Getting Started

Hello.java

public class Hello {
  // main method
  public static void main(String[] args)
  {
    // Output: Hello, world!
    System.out.println("Hello, world!");
  }
}

Compiling and running

$ javac Hello.java
$ java Hello
Hello, world!

Variables

int num = 5;
float floatNum = 5.99f;
char letter = 'D';
boolean bool = true;
String site = "cheatsheets.zip";

Primitive Data Types

Data Type	Size	Default	Range
byte	1 byte	0	-128 ^to^ 127
short	2 byte	0	-2^15^ ^to^ 2^15^-1
int	4 byte	0	-2^31^ ^to^ 2^31^-1
long	8 byte	0	-2^63^ ^to^ 2^63^-1
float	4 byte	0.0f	N/A
double	8 byte	0.0d	N/A
char	2 byte	\u0000	0 ^to^ 65535
boolean	N/A	false	true / false

Strings

String first = "John";
String last = "Doe";
String name = first + " " + last;
System.out.println(name);

Loops

String word = "CheatSheets";
for (char c: word.toCharArray()) {
  System.out.print(c + "-");
}
// Outputs: C-h-e-a-t-S-h-e-e-t-s-

Arrays

char[] chars = new char[10];
chars[0] = 'a'
chars[1] = 'b'

String[] letters = {"A", "B", "C"};
int[] mylist = {100, 200};
boolean[] answers = {true, false};

Swap

int a = 1;
int b = 2;
System.out.println(a + " " + b); // 1 2

int temp = a;
a = b;
b = temp;
System.out.println(a + " " + b); // 2 1

Type Casting

// Widening
// byte<short<int<long<float<double
int i = 10;
long l = i;               // 10

// Narrowing
double d = 10.02;
long l = (long)d;         // 10

String.valueOf(10);       // "10"
Integer.parseInt("10");   // 10
Double.parseDouble("10"); // 10.0

Conditionals

int j = 10;

if (j == 10) {
  System.out.println("I get printed");
} else if (j > 10) {
  System.out.println("I don't");
} else {
  System.out.println("I also don't");
}

User Input

Scanner in = new Scanner(System.in);
String str = in.nextLine();
System.out.println(str);

int num = in.nextInt();
System.out.println(num);

Comments