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