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Java17 Cheat Sheet (DRAFT) by

Complete Cheat Sheet on Java 17, WIP

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Primitive Data types

Name
Size
Values
 
byte
1 B
-128 → 127
 
short
2 B
-32768 → 32767
 
int
4 B
-2∙10⁹ → 2∙10⁹
 
long
8 B
-9∙10¹⁵ → 9∙10¹⁵
 
float
4 B
6-7 decimal digits
 
double
8 B
15 decimal digits
 
boolean
1 b
 
true
or 
false
 
char
2 B
Characters or ASCII

Comparison Operators

Equal
 
x == y
Not equal
 
x != y
Less
 
x < y
Less or equal
 
x <= y
Greater
 
x > y
Greater or equal
 
x >= y

String Methods

Usage
Descri­ption
 
s.leng­th();
Length of a string
 
s.char­At(x);
Extract the nth character
 
s.toUp­per­Case();
Returns a copy of a string in ALL CAPS
 
s.toLo­wer­Case();
Returns a copy of a string in lowecase
 
s.inde­xOf(c);
Returns the first occurring index of c
 
s.repl­ace­(old, new);
Search and Replace
 
s.split(RegEx);
Splits string into tokens
 
s.equa­ls(s2);
Compares strings (true if equal)
 
s.comp­are­To(s2);
Returns "­0" if equal, "­+" if s > s2, "­-" if s < s2

IF Statement

// conditional handler
if (condition) {
// pose a condition ...
} else if (other condition) {
// if first condition not met pose another one ...
} else {
// if second condition not met finally ... 
}

FOR EACH Statement

// self-iterating loop for iterable objects
for (var: collection) {
  statements
}

FOR Statement

// structured iterator for non-iterable objects
for (int i = 0; i < max; i++) {}

Continue

for (int i = 0; i < max; i++) {
  // jump to the next iteration
  continue;
}

Break

while (true) {
  // exit early a loop
  break;
}

TRY-CATCH Statement

// exception handler
try {
// try to do something
} catch (exception e) {
// when you fail do...
} finally {
// when you are done do...
}

Enumerals

// "class" containing a group of constants
enum Name {
  NAME1,
  NAME2,
  NAME3
}
// constant are accessible per dot notation
// they are typed as the declared enum
Name var = Name.NAME1;

Object Example

public class Object {
  private <type> attr1;
  public Object (<type> attr1) {
    this.attr1 = attr1;
  }
  public void method() {
    // ...
  }
}

Interfaces

// An interface manages accessibility of object
// it is an abstract object
public interface Intrf {
  String const = "CIAO";
  public void method1();
}
public class Obj implements Intrf {
  public void Obj() { }
  @override
  public void method1() {}
}

Parent Class

// parent class
public class Parent implements Intrf {}
// child class, inherits Parents characteristics
public class Child extends Parent {}

ArrayList Methods

Usage
Descri­ption
 
list.add(item);
Add item to the list
 
list.get(n);
Return the nth item
 
list.s­ize();
Return number of item
 
list.r­emove(n);
Remove the nth item
 
list.set(n, value);
Put at the nth position value
 

Non-Pr­imitive Data Types

Name
Declar­ation
String
 
String name = "­Luc­a";
Array
 
type[] = {x, y};
Class
 
access class Obj { ... }
Interface
 
public interface ObjInt­erface { ... }

Boolean Operators

NOT
 
! x
AND
 
x && y
OR
 
x || y
XOR
 
x ^ y

Arithmetic Operators

Addition
 
x + y
Subtra­ction
 
x - y
Multip­lic­ation
 
x * y
Division
 
x / y
Increment
 
x++  ++x
Decrement
 
x--  --x
Modulus
 
x % y

WHILE Statement

// pre-conditioned iterator ...
while (condition) {}

Scanner

import java.utils.Scanner;
public class Obj{
  public static void main (String[] args) {
      Scanner input = new Scanner(System.in);
      String str = input.nextLine();
      int i = input.nextInt();
      input.close();
  }
}

DO-WHILE Statement

// post-conditional iterator
do { } while (condition);

SWITCH Statement

// case-oriented condition handler
switch (condition) {
  case value0:
    statements
    break;
  case value1:
    statements
    break;
  default:
    statements
}

Ternary operator

// same as an if, but inline!
condition ? iftrue : iffalse;

Type conversion

String
to any number
 
int i = Intege­r.p­ars­eInt(str);
Any type to 
String
 
String s = String.va­lueOf(value);
Between numeric types
 
int i = (int) number;

Array Utilities

Usage
Descri­ption
Arrays.co­pyOf(og, og.length);`
Copy array onto another one

Recursion

// method that sums all the number 1..10
// attention! recursion can incur in an infinite loop
// handle it well
public static int sum(int start, int end) {
  if (end > start) {
    return end + sum(start, end - 1);
  } else {
    return end;
  }
}

Access Defini­tions

  
public
 
protected
 
package
 
private
Class
•
•
•
•
Package
•
•
•
‎
Subclasses
•
•
‎
‎
Classes
•
‎
‎
‎

Usage of an Object

// to use an object
public static void main (String[] args) {
    // declaration meets constructor method
    Object obj = new Object(attr1);
    obj.method();
}

Dynamic Data Types

import java.util.*;

L​ist​<type> names = ​ new ArrayL​ist​<type>();

Vector<type> vec = new Vector<type>(); 

Stack<type> stk= new Stack<>();

Dynamic Data Types Methods

Vector
 
vec.add(value);
Add an element to the end
 
vec.add(i, value);
Add an element to a specific index
 
vec.ca­pac­ity();
Returns the current capacity
 
vec.cl­ear();
Empty vector
 
vec.cl­one();
Clone the vector
Stack
Inherits from Vector
 
stk.em­pty();
Returns true if head is empty
 
stk.pe­ek();
Returns head of stack
 
stk.pop();
Delete head of stack
 
stk.push(value);
Add an element at head
 
stk.se­arch(value);
Returns index of element