learnxinyminutes-docs/java.html.markdown

---

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
///////////////////////////////////////
// General
///////////////////////////////////////
	// Single-line comments start with //
	/*
	Multi-line comments look like this.
	*/
	
	// Import Packages
	import java.util.ArrayList;
	import package.path.here;
	// Import all "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, and forcing a new line on next print = println()
	System.out.println("Hello World");
	System.out.println("Integer: "+10+"Double: "+3.14+ "Boolean: "+true);
	// Printing, without forcing a new line on next print = print()
	System.out.print("Hello World");
	System.out.print("Integer: "+10+"Double: "+3.14+ "Boolean: "+true);

///////////////////////////////////////
// Types
///////////////////////////////////////

	// Byte - 8-bit signed two's complement integer (-128 <= byte <= 127)
	byte foo = 100;
	
	// Short - 16-bit signed two's complement integer (-32,768 <= short <= 32,767)
	short bar = 10000;
	
	//Integer - 32-bit signed two's complement integer (-2,147,483,648 <= int <= 2,147,483,647)
	int foo = 1;
	
	//Long - 64-bit signed two's complement integer (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
	long bar = 100000L;
	
	//Float - Single-precision 32-bit IEEE 754 Floating Point
	float foo = 234.5f;
	
	//Double - Double-precision 64-bit IEEE 754 Floating Point
	double bar = 123.4;
	
	//Boolean - True & False
	boolean foo = true;
	boolean bar = false;
	
	//Char - A single 16-bit Unicode character
	char foo = 'A';
	
	//Strings
	String foo = "Hello World!";
	// \n is an escaped character that starts a new line
	String foo = "Hello World!\nLine2!";
	System.out.println(foo);
	//Hello World!
	//Line2!
	
	//Arrays
	//The array size must be decided upon declaration
	//The format for declaring an array is follows:
	//<datatype> [] <var name> = new <datatype>[<array size>];
	int [] array = new int[10];
	String [] array = new String[1];
	boolean [] array = new boolean[100];
	
	// Indexing an array - Accessing an element
	array[0];
	
	// Arrays are mutable; it's just memory!
	array[1] = 1;
	System.out.println(array[1]); // => 1
	array[1] = 2;
	printf("%d\n", my_array[1]); // => 2
		
	//Others to check out
	//ArrayLists - Like arrays except more functionality is offered, and the size is mutable
	//LinkedLists
	//Maps
	//HashMaps

///////////////////////////////////////
// Operators
///////////////////////////////////////

	int i1 = 1, i2 = 2; // Shorthand for multiple declarations
	
	// Arithmetic is straightforward
	i1 + i2; // => 3
	i2 - i1; // => 1
	i2 * i1; // => 2
	i1 / i2; // => 0 (0.5, but truncated towards 0)
	
	// Modulo
	11 % 3; // => 2
	
	// Comparison operators
	3 == 2; // => 0 (false)
	3 != 2; // => 1 (true)
	3 > 2; // => 1
	3 < 2; // => 0
	2 <= 2; // => 1
	2 >= 2; // => 1
	
	// Bitwise operators!
	~       Unary bitwise complement
	<<      Signed left shift
	>>      Signed right shift
	>>>     Unsigned right shift
	&       Bitwise AND
	^       Bitwise exclusive OR
	|       Bitwise inclusive OR

///////////////////////////////////////
// Control Structures
///////////////////////////////////////

	if (false) {
		  System.out.println("I never run");
		} else if (false) {
		  System.out.println("I am also never run");
		} else {
		  System.out.println("I print");
		}
	}

	// 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.