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Java Cheatsheet
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Raúl Villora Valencia
2019-01-23 12:29:20 +01:00
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Java Cheatsheet
HELLO WORLD
//Text file name HelloWorld.java
public class HelloWorld {
// main() is the method
public static void main (String[] arfs)
//Prints "Hello World" in the terminal window.
System.out.pritn("Hello World");
}
DATA TYPES
Type Set of values Values Operators
int integers between -2^31 and + (2^31)-1 + - * / %
double floating-point numbers real numbers + - * /
boolean boolean values true or false && || !
char characters
String sequences of characters
DECLARATION AND ASSIGNMENT STATEMENTS
//Declaration statement
int a,b;
//Assignment statement
a = 13212; //a is the variable name; 13212 is the literal which is assign to the variable a
//Initialization statement
int c = a + b;
COMPARISON OPERATORS
Operation Meaning
== equal
!= not equal
< less than
> greater than
<= less than or equal
>= greater than or equal
PRINTING
void System.out.print(String s) //print s
void System.out.println(String s) //print s, followed by a newline
void System.out.println() //print a newline
PARSING COMMAND-LINE ARGUMENTS
int Integer.parseInt(String s) //convert s to an int value
double Double.parseDouble(String) //convert s to a double value
long Long.parseLong(String s) // convert s to a long value
MATH LIBRARY
Public Class Math
double abs(double a) // absolute value of a
double max(double a, double b) //maximum of a and b
double min(double a, dobule a) //minimum of a and b
double sin(double theta) //sine of theta
double cos(double theta) //cosine of theta
double tan(double theta) //tangent of theta
double toRadians(double degrees) // convert angle from degrees to radians
double toDegreestouble radians) // convert angle from radians to degrees
double exp(doube a) // exponential (e^a)
double pow(double a, double p) //raise a to the bth power (a^b)
double random() //random in [0,1)
double sqrt(double a) //square root of a
EXAMPLES OF TYPE CONVERSION
Expression Expression type Expression value
(1 + 2 + 3 + 4) / 4.0 double 2.5
Math.sqrt(4) double 2.0
"123343" + 99 String "12334399"
11 * 0.25 double 2.75
(int) 11 * 0.25 double 2.75
11 * (int) 0.25 int 0
(int) (11 * 0.25) int 2
ANATOMY OF AN IF STATEMENT
if (x>y) { // x > y is the boolean expression
//Sequence of statements
x = y;
}
IF AND IF-ELSE STATEMENT
if (BOOLEAN EXPRESSION) {
//Sequence of statements
} else {
//Sequence of statements
}
ANATOMY OF A WHILE LOOP
//Initialization is a separate statement
int power = 1;
while ( power <= n/2 ) // power <= n/2 is an example of the loop-continuation condition
{
//Statements
}
ANATOMY OF A WHILE LOOP
//Initialize another variable in a separate statement
int power = 1;
for (declare and initialize a loop control variable; loop-continuation condition/s; increment or decrement of the variable of control)
{
//Statement
}
Example:
for (int i = 0; i <= n; i++) {
//Statement
}
DO-WHILE LOOP
do{
//Statement
} while(lopp-continuation condition);
SWITCH STATEMENT
switch (VARIABLE TO EVALUATE ITS VALUE) {
case value: Statement; break;
...
default: Statement; break;
}
Example:
int month = 8;
String monthString;
switch (month) {
case 1: monthString = "January";
break;
case 2: monthString = "February";
break;
case 3: monthString = "March";
break;
case 4: monthString = "April";
break;
case 5: monthString = "May";
break;
case 6: monthString = "June";
break;
case 7: monthString = "July";
break;
case 8: monthString = "August";
break;
case 9: monthString = "September";
break;
case 10: monthString = "October";
break;
case 11: monthString = "November";
break;
case 12: monthString = "December";
break;
default: monthString = "Invalid month";
break;
}
ARRAY DECLARATION
int[] ai; // array of int
short[][] as; // array of array of short
short s, // scalar short
aas[][]; // array of array of short
Object[] ao, // array of Object
otherAo; // array of Object
Collection<?>[] ca; // array of Collection of unknown type
DECLARATION OF ARRAY VARIABLE
Exception ae[] = new Exception[3];
Object aao[][] = new Exception[2][3];
int[] factorial = { 1, 1, 2, 6, 24, 120, 720, 5040 };
char ac[] = { 'n', 'o', 't', ' ', 'a', ' ',
'S', 't', 'r', 'i', 'n', 'g' };
String[] aas = { "array", "of", "String", };
FUNCTIONS
public static double sum (int a, int b) { //double is the return type, sum is the method's name, a and b are two arguments of type int;
int result; //local variable
result = a + b;
return result;//return statement;
}
USING AN OBJECT
//Declare a a variable, object name
String s;
//Invoke a contructor to create an object
s = new String ("Hello World");
//Invoke an instance method that operates on the object's value
char c = s.chartAt(4);
INSTANCE VARIABLES
public class Charge {
//Instance variable declarations
private final double rx, ry;
private final double q;
}
//private, final and public are access modifiers
CONSTRUCTORS
//A class contains constructors that are invoked to create objects from the class blueprint.
//Constructor declarations look like method declarations—except that they use the name of the class and have no return type
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
DECLARING CLASSESS
class MyClass {
// field, constructor, and
// method declarations
}
Example:
public class Bicycle {
// the Bicycle class has
// three fields
public int cadence;
public int gear;
public int speed;
// the Bicycle class has
// one constructor
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
// the Bicycle class has
// four methods
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void applyBrake(int decrement) {
speed -= decrement;
}
public void speedUp(int increment) {
speed += increment;
}
DECLARING CLASSESS IMPLEMENTATING AN INTERFACE
class MyClass extends MySuperClass implements YourInterface {
// field, constructor, and
// method declarations
}
// MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.
STACK DATA TYPE
public class Stack<Item> implements Iterable <Item>
Stack() //create an empty stack
boolean isEmpthy() //return if the stack empthy
void push(Item item) // push an item onto the stack
Item pop() //return and remove the item that was inserted most recently
int size() //number of item on stack
QUEUE DATA TYPE
public class Queue<Item> implements Iterable<Item>
Queue() //create an emptyh queue
boolean isEmpthy() //return if the queue empthy
void enqueue(Item item) // insert an item onto queue
Item dequeue() //return and remove the item that was inserted least recently
int size() //number of item on queue
ITERABLE
//import Iterator
import java.util.Iterator;
public class Queue<Item> implements Iterable <Item> {
//FIFO queue
private Node first;
private Node last;
private class Node {
Item item;
Node next;
}
public void enqueue (Item item)
...
public Item dequeue()
...
}
SYMBOL TABLE DATA TYPE
public class ST<Key extends Comparable<Key>, Value>
ST() //create and empthy symbol table
void put(Key key, Value val) //associate val with key
Value get(Key key) //value associated with key
void remove(Key key) //remove key (and its associated value)
boolean contains (Key key) //return if there is a value associated with key
int size() //number of key-value pairs
Iterable<Key> keys() // all keys in the symbol table
SET DATA TYPE
public class SET<Key extends Comparable<Key>> implements Iterable<Key>
SET() //create an empthy set
boolean isEmpthy() //return if the set is empthy
void add (Key key) //add key to the set
void remove(Key key) //remove key from set
boolean contains(Key key) //return if the key is in the set
int size() //number of elements in set