Java Quick Reference Guide

Hello, World.

Editing, compiling, and executing.

Built-in data types.

Declaration and assignment statements.

Integers.

Floating point numbers.

Booleans.

Comparison operators.

Math library.

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 statements.

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 and Case 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 class & 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

Java 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.