JAVA Cheat Sheet

One glance JAVA Cheat Sheet

Boilerplate

class Main
{
        public static void main(String args[])
        {
              Scannner sc=new Scanner(System.in);
              int num=sc.nextInt();
              for(int i=0;i<num;i++){
	              System.out.println(num);
              }
              System.out.println("Hello World!");
        }
}

Types

We use variables to temporarily store data in computer’s memory. In Java, the type of a variable should be specified at the time of declaration. In Java, we have two categories of types:

Primitives: for storing simple values like numbers, strings and booleans.
Reference Types: for storing complex objects like email messages.(Objects)

Primitive Types

#	Type	Bytes	Range
1	byte	1	[-128,127]
2	short	2	[-32k,32k]
3	int	4	[-2B,2B]
4	long	8
5	float	4
6	double	8
7	char	2	A,B,C, ...
8	boolean	1	true/false

class Main
{
        public static void main(String args[])
        {
              byte age = 30; 
              long viewsCount = 3_123_456L; 
              float price = 10.99F; 
              char letter = ‘A’; 
              boolean isEligible = true;
              System.out.println("Hello World!");
        }
}

Reference Types

Variables other than the above 8 Primitive Types.
Objects

class Main
{
        public static void main(String args[])
        {
              Object obj = new Object();
              Scanner sc = new Scanner(System.in);
              String name = “Pratyay”;
              //Internally: String name = new String("Pratyay");
        }
}

Strings

String name = “Pratyay”;

Useful String Methods: The String class in Java provides a number of useful methods:

startsWith(“a”)
endsWith(“a”)
length()
indexOf(“a”)
replace(“a”, “b”)
toUpperCase()
toLowerCase()

class Main
{
        public static void main(String args[])
        {
              String s1="Hello World!";
              System.out.println("string length is: "+s1.length());
        }
}

Strings are immutable, which means once we initialize them, their value cannot be changed. All methods that modify a string (like toUpperCase) return a new string object. The original string remains unaffected.

Escape Sequences: If you need to use a backslash or a double quotation mark in a string, you need to prefix it with a backslash. This is called escaping. Common escape sequences: