Are you an aspiring Java developer? Well, if you are, then I bet you can make use of this Java Cheat Sheet. Java is known for its pre-built classes and libraries and sometimes, keeping a track of them becomes a little tricky. So, here I bring you the Core Java Cheat Sheet.
This cheat sheet will act as a crash course for Java beginners and help you with various fundamentals of Java.
Core Java Cheat Sheet
Java is an open source programming language that has been changing the face of the IT market since ages. It is widely preferred by the programmers as the code written in Java can be executed securely on any platform, irrespective of the operating system or architecture of the device. The only requirement is, Java Runtime Environment (JRE) installed on the system.
Primitive Data Types
Let’s start off by learning the primitive data types that Java offers:
| Data Type | Size | Range |
| byte | 8 | -128..127 |
| short | 16 | -32,768..32,767 |
| int | 32 | -2,147,483,648.. 2,147,483,647 |
| long | 64 | -9,223,372,036,854,775,808.. 9,223,372,036,854,775,807 |
| float | 32 | 3.4e-0.38.. 3.4e+0.38 |
| double | 64 | 1.7e-308.. 1.7e+308 |
| char | 16 | Complete Unicode Character Set |
| Boolean | 1 | True, False |
Java Operators
There are mainly 8 different types of operators available in Java:
| Operator Type | Operators |
| Arithmetic | +, – , *, ? , % |
| Assignment | =, +=, -=, *=, /=, %=, &=, ^=, |=, <<=, >>=, >>>= |
| Bitwise | ^, &, | |
| Logical | &&, || |
| Relational | <, >, <=, >=,==, != |
| Shift | <<, >>, >>> |
| Ternary | ?: |
| Unary | ++x, –x, x++, x–, +x, –x, !, ~ |
Java Variables
Variables in Java refers to the name of the reserved memory area. You need variables to store any value for the computational or reference purpose.
There are 3 types of variable in Java:
- Local Variables
- Instance Variables
- Static Variables
{public | private} [static] type name [= expression | value];Java Methods
A method is a set of code that is grouped together to perform a specific operation. A method is completed in two steps:
- Method Initialization
- Method Invocation
A method can be invoked either by calling it by reference or by value.
{public | private} [static] {type | void} name(arg1, ..., argN ){statements} Data Conversion
The process of changing a value from one data type to another type is known as data type conversion. Data Type conversion is of two types:
- Widening: The lower size datatype is converted into a higher size data type without loss of information.
- Narrowing: The higher size datatype is converted into a lower size data type with a loss of information.
// Widening (byte<short<int<long<float<double)
int i = 10; //int--> long
long l = i; //automatic type conversion
// Narrowing
double d = 10.02;
long l = (long)d; //explicit type casting
// Numeric values to String
String str = String.valueOf(value);
// String to Numeric values
int i = Integer.parseInt(str);
double d = Double.parseDouble(str);
User Input
Java provides three ways to take an input from the user/ console:
- Using BufferReader class
- Using Scanner class
- Using Console class
// Using BufferReader
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
String name = reader.readLine();
// Using Scanner
Scanner in = new Scanner(System.in);
String s = in.nextLine();
int a = in.nextInt();
// Using Console
String name = System.console().readLine();Basic Java Program
A basic program in Java will consist of at least the following components:
- Classes & Objects
- Methods
- Variables
public class Demo{ public static void main(String[] args) { System.out.println("Hello from edureka!");} }
Compile a Java Program
You need to save your Java Program by the name of the class containing main() method along with .java extension.
className.javaCall the compiler using javac command.
javac classNameFinally, execute the program using below code:
java classNameFlow Of Control
Iterative Statements
Iterative statements are used when you need to repeat a set of statements until the condition for termination is not met.
// for loop
for (condition) {expression}
// for each loop
for (int i: someArray) {}
// while loop
while (condition) {expression}
// do while loop
do {expression} while(condition)Generating a Fibonacci series.
for (i = 1; i <= n; ++i) {System.out.print(t1 + " + "); int sum = t1 + t2; t1 = t2; t2 = sum;}
Creating a pyramid pattern.
k = 2*n - 2;
for(i=0; i<n; i++)
{ for(j=0; j<k; j++){System.out.print(" ");}
k = k - 1;
for(j=0; j<=i; j++ ){System.out.print("* ");}
System.out.println(); }Decisive Statements
Selection statements used when you need to choose between alternative actions during execution of the program.
//if statement
if (condition) {expression}
//if-else statement
if (condition) {expression} else {expression}
//switch statement
switch (var)
{ case 1: expression; break; default: expression; break; } Checking the given number is prime or not.
if (n < 2) { return false; } for (int i=2; i <= n/i; i++) {if (n%i == 0) return false;} return true;
Finding the factorial using recursion function.
int factorial(int n) { if (n == 0) {return 1;} else {return(n * factorial(n-1));} }
Java Arrays
Single Dimensional (1-D)
Single Dimensional or 1-D array is a type of linear array in which elements are stored in a continuous row.
// Initializing type[] varName= new type[size]; // Declaring type[] varName= new type[]{values1, value2,...};
Creating an array with random values.
double[] arr = new double[n];
for (int i=0; i<n; i++)
{a[i] = Math.random();}Searching the max value in the array.
double max = 0;
for(int i=0; i<arr.length(); i++)
{ if(a[i] > max) max = a[i]; }
Reversing an array.
for(int i=0; i<(arr.length())/2; i++)
{ double temp = a[i];
a[i] = a[n-1-i];
a[n-1-i] = temp;
}
Multi Dimensional (2-D)
Two Dimensional or 2-D array is an array of an array where elements are stored in rows and columns.
// Initializing
datatype[][] varName = new dataType[row][col];
// Declaring
datatype[][] varName = {{value1, value2....},{value1, value2....}..};Transposing a matrix.
for(i = 0; i < row; i++)
{ for(j = 0; j < column; j++)
{ System.out.print(array[i][j]+" "); }
System.out.println(" ");
}Multiplying two matrices.
for (i = 0; i < row1; i++)
{ for (j = 0; j < col2; j++)
{ for (k = 0; k < row2; k++)
{ sum = sum + first[i][k]*second[k][j]; }
multiply[i][j] = sum;
sum = 0;
}
}Java Strings
Creating a String
String in Java is an object that represents a sequence of char values. A String can be created in two ways:
- Using a literal
- Using ‘new’ keyword
String str1 = “Welcome”; // Using literal
String str2 = new String(”Edureka”); // Using new keywordThe java.lang.String class implements Serializable, Comparable and CharSequence interfaces. Since the String object is immutable in nature Java provides two utility classes:
- StringBuffer: It is a mutable class that is thread-safe and synchronized.
- StringBuilder: It is a mutable class that is not thread-safe but is faster and is used in a single threaded environment.
String Methods
Few of the most important and frequently used String methods are listed below:
str1==str2 //compares address;
String newStr = str1.equals(str2); //compares the values
String newStr = str1.equalsIgnoreCase() //compares the values ignoring the case
newStr = str1.length() //calculates length
newStr = str1.charAt(i) //extract i'th character
newStr = str1.toUpperCase() //returns string in ALL CAPS
newStr = str1.toLowerCase() //returns string in ALL LOWERvCASE
newStr = str1.replace(oldVal, newVal) //search and replace
newStr = str1.trim() //trims surrounding whitespace
newStr = str1.contains("value"); //check for the values
newStr = str1.toCharArray(); // convert String to character type array
newStr = str1.IsEmpty(); //Check for empty String
newStr = str1.endsWith(); //Checks if string ends with the given suffix