Java cheat sheet

The following Java cheat sheet will help review the basic syntax of the language.

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Main function

The entry point of a Java program is the main() function. It has the following signature:

public static void main(String args[]) {
    // Code goes here...
}

Primitive data types

class HelloWorld {

    public static void main(String args[]) {

        byte b = 1;

        System.out.println("Byte: " + b);



        short s = 50;

        System.out.println("short: " + s);



        int i = 255;

        System.out.println("Integer: " + i);



        long l = 1500;

        System.out.println("Long: " + l);



        float f = 250.6f;

        System.out.println("Float: " + f);



        double d = 134567.89;

        System.out.println("Double: " + d);



        char c = 'k';

        System.out.println("Char: " + c);



        boolean flag = false;

        System.out.println("Boolean: " + flag);

    }

}
Type Minimum Value Maximum Value
byte −27(−128)-2^7(-128) 27−1(127)2^7-1 (127)
short −215(−32,768)-2^{15} (-32,768) 215−1(32,767)2^{15}-1 (32,767)
int −231-2^{31} 231−12^{31}-1
long −263-2^{63} 263−12^{63}-1

Non-primitive data types

1. String

String is the most commonly used data type in Java. Some frequently used methods of this class are demonstrated below:

class HelloWorld {

    public static void main( String args[] ) {

        String str = new String(" Hello World!  ");



        // Remove leading and trailing whitespaces:

        str = str.trim();



        // Get index of a character:

        int i = str.indexOf('W');



        // Get length of string:

        int len = str.length();



        // Replace each occurrence of a character:

        String s = str.replace("!", " :)");



        // Convert to lowercase:

        s = s.toLowerCase(); // Opposite to toUpperCase()



        System.out.println("Original: " + str +

                           "\nLength: " + len +

                           "\nChanged: " + s);

    }

}

2. Arrays

Arrays can be defined and referenced using the following syntax:

In Java, Arrays can be easily converted into a String using the static toString() method in the Arrays class.

import java.util.*;



class HelloWorld {

    public static void main( String args[] ) {

        // Declare an int array of size 10:

        int[] seq = new int[10];



        // Populate the array:

        for(int i = 0; i < seq.length; ++i)

            seq[i] = i;



        // Print the array:

        System.out.println("int array: " + Arrays.toString(seq));



        // Declare and fill a char array:

        char[] s = {'a', 'b', 'c', 'd'};



        // Print the char array:

        System.out.println(s);

    }

}

Conditional statements

1. If-else

class HelloWorld {

    public static void main(String args[]) {

        int i = 0;



        if(i < 0)

            System.out.println("Negative int");

        else if (i > 0)

            System.out.println("Positive int");

        else

            System.out.println("int = 0");

    }

}

2. Switch statements

class HelloWorld {

    public static void main(String args[]) {

        int i = 0;



        switch(i){

            case 0:

                System.out.println("int = 0");

                break;

            default:

                System.out.println("int != 0");

        }

    }

}

Classes and interface

// Interface:

interface Workable{

    public void powerOn();

    public void powerOff();

}



// Abstract class:

abstract class Machine implements Workable{

    private boolean state;

    protected String name;



    // Constructor:

    Machine(){

        state = false;

        setName();

    }



    // Abstract class:

    public abstract void setName();



    // Implemented interface's functions:

    @Override

    public void powerOn(){

        System.out.println("Powering on " + name);

        state = true;

    }

    @Override

    public void powerOff(){

        System.out.println("Powering off " + name);

        state = false;

    }

}



// Concrete class:

class Computer extends Machine{

    @Override

    public void setName(){

        name = "PC";

    }



    // Entry point:

    public static void main(String args[]) {

        Computer pc = new Computer();

        pc.powerOn();

        pc.powerOff();

    }

}

Loops

class HelloWorld {

    public static void main( String args[] ) {

        // while loop:

        System.out.print("while loop: ");

        int i = 0;

        while(i < 10){

            System.out.print(i + " ");

            ++i;

        }



        // for loop:

        System.out.print("\nfor loop: ");

        for(; i < 20; ++i)

            System.out.print(i + " ");

        

        // for-each loop:

        System.out.print("\nfor-each loop: ");

        char[] arr = {'s','e','n','t','e','n','c','e'};

        for(char c: arr)

            System.out.print(c);

    }

}

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