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Java cheat sheet

Educative Answers Team

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) 271(127)2^7-1 (127)
short 215(32,768)-2^{15} (-32,768) 2151(32,767)2^{15}-1 (32,767)
int 231-2^{31} 23112^{31}-1
long 263-2^{63} 26312^{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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