java

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+---
+language: java
+author: Jake Prather
+author_url: http://github.com/JakeHP
+---
+
+Java is a general-purpose, concurrent, class-based, object-oriented computer programming language.
+Read more here: https://en.wikipedia.org/wiki/Java_(programming_language)
+
+```java
+// Single-line comments start with //
+/*
+Multi-line comments look like this.
+*/
+
+// Import Packages
+import java.util.ArrayList;
+import package.path.here;
+// Import "sub-packages"
+import java.lang.Math.*;
+
+// Your program's entry point is a function called main
+public class Main
+{
+	public static void main (String[] args) throws java.lang.Exception
+	{
+		//stuff here
+	}
+}
+
+// Printing
+System.out.println("Hello World");
+System.out.println("Integer: "+10+"Double: "+3.14+ "Boolean: "+true);
+
+///////////////////////////////////////
+// Types
+///////////////////////////////////////
+
+// You have to declare variables before using them. A variable declaration
+// requires you to specify its type; a variable's type determines its size
+// in bytes.
+
+// Integers
+int x_int = 0;
+
+// shorts are usually 2 bytes
+short x_short = 0;
+
+// chars are guaranteed to be 1 byte
+char x_char = 0;
+char y_char = 'y'; // Char literals are quoted with ''
+
+// longs are often 4 to 8 bytes; long longs are guaranteed to be at least
+// 64 bits
+long x_long = 0;
+long long x_long_long = 0; 
+
+// floats are usually 32-bit floating point numbers
+float x_float = 0.0;
+
+// doubles are usually 64-bit floating-point numbers
+double x_double = 0.0;
+
+// Integral types may be unsigned. This means they can't be negative, but
+// the maximum value of an unsigned variable is greater than the maximum
+// value of the same size.
+unsigned char ux_char;
+unsigned short ux_short;
+unsigned int ux_int;
+unsigned long long ux_long_long;
+
+// Other than char, which is always 1 byte, these types vary in size depending
+// on your machine. sizeof(T) gives you the size of a variable with type T in 
+// bytes so you can express the size of these types in a portable way.
+// For example,
+printf("%d\n", sizeof(int)); // => 4 (on machines with 4-byte words)
+
+// Arrays must be initialized with a concrete size.
+char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
+int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
+                      // (assuming 4-byte words)
+
+
+// You can initialize an array to 0 thusly:
+char my_array[20] = {0};
+
+// Indexing an array is like other languages -- or,
+// rather, other languages are like C
+my_array[0]; // => 0
+
+// Arrays are mutable; it's just memory!
+my_array[1] = 2;
+printf("%d\n", my_array[1]); // => 2
+
+// Strings are just arrays of chars terminated by a NUL (0x00) byte,
+// represented in strings as the special character '\0'.
+// (We don't have to include the NUL byte in string literals; the compiler
+//  inserts it at the end of the array for us.)
+char a_string[20] = "This is a string";
+printf("%s\n", a_string); // %s formats a string
+
+/*
+You may have noticed that a_string is only 16 chars long.
+Char #17 is the NUL byte. 
+Chars #18, 19 and 20 have undefined values.
+*/
+
+printf("%d\n", a_string[16]); => 0
+
+///////////////////////////////////////
+// Operators
+///////////////////////////////////////
+
+int i1 = 1, i2 = 2; // Shorthand for multiple declaration
+float f1 = 1.0, f2 = 2.0;
+
+// Arithmetic is straightforward
+i1 + i2; // => 3
+i2 - i1; // => 1
+i2 * i1; // => 2
+i1 / i2; // => 0 (0.5, but truncated towards 0)
+
+f1 / f2; // => 0.5, plus or minus epsilon
+
+// Modulo is there as well
+11 % 3; // => 2
+
+// Comparison operators are probably familiar, but
+// there is no boolean type in c. We use ints instead.
+// 0 is false, anything else is true. (The comparison 
+// operators always return 0 or 1.)
+3 == 2; // => 0 (false)
+3 != 2; // => 1 (true)
+3 > 2; // => 1
+3 < 2; // => 0
+2 <= 2; // => 1
+2 >= 2; // => 1
+
+// Logic works on ints
+!3; // => 0 (Logical not)
+!0; // => 1
+1 && 1; // => 1 (Logical and)
+0 && 1; // => 0
+0 || 1; // => 1 (Logical or)
+0 || 0; // => 0
+
+// Bitwise operators!
+~0x0F; // => 0xF0 (bitwise negation)
+0x0F & 0xF0; // => 0x00 (bitwise AND)
+0x0F | 0xF0; // => 0xFF (bitwise OR)
+0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
+0x01 << 1; // => 0x02 (bitwise left shift (by 1))
+0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
+
+///////////////////////////////////////
+// Control Structures
+///////////////////////////////////////
+
+if (0) {
+  printf("I am never run\n");
+} else if (0) {
+  printf("I am also never run\n");
+} else {
+  printf("I print\n");
+}
+
+// While loops exist
+int ii = 0;
+while (ii < 10) {
+    printf("%d, ", ii++); // ii++ increments ii in-place, after using its value.
+} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+printf("\n");
+
+int kk = 0;
+do {
+    printf("%d, ", kk);
+} while (++kk < 10); // ++kk increments kk in-place, before using its value
+// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+printf("\n");
+
+// For loops too
+int jj;
+for (jj=0; jj < 10; jj++) {
+    printf("%d, ", jj);
+} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+printf("\n");
+
+///////////////////////////////////////
+// Typecasting
+///////////////////////////////////////
+
+// Every value in C has a type, but you can cast one value into another type
+// if you want.
+
+int x_hex = 0x01; // You can assign vars with hex literals
+
+// Casting between types will attempt to preserve their numeric values
+printf("%d\n", x_hex); // => Prints 1
+printf("%d\n", (short) x_hex); // => Prints 1
+printf("%d\n", (char) x_hex); // => Prints 1
+
+// Types will overflow without warning
+printf("%d\n", (char) 257); // => 1 (Max char = 255)
+
+// Integral types can be cast to floating-point types, and vice-versa.
+printf("%f\n", (float)100); // %f formats a float
+printf("%lf\n", (double)100); // %lf formats a double
+printf("%d\n", (char)100.0);
+
+///////////////////////////////////////
+// Pointers
+///////////////////////////////////////
+
+// A pointer is a variable declared to store a memory address. Its declaration will
+// also tell you the type of data it points to. You can retrieve the memory address 
+// of your variables, then mess with them.
+
+int x = 0;
+printf("%p\n", &x); // Use & to retrieve the address of a variable
+// (%p formats a pointer)
+// => Prints some address in memory;
+
+// Pointer types end with * in their declaration
+int* px; // px is a pointer to an int
+px = &x; // Stores the address of x in px
+printf("%p\n", px); // => Prints some address in memory
+
+// To retreive the value at the address a pointer is pointing to,
+// put * in front to de-reference it.
+printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
+
+// You can also change the value the pointer is pointing to.
+// We'll have to wrap the de-reference in parenthesis because
+// ++ has a higher precedence than *.
+(*px)++; // Increment the value px is pointing to by 1
+printf("%d\n", *px); // => Prints 1
+printf("%d\n", x); // => Prints 1
+
+int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
+int xx;
+for (xx=0; xx<20; xx++) {
+    x_array[xx] = 20 - xx;
+} // Initialize x_array to 20, 19, 18,... 2, 1
+
+// Declare a pointer of type int and initialize it to point to x_array
+int* x_ptr = x_array;
+// x_ptr now points to the first element in the array (the integer 20). 
+// This works because arrays are actually just pointers to their first element.
+
+// Arrays are pointers to their first element
+printf("%d\n", *(x_ptr)); // => Prints 20
+printf("%d\n", x_array[0]); // => Prints 20
+
+// Pointers are incremented and decremented based on their type
+printf("%d\n", *(x_ptr + 1)); // => Prints 19
+printf("%d\n", x_array[1]); // => Prints 19
+
+// You can also dynamically allocate contiguous blocks of memory with the
+// standard library function malloc, which takes one integer argument 
+// representing the number of bytes to allocate from the heap.
+int* my_ptr = (int*) malloc(sizeof(int) * 20);
+for (xx=0; xx<20; xx++) {
+    *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here
+} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
+
+// Dereferencing memory that you haven't allocated gives
+// unpredictable results
+printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?
+
+// When you're done with a malloc'd block of memory, you need to free it, 
+// or else no one else can use it until your program terminates
+free(my_ptr);
+
+// Strings can be char arrays, but are usually represented as char
+// pointers:
+char* my_str = "This is my very own string";
+
+printf("%c\n", *my_str); // => 'T'
+
+function_1();
+} // end main function
+
+///////////////////////////////////////
+// Functions
+///////////////////////////////////////
+
+// Function declaration syntax:
+// <return type> <function name>(<args>)
+
+int add_two_ints(int x1, int x2){
+    return x1 + x2; // Use return to return a value
+}
+
+/*
+Functions are pass-by-value, but you can make your own references
+with pointers so functions can mutate their values.
+
+Example: in-place string reversal
+*/
+
+// A void function returns no value
+void str_reverse(char* str_in){
+    char tmp;
+    int ii=0, len = strlen(str_in); // Strlen is part of the c standard library
+    for(ii=0; ii<len/2; ii++){
+        tmp = str_in[ii];
+        str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
+        str_in[len - ii - 1] = tmp;
+    }
+}
+
+/*
+char c[] = "This is a test.";
+str_reverse(c);
+printf("%s\n", c); // => ".tset a si sihT"
+*/
+
+///////////////////////////////////////
+// User-defined types and structs
+///////////////////////////////////////
+
+// Typedefs can be used to create type aliases
+typedef int my_type;
+my_type my_type_var = 0;
+
+// Structs are just collections of data
+struct rectangle {
+    int width;
+    int height;
+};
+
+
+void function_1(){
+
+    struct rectangle my_rec;
+
+    // Access struct members with .
+    my_rec.width = 10;
+    my_rec.height = 20;
+
+    // You can declare pointers to structs
+    struct rectangle* my_rec_ptr = &my_rec;
+
+    // Use dereferencing to set struct pointer members...
+    (*my_rec_ptr).width = 30;
+
+    // ... or use the -> shorthand
+    my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
+}
+
+// You can apply a typedef to a struct for convenience
+typedef struct rectangle rect;
+
+int area(rect r){
+    return r.width * r.height;
+}
+
+```
+
+## Further Reading
+
+Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://en.wikipedia.org/wiki/The_C_Programming_Language)
+
+Another good resource is [Learn C the hard way](http://c.learncodethehardway.org/book/)
+
+Other than that, Google is your friend.