command-line
String c = args[2]; // read String from command-line

Type conversion.

If and if-else statements.

Nested if-else statement.

While and for loops.

Nested for loops.

If-else nested inside a while loop.

// how many fair bets until you go broke?
int bets = 0;
while (stake > 0) {
 bets++;
 if (Math.random() < 0.5) stake++;
 else stake--;
}

Deeper nesting.

// print out Pythagorean triples (i, j, k) such that i^2 + j^2 = k^2
for (int i = 1; i <= N; i++) {
 for (int j = i; j <= N; j++) {
 for (int k = j; k <= N; k++) {
 if (i*i + j*j == k*k) {
 System.out.println("(" + i + ", " + j + ", " + k + ")");
 }
 }
 }
}

Break statement.

Do-while loop.

Switch statement.

Arrays.

// declare and compile-time initialize an array
int[] a = { 3, 1, 4, 1, 5, 9 };
double[] b = { 3.0, 1.0, 4.0, 1.0, 5.0, 9.0 };
String[] suits = { "Clubs", "Hearts", "Diamonds", "Spades" };

// declare and run-time initialize an array of integers
int N = 100;
int[] c = new int[N];
for (int i = 0; i < N; i++) {
 c[i] = i;
}

double[] d = new double[N];
for (int i = 0; i < N; i++) {
 d[i] = i;
}

// compute the average of the elements in the array d[]
double sum = 0.0;
for (int i = 0; i < d.length; i++) {
 sum = sum + d[i];
}
double average = sum / d.length;

Two-dimensional arrays.

// declare and compile-time initialize a 5-by-5 array of doubles
double[][] p = { 
 { .02, .92, .02, .02, .02 }, 
 { .02, .02, .32, .32, .32 }, 
 { .02, .02, .02, .92, .02 }, 
 { .92, .02, .02, .02, .02 }, 
 { .47, .02, .47, .02, .02 }, 
}; 


// declare and run-time initialize an M-by-N array of doubles
double[][] b = new double[M][N];
for (int i = 0; i < M; i++) {
 for (int j = 0; j < N; j++) {
 b[i][j] = 0.0;
 }
}

The String data type.

The full String API.

String s = "Hello";
String t = "World";
String u = s + " " + t; // "Hello World"
int length = s.length(); // 5 (length of string)
char c = t.charAt(2); // 'r' (character indices start at 0)
s.equals("Hello"); // compare string for equality

Our standard output library.

Our standard input library.

Our standard drawing library.

A few extra commands

StdDraw.mouseX() StdDraw.mouseY() StdDraw.mousePressed()
StdDraw.text() StdDraw.picture() StdDraw.setCanvasSize()

Our standard audio library.

Redirection and piping.

Constructors.

Instance variables.

Instance methods.

Classes.

Compile-time and run-time errors.

Here's a list of errors compiled by Mordechai Ben-Ari. It includes a list of common error message and typical mistakes that give rise to them.

javacheat